Debugging with GDB: Extending GDB

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# 23. Extending GDB

GDB provides several mechanisms for extension. GDB also provides the ability to automatically load extensions when it reads a file for debugging. This allows the user to automatically customize GDB for the program being debugged.

> 23.1 Canned Sequences of Commands |  | Canned Sequences of GDB Commands
> 23.2 Extending GDB using Python |  |
> 23.3 Auto-loading extensions |  | Automatically loading extensions
> 23.4 Creating new spellings of existing commands |  |

To facilitate the use of extension languages, GDB is capable of evaluating the contents of a file. When doing so, GDB can recognize which extension language is being used by looking at the filename extension. Files with an unrecognized filename extension are always treated as a GDB Command Files. See section Command files.

You can control how GDB evaluates these files with the following setting:

`set script-extension off`
  All scripts are always evaluated as GDB Command Files.

`set script-extension soft`
  The debugger determines the scripting language based on filename extension. If this scripting language is supported, GDB evaluates the script using that language. Otherwise, it evaluates the file as a GDB Command File.

`set script-extension strict`
  The debugger determines the scripting language based on filename extension, and evaluates the script using that language. If the language is not supported, then the evaluation fails.

`show script-extension`
  Display the current value of the `script-extension` option.

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## 23.1 Canned Sequences of Commands

Aside from breakpoint commands (see section Breakpoint Command Lists), GDB provides two ways to store sequences of commands for execution as a unit: user-defined commands and command files.

> 23.1.1 User-defined Commands |  | How to define your own commands
> 23.1.2 User-defined Command Hooks |  | Hooks for user-defined commands
> 23.1.3 Command Files |  | How to write scripts of commands to be stored in a file
> 23.1.4 Commands for Controlled Output |  | Commands for controlled output
> 23.1.5 Controlling auto-loading native GDB scripts |  | Controlling auto-loaded command files

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### 23.1.1 User-defined Commands

A user-defined command is a sequence of GDB commands to which you assign a new name as a command. This is done with the `define` command. User commands may accept up to 10 arguments separated by whitespace. Arguments are accessed within the user command via `$arg0...$arg9`. A trivial example:

    define adder
      print $arg0 + $arg1 + $arg2
    end

To execute the command use:

    adder 1 2 3

This defines the command `adder`, which prints the sum of its three arguments. Note the arguments are text substitutions, so they may reference variables, use complex expressions, or even perform inferior functions calls.

In addition, `$argc` may be used to find out how many arguments have been passed. This expands to a number in the range 0...10.

    define adder
      if $argc == 2
        print $arg0 + $arg1
      end
      if $argc == 3
        print $arg0 + $arg1 + $arg2
      end
    end

`define commandname`
  Define a command named commandname. If there is already a command by that name, you are asked to confirm that you want to redefine it. commandname may be a bare command name consisting of letters, numbers, dashes, and underscores. It may also start with any predefined prefix command. For example, ``define target my-target'` creates a user-defined ``target my-target'` command.

  The definition of the command is made up of other GDB command lines, which are given following the `define` command. The end of these commands is marked by a line containing `end`.

`document commandname`
  Document the user-defined command commandname, so that it can be accessed by `help`. The command commandname must already be defined. This command reads lines of documentation just as `define` reads the lines of the command definition, ending with `end`. After the `document` command is finished, `help` on command commandname displays the documentation you have written.

  You may use the `document` command again to change the documentation of a command. Redefining the command with `define` does not change the documentation.

`dont-repeat`
  Used inside a user-defined command, this tells GDB that this command should not be repeated when the user hits `RET` (see section repeat last command).

`help user-defined`
  List all user-defined commands and all python commands defined in class COMAND_USER. The first line of the documentation or docstring is included (if any).

`show user`
`show user commandname`
  Display the GDB commands used to define commandname (but not its documentation). If no commandname is given, display the definitions for all user-defined commands. This does not work for user-defined python commands.

`show max-user-call-depth`
`set max-user-call-depth`
  The value of `max-user-call-depth` controls how many recursion levels are allowed in user-defined commands before GDB suspects an infinite recursion and aborts the command. This does not apply to user-defined python commands.

In addition to the above commands, user-defined commands frequently use control flow commands, described in 23.1.3 Command Files.

When user-defined commands are executed, the commands of the definition are not printed. An error in any command stops execution of the user-defined command.

If used interactively, commands that would ask for confirmation proceed without asking when used inside a user-defined command. Many GDB commands that normally print messages to say what they are doing omit the messages when used in a user-defined command.

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### 23.1.2 User-defined Command Hooks

You may define hooks, which are a special kind of user-defined command. Whenever you run the command ``foo'`, if the user-defined command ``hook-foo'` exists, it is executed (with no arguments) before that command.

A hook may also be defined which is run after the command you executed. Whenever you run the command ``foo'`, if the user-defined command ``hookpost-foo'` exists, it is executed (with no arguments) after that command. Post-execution hooks may exist simultaneously with pre-execution hooks, for the same command.

It is valid for a hook to call the command which it hooks. If this occurs, the hook is not re-executed, thereby avoiding infinite recursion.

In addition, a pseudo-command, ``stop'` exists. Defining (``hook-stop'`) makes the associated commands execute every time execution stops in your program: before breakpoint commands are run, displays are printed, or the stack frame is printed.

For example, to ignore `SIGALRM` signals while single-stepping, but treat them normally during normal execution, you could define:

    define hook-stop
    handle SIGALRM nopass
    end

    define hook-run
    handle SIGALRM pass
    end

    define hook-continue
    handle SIGALRM pass
    end

As a further example, to hook at the beginning and end of the `echo` command, and to add extra text to the beginning and end of the message, you could define:

    define hook-echo
    echo <<<---
    end

    define hookpost-echo
    echo --->>>\n
    end

    (gdb) echo Hello World
    <<<---Hello World--->>>
    (gdb)

You can define a hook for any single-word command in GDB, but not for command aliases; you should define a hook for the basic command name, e.g. `backtrace` rather than `bt`. You can hook a multi-word command by adding `hook-` or `hookpost-` to the last word of the command, e.g. ``define target hook-remote'` to add a hook to ``target remote'`.

If an error occurs during the execution of your hook, execution of GDB commands stops and GDB issues a prompt (before the command that you actually typed had a chance to run).

If you try to define a hook which does not match any known command, you get a warning from the `define` command.

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### 23.1.3 Command Files

A command file for GDB is a text file made of lines that are GDB commands. Comments (lines starting with `#`) may also be included. An empty line in a command file does nothing; it does not mean to repeat the last command, as it would from the terminal.

You can request the execution of a command file with the `source` command. Note that the `source` command is also used to evaluate scripts that are not Command Files. The exact behavior can be configured using the `script-extension` setting. See section Extending GDB.

`source [-s] [-v] filename`
  Execute the command file filename.

The lines in a command file are generally executed sequentially, unless the order of execution is changed by one of the flow-control commands described below. The commands are not printed as they are executed. An error in any command terminates execution of the command file and control is returned to the console.

GDB first searches for filename in the current directory. If the file is not found there, and filename does not specify a directory, then GDB also looks for the file on the source search path (specified with the ``directory'` command); except that ``$cdir'` is not searched because the compilation directory is not relevant to scripts.

If `-s` is specified, then GDB searches for filename on the search path even if filename specifies a directory. The search is done by appending filename to each element of the search path. So, for example, if filename is ``mylib/myscript'` and the search path contains ``/home/user'` then GDB will look for the script ``/home/user/mylib/myscript'`. The search is also done if filename is an absolute path. For example, if filename is ``/tmp/myscript'` and the search path contains ``/home/user'` then GDB will look for the script ``/home/user/tmp/myscript'`. For DOS-like systems, if filename contains a drive specification, it is stripped before concatenation. For example, if filename is ``d:myscript'` and the search path contains ``c:/tmp'` then GDB will look for the script ``c:/tmp/myscript'`.

If `-v`, for verbose mode, is given then GDB displays each command as it is executed. The option must be given before filename, and is interpreted as part of the filename anywhere else.

Commands that would ask for confirmation if used interactively proceed without asking when used in a command file. Many GDB commands that normally print messages to say what they are doing omit the messages when called from command files.

GDB also accepts command input from standard input. In this mode, normal output goes to standard output and error output goes to standard error. Errors in a command file supplied on standard input do not terminate execution of the command file--execution continues with the next command.

    gdb < cmds > log 2>&1

(The syntax above will vary depending on the shell used.) This example will execute commands from the file ``cmds'`. All output and errors would be directed to ``log'`.

Since commands stored on command files tend to be more general than commands typed interactively, they frequently need to deal with complicated situations, such as different or unexpected values of variables and symbols, changes in how the program being debugged is built, etc. GDB provides a set of flow-control commands to deal with these complexities. Using these commands, you can write complex scripts that loop over data structures, execute commands conditionally, etc.

`if`
`else`
  This command allows to include in your script conditionally executed commands. The `if` command takes a single argument, which is an expression to evaluate. It is followed by a series of commands that are executed only if the expression is true (its value is nonzero). There can then optionally be an `else` line, followed by a series of commands that are only executed if the expression was false. The end of the list is marked by a line containing `end`.

`while`
  This command allows to write loops. Its syntax is similar to `if`: the command takes a single argument, which is an expression to evaluate, and must be followed by the commands to execute, one per line, terminated by an `end`. These commands are called the body of the loop. The commands in the body of `while` are executed repeatedly as long as the expression evaluates to true.

`loop_break`
  This command exits the `while` loop in whose body it is included. Execution of the script continues after that `while`s `end` line.

`loop_continue`
  This command skips the execution of the rest of the body of commands in the `while` loop in whose body it is included. Execution branches to the beginning of the `while` loop, where it evaluates the controlling expression.

`end`
  Terminate the block of commands that are the body of `if`, `else`, or `while` flow-control commands.

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### 23.1.4 Commands for Controlled Output

During the execution of a command file or a user-defined command, normal GDB output is suppressed; the only output that appears is what is explicitly printed by the commands in the definition. This section describes three commands useful for generating exactly the output you want.

`echo text`
  Print text. Nonprinting characters can be included in text using C escape sequences, such as ``\n'` to print a newline. No newline is printed unless you specify one. In addition to the standard C escape sequences, a backslash followed by a space stands for a space. This is useful for displaying a string with spaces at the beginning or the end, since leading and trailing spaces are otherwise trimmed from all arguments. To print `` and foo = '`, use the command ``echo \ and foo = \ '`.

  A backslash at the end of text can be used, as in C, to continue the command onto subsequent lines. For example,

      echo This is some text\n\
      which is continued\n\
      onto several lines.\n

  produces the same output as

      echo This is some text\n
      echo which is continued\n
      echo onto several lines.\n

`output expression`
  Print the value of expression and nothing but that value: no newlines, no ``$nn = '`. The value is not entered in the value history either. See section Expressions, for more information on expressions.

`output/fmt expression`
  Print the value of expression in format fmt. You can use the same formats as for `print`. See section Output Formats, for more information.

`printf template, expressions...`
  Print the values of one or more expressions under the control of the string template. To print several values, make expressions be a comma-separated list of individual expressions, which may be either numbers or pointers. Their values are printed as specified by template, exactly as a C program would do by executing the code below:

      printf (template, expressions...);

  As in `C` `printf`, ordinary characters in template are printed verbatim, while conversion specification introduced by the ``%'` character cause subsequent expressions to be evaluated, their values converted and formatted according to type and style information encoded in the conversion specifications, and then printed.

  For example, you can print two values in hex like this:

      printf "foo, bar-foo = 0x%x, 0x%x\n", foo, bar-foo

  `printf` supports all the standard `C` conversion specifications, including the flags and modifiers between the ``%'` character and the conversion letter, with the following exceptions:

  - The argument-ordering modifiers, such as ``2$'`, are not supported.

  - The modifier ``*'` is not supported for specifying precision or width.

  - The ``''` flag (for separation of digits into groups according to `LC_NUMERIC'`) is not supported.

  - The type modifiers ``hh'`, ``j'`, ``t'`, and ``z'` are not supported.

  - The conversion letter ``n'` (as in ``%n'`) is not supported.

  - The conversion letters ``a'` and ``A'` are not supported.

  Note that the ``ll'` type modifier is supported only if the underlying `C` implementation used to build GDB supports the `long long int` type, and the ``L'` type modifier is supported only if `long double` type is available.

  As in `C`, `printf` supports simple backslash-escape sequences, such as `\n`, ``\t'`, ``\\'`, ``\"'`, ``\a'`, and ``\f'`, that consist of backslash followed by a single character. Octal and hexadecimal escape sequences are not supported.

  Additionally, `printf` supports conversion specifications for DFP (Decimal Floating Point) types using the following length modifiers together with a floating point specifier. letters:

  - ``H'` for printing `Decimal32` types.

  - ``D'` for printing `Decimal64` types.

  - ``DD'` for printing `Decimal128` types.

  If the underlying `C` implementation used to build GDB has support for the three length modifiers for DFP types, other modifiers such as width and precision will also be available for GDB to use.

  In case there is no such `C` support, no additional modifiers will be available and the value will be printed in the standard way.

  Here's an example of printing DFP types using the above conversion letters:

      printf "D32: %Hf - D64: %Df - D128: %DDf\n",1.2345df,1.2E10dd,1.2E1dl

`eval template, expressions...`
  Convert the values of one or more expressions under the control of the string template to a command line, and call it.

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### 23.1.5 Controlling auto-loading native GDB scripts

When a new object file is read (for example, due to the `file` command, or because the inferior has loaded a shared library), GDB will look for the command file ``objfile-gdb.gdb'`. See section 23.3 Auto-loading extensions.

Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be printed.

`set auto-load gdb-scripts [on|off]`
  Enable or disable the auto-loading of canned sequences of commands scripts.

`show auto-load gdb-scripts`
  Show whether auto-loading of canned sequences of commands scripts is enabled or disabled.

`info auto-load gdb-scripts [regexp]`
  Print the list of all canned sequences of commands scripts that GDB auto-loaded.

If regexp is supplied only canned sequences of commands scripts with matching names are printed.

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## 23.2 Extending GDB using Python

You can extend GDB using the Python programming language. This feature is available only if GDB was configured using ``--with-python'`.

Python scripts used by GDB should be installed in ``data-directory/python'`, where data-directory is the data directory as determined at GDB startup (see section 18.6 GDB Data Files). This directory, known as the python directory, is automatically added to the Python Search Path in order to allow the Python interpreter to locate all scripts installed at this location.

Additionally, GDB commands and convenience functions which are written in Python and are located in the ``data-directory/python/gdb/command'` or ``data-directory/python/gdb/function'` directories are automatically imported when GDB starts.

> 23.2.1 Python Commands |  | Accessing Python from GDB.
> 23.2.2 Python API |  | Accessing GDB from Python.
> 23.2.3 Python Auto-loading |  | Automatically loading Python code.
> 23.2.4 Python modules |  | Python modules provided by GDB.

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### 23.2.1 Python Commands

GDB provides two commands for accessing the Python interpreter, and one related setting:

`python-interactive [command]`
`pi [command]`
  Without an argument, the `python-interactive` command can be used to start an interactive Python prompt. To return to GDB, type the `EOF` character (e.g., `Ctrl-D` on an empty prompt).

  Alternatively, a single-line Python command can be given as an argument and evaluated. If the command is an expression, the result will be printed; otherwise, nothing will be printed. For example:

      (gdb) python-interactive 2 + 3
      5

`python [command]`
`py [command]`
  The `python` command can be used to evaluate Python code.

  If given an argument, the `python` command will evaluate the argument as a Python command. For example:

      (gdb) python print 23
      23

  If you do not provide an argument to `python`, it will act as a multi-line command, like `define`. In this case, the Python script is made up of subsequent command lines, given after the `python` command. This command list is terminated using a line containing `end`. For example:

      (gdb) python
      Type python script
      End with a line saying just "end".
      >print 23
      >end
      23

`set python print-stack`
  By default, GDB will print only the message component of a Python exception when an error occurs in a Python script. This can be controlled using `set python print-stack`: if `full`, then full Python stack printing is enabled; if `none`, then Python stack and message printing is disabled; if `message`, the default, only the message component of the error is printed.

It is also possible to execute a Python script from the GDB interpreter:

`source ``script-name'``
  The script name must end with ``.py'` and GDB must be configured to recognize the script language based on filename extension using the `script-extension` setting. See section Extending GDB.

`python execfile ("script-name")`
  This method is based on the `execfile` Python built-in function, and thus is always available.

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### 23.2.2 Python API

You can get quick online help for GDB's Python API by issuing the command `python help (gdb)`.

Functions and methods which have two or more optional arguments allow them to be specified using keyword syntax. This allows passing some optional arguments while skipping others. Example: `gdb.some_function ('foo', bar = 1, baz = 2)`.

> 23.2.2.1 Basic Python |  | Basic Python Functions.
> 23.2.2.2 Exception Handling |  | How Python exceptions are translated.
> 23.2.2.3 Values From Inferior |  | Python representation of values.
> 23.2.2.4 Types In Python |  | Python representation of types.
> 23.2.2.5 Pretty Printing API |  | Pretty-printing values.
> 23.2.2.6 Selecting Pretty-Printers |  | How GDB chooses a pretty-printer.
> 23.2.2.7 Writing a Pretty-Printer |  |
> 23.2.2.8 Type Printing API |  | Pretty-printing types.
> 23.2.2.9 Filtering Frames. |  |
> 23.2.2.10 Decorating Frames. |  |
> 23.2.2.11 Writing a Frame Filter |  |
> 23.2.2.12 Inferiors In Python |  | Python representation of inferiors (processes)
> 23.2.2.13 Events In Python |  | Listening for events from GDB.
> 23.2.2.14 Threads In Python |  | Accessing inferior threads from Python.
> 23.2.2.15 Commands In Python |  | Implementing new commands in Python.
> 23.2.2.16 Parameters In Python |  | Adding new GDB parameters.
> 23.2.2.17 Writing new convenience functions |  |
> 23.2.2.18 Program Spaces In Python |  | Program spaces.
> 23.2.2.19 Objfiles In Python |  | Object files.
> 23.2.2.20 Accessing inferior stack frames from Python. |  |
> 23.2.2.21 Accessing blocks from Python. |  |
> 23.2.2.22 Python representation of Symbols. |  | Python representation of symbols.
> 23.2.2.23 Symbol table representation in Python. |  | Python representation of symbol tables.
> 23.2.2.24 Manipulating line tables using Python |  | Python representation of line tables.
> 23.2.2.25 Manipulating breakpoints using Python |  |
> 23.2.2.26 Finish Breakpoints |  | Setting Breakpoints on function return using Python.
> 23.2.2.27 Python representation of lazy strings. |  |
> 23.2.2.28 Python representation of architectures |  |

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#### 23.2.2.1 Basic Python

At startup, GDB overrides Python's `sys.stdout` and `sys.stderr` to print using GDB's output-paging streams. A Python program which outputs to one of these streams may have its output interrupted by the user (see section 22.4 Screen Size). In this situation, a Python `KeyboardInterrupt` exception is thrown.

Some care must be taken when writing Python code to run in GDB. Two things worth noting in particular:

- GDB install handlers for `SIGCHLD` and `SIGINT`. Python code must not override these, or even change the options using `sigaction`. If your program changes the handling of these signals, GDB will most likely stop working correctly. Note that it is unfortunately common for GUI toolkits to install a `SIGCHLD` handler.

- GDB takes care to mark its internal file descriptors as close-on-exec. However, this cannot be done in a thread-safe way on all platforms. Your Python programs should be aware of this and should both create new file descriptors with the close-on-exec flag set and arrange to close unneeded file descriptors before starting a child process.

GDB introduces a new Python module, named `gdb`. All methods and classes added by GDB are placed in this module. GDB automatically `import`s the `gdb` module for use in all scripts evaluated by the `python` command.

Variable: gdb.PYTHONDIR
  A string containing the python directory (see section 23.2 Extending GDB using Python).

Function: gdb.execute (command [, from_tty [, to_string]])
  Evaluate command, a string, as a GDB CLI command. If a GDB exception happens while command runs, it is translated as described in Exception Handling.

  from_tty specifies whether GDB ought to consider this command as having originated from the user invoking it interactively. It must be a boolean value. If omitted, it defaults to `False`.

  By default, any output produced by command is sent to GDB's standard output. If the to_string parameter is `True`, then output will be collected by `gdb.execute` and returned as a string. The default is `False`, in which case the return value is `None`. If to_string is `True`, the GDB virtual terminal will be temporarily set to unlimited width and height, and its pagination will be disabled; see section 22.4 Screen Size.

Function: gdb.breakpoints ()
  Return a sequence holding all of GDB's breakpoints. See section 23.2.2.25 Manipulating breakpoints using Python, for more information.

Function: gdb.parameter (parameter)
  Return the value of a GDB parameter. parameter is a string naming the parameter to look up; parameter may contain spaces if the parameter has a multi-part name. For example, ``print object'` is a valid parameter name.

  If the named parameter does not exist, this function throws a `gdb.error` (see section 23.2.2.2 Exception Handling). Otherwise, the parameter's value is converted to a Python value of the appropriate type, and returned.

Function: gdb.history (number)
  Return a value from GDB's value history (see section 10.10 Value History). number indicates which history element to return. If number is negative, then GDB will take its absolute value and count backward from the last element (i.e., the most recent element) to find the value to return. If number is zero, then GDB will return the most recent element. If the element specified by number doesn't exist in the value history, a `gdb.error` exception will be raised.

  If no exception is raised, the return value is always an instance of `gdb.Value` (see section 23.2.2.3 Values From Inferior).

Function: gdb.parse_and_eval (expression)
  Parse expression as an expression in the current language, evaluate it, and return the result as a `gdb.Value`. expression must be a string.

  This function can be useful when implementing a new command (see section 23.2.2.15 Commands In Python), as it provides a way to parse the command's argument as an expression. It is also useful simply to compute values, for example, it is the only way to get the value of a convenience variable (see section 10.11 Convenience Variables) as a `gdb.Value`.

Function: gdb.find_pc_line (pc)
  Return the `gdb.Symtab_and_line` object corresponding to the pc value. See section 23.2.2.23 Symbol table representation in Python.. If an invalid value of pc is passed as an argument, then the `symtab` and `line` attributes of the returned `gdb.Symtab_and_line` object will be `None` and 0 respectively.

Function: gdb.post_event (event)
  Put event, a callable object taking no arguments, into GDB's internal event queue. This callable will be invoked at some later point, during GDB's event processing. Events posted using `post_event` will be run in the order in which they were posted; however, there is no way to know when they will be processed relative to other events inside GDB.

  GDB is not thread-safe. If your Python program uses multiple threads, you must be careful to only call GDB-specific functions in the main GDB thread. `post_event` ensures this. For example:

      (gdb) python
      >import threading
      >
      >class Writer():
      > def __init__(self, message):
      >        self.message = message;
      > def __call__(self):
      >        gdb.write(self.message)
      >
      >class MyThread1 (threading.Thread):
      > def run (self):
      >        gdb.post_event(Writer("Hello "))
      >
      >class MyThread2 (threading.Thread):
      > def run (self):
      >        gdb.post_event(Writer("World\n"))
      >
      >MyThread1().start()
      >MyThread2().start()
      >end
      (gdb) Hello World

Function: gdb.write (string [, stream{]})
  Print a string to GDB's paginated output stream. The optional stream determines the stream to print to. The default stream is GDB's standard output stream. Possible stream values are:

  `gdb.STDOUT`
    GDB's standard output stream.

  `gdb.STDERR`
    GDB's standard error stream.

  `gdb.STDLOG`
    GDB's log stream (see section 2.4 Logging Output).

  Writing to `sys.stdout` or `sys.stderr` will automatically call this function and will automatically direct the output to the relevant stream.

Function: gdb.flush ()
  Flush the buffer of a GDB paginated stream so that the contents are displayed immediately. GDB will flush the contents of a stream automatically when it encounters a newline in the buffer. The optional stream determines the stream to flush. The default stream is GDB's standard output stream. Possible stream values are:

  `gdb.STDOUT`
    GDB's standard output stream.

  `gdb.STDERR`
    GDB's standard error stream.

  `gdb.STDLOG`
    GDB's log stream (see section 2.4 Logging Output).

  Flushing `sys.stdout` or `sys.stderr` will automatically call this function for the relevant stream.

Function: gdb.target_charset ()
  Return the name of the current target character set (see section 10.20 Character Sets). This differs from `gdb.parameter('target-charset')` in that ``auto'` is never returned.

Function: gdb.target_wide_charset ()
  Return the name of the current target wide character set (see section 10.20 Character Sets). This differs from `gdb.parameter('target-wide-charset')` in that ``auto'` is never returned.

Function: gdb.solib_name (address)
  Return the name of the shared library holding the given address as a string, or `None`.

Function: gdb.decode_line [expression]
  Return locations of the line specified by expression, or of the current line if no argument was given. This function returns a Python tuple containing two elements. The first element contains a string holding any unparsed section of expression (or `None` if the expression has been fully parsed). The second element contains either `None` or another tuple that contains all the locations that match the expression represented as `gdb.Symtab_and_line` objects (see section 23.2.2.23 Symbol table representation in Python.). If expression is provided, it is decoded the way that GDB's inbuilt `break` or `edit` commands do (see section 9.2 Specifying a Location).

Function: gdb.prompt_hook (current_prompt)

  If prompt_hook is callable, GDB will call the method assigned to this operation before a prompt is displayed by GDB.

  The parameter `current_prompt` contains the current GDB prompt. This method must return a Python string, or `None`. If a string is returned, the GDB prompt will be set to that string. If `None` is returned, GDB will continue to use the current prompt.

  Some prompts cannot be substituted in GDB. Secondary prompts such as those used by readline for command input, and annotation related prompts are prohibited from being changed.

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#### 23.2.2.2 Exception Handling

When executing the `python` command, Python exceptions uncaught within the Python code are translated to calls to GDB error-reporting mechanism. If the command that called `python` does not handle the error, GDB will terminate it and print an error message containing the Python exception name, the associated value, and the Python call stack backtrace at the point where the exception was raised. Example:

    (gdb) python print foo
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
    NameError: name 'foo' is not defined

GDB errors that happen in GDB commands invoked by Python code are converted to Python exceptions. The type of the Python exception depends on the error.

`gdb.error`
  This is the base class for most exceptions generated by GDB. It is derived from `RuntimeError`, for compatibility with earlier versions of GDB.

  If an error occurring in GDB does not fit into some more specific category, then the generated exception will have this type.

`gdb.MemoryError`
  This is a subclass of `gdb.error` which is thrown when an operation tried to access invalid memory in the inferior.

`KeyboardInterrupt`
  User interrupt (via `C-c` or by typing `q` at a pagination prompt) is translated to a Python `KeyboardInterrupt` exception.

In all cases, your exception handler will see the GDB error message as its value and the Python call stack backtrace at the Python statement closest to where the GDB error occured as the traceback.

When implementing GDB commands in Python via `gdb.Command`, it is useful to be able to throw an exception that doesn't cause a traceback to be printed. For example, the user may have invoked the command incorrectly. Use the `gdb.GdbError` exception to handle this case. Example:

    (gdb) python
    >class HelloWorld (gdb.Command):
    >  """Greet the whole world."""
    >  def __init__ (self):
    >    super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)
    >  def invoke (self, args, from_tty):
    >    argv = gdb.string_to_argv (args)
    >    if len (argv) != 0:
    >      raise gdb.GdbError ("hello-world takes no arguments")
    >    print "Hello, World!"
    >HelloWorld ()
    >end
    (gdb) hello-world 42
    hello-world takes no arguments

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#### 23.2.2.3 Values From Inferior

GDB provides values it obtains from the inferior program in an object of type `gdb.Value`. GDB uses this object for its internal bookkeeping of the inferior's values, and for fetching values when necessary.

Inferior values that are simple scalars can be used directly in Python expressions that are valid for the value's data type. Here's an example for an integer or floating-point value `some_val`:

    bar = some_val + 2

As result of this, `bar` will also be a `gdb.Value` object whose values are of the same type as those of `some_val`.

Inferior values that are structures or instances of some class can be accessed using the Python dictionary syntax. For example, if `some_val` is a `gdb.Value` instance holding a structure, you can access its `foo` element with:

    bar = some_val['foo']

Again, `bar` will also be a `gdb.Value` object. Structure elements can also be accessed by using `gdb.Field` objects as subscripts (see section 23.2.2.4 Types In Python, for more information on `gdb.Field` objects). For example, if `foo_field` is a `gdb.Field` object corresponding to element `foo` of the above structure, then `bar` can also be accessed as follows:

    bar = some_val[foo_field]

A `gdb.Value` that represents a function can be executed via inferior function call. Any arguments provided to the call must match the function's prototype, and must be provided in the order specified by that prototype.

For example, `some_val` is a `gdb.Value` instance representing a function that takes two integers as arguments. To execute this function, call it like so:

    result = some_val (10,20)

Any values returned from a function call will be stored as a `gdb.Value`.

The following attributes are provided:

Variable: Value.address
  If this object is addressable, this read-only attribute holds a `gdb.Value` object representing the address. Otherwise, this attribute holds `None`.

Variable: Value.is_optimized_out
  This read-only boolean attribute is true if the compiler optimized out this value, thus it is not available for fetching from the inferior.

Variable: Value.type
  The type of this `gdb.Value`. The value of this attribute is a `gdb.Type` object (see section 23.2.2.4 Types In Python).

Variable: Value.dynamic_type
  The dynamic type of this `gdb.Value`. This uses C`++` run-time type information (RTTI) to determine the dynamic type of the value. If this value is of class type, it will return the class in which the value is embedded, if any. If this value is of pointer or reference to a class type, it will compute the dynamic type of the referenced object, and return a pointer or reference to that type, respectively. In all other cases, it will return the value's static type.

  Note that this feature will only work when debugging a C`++` program that includes RTTI for the object in question. Otherwise, it will just return the static type of the value as in `ptype foo` (see section ptype).

Variable: Value.is_lazy
  The value of this read-only boolean attribute is `True` if this `gdb.Value` has not yet been fetched from the inferior. GDB does not fetch values until necessary, for efficiency. For example:

      myval = gdb.parse_and_eval ('somevar')

  The value of `somevar` is not fetched at this time. It will be fetched when the value is needed, or when the `fetch_lazy` method is invoked.

The following methods are provided:

Function: Value.__init__ (val)
  Many Python values can be converted directly to a `gdb.Value` via this object initializer. Specifically:

  Python boolean
    A Python boolean is converted to the boolean type from the current language.

  Python integer
    A Python integer is converted to the C `long` type for the current architecture.

  Python long
    A Python long is converted to the C `long long` type for the current architecture.

  Python float
    A Python float is converted to the C `double` type for the current architecture.

  Python string
    A Python string is converted to a target string, using the current target encoding.

  `gdb.Value`
    If `val` is a `gdb.Value`, then a copy of the value is made.

  `gdb.LazyString`
    If `val` is a `gdb.LazyString` (see section 23.2.2.27 Python representation of lazy strings.), then the lazy string's `value` method is called, and its result is used.

Function: Value.cast (type)
  Return a new instance of `gdb.Value` that is the result of casting this instance to the type described by type, which must be a `gdb.Type` object. If the cast cannot be performed for some reason, this method throws an exception.

Function: Value.dereference ()
  For pointer data types, this method returns a new `gdb.Value` object whose contents is the object pointed to by the pointer. For example, if `foo` is a C pointer to an `int`, declared in your C program as

      int *foo;

  then you can use the corresponding `gdb.Value` to access what `foo` points to like this:

      bar = foo.dereference ()

  The result `bar` will be a `gdb.Value` object holding the value pointed to by `foo`.

  A similar function `Value.referenced_value` exists which also returns `gdb.Value` objects corresonding to the values pointed to by pointer values (and additionally, values referenced by reference values). However, the behavior of `Value.dereference` differs from `Value.referenced_value` by the fact that the behavior of `Value.dereference` is identical to applying the C unary operator `*` on a given value. For example, consider a reference to a pointer `ptrref`, declared in your C`++` program as

      typedef int *intptr;
      ...
      int val = 10;
      intptr ptr = &val;
      intptr &ptrref = ptr;

  Though `ptrref` is a reference value, one can apply the method `Value.dereference` to the `gdb.Value` object corresponding to it and obtain a `gdb.Value` which is identical to that corresponding to `val`. However, if you apply the method `Value.referenced_value`, the result would be a `gdb.Value` object identical to that corresponding to `ptr`.

      py_ptrref = gdb.parse_and_eval ("ptrref")
      py_val = py_ptrref.dereference ()
      py_ptr = py_ptrref.referenced_value ()

  The `gdb.Value` object `py_val` is identical to that corresponding to `val`, and `py_ptr` is identical to that corresponding to `ptr`. In general, `Value.dereference` can be applied whenever the C unary operator `*` can be applied to the corresponding C value. For those cases where applying both `Value.dereference` and `Value.referenced_value` is allowed, the results obtained need not be identical (as we have seen in the above example). The results are however identical when applied on `gdb.Value` objects corresponding to pointers (`gdb.Value` objects with type code `TYPE_CODE_PTR`) in a C/C`++` program.

Function: Value.referenced_value ()
  For pointer or reference data types, this method returns a new `gdb.Value` object corresponding to the value referenced by the pointer/reference value. For pointer data types, `Value.dereference` and `Value.referenced_value` produce identical results. The difference between these methods is that `Value.dereference` cannot get the values referenced by reference values. For example, consider a reference to an `int`, declared in your C`++` program as

      int val = 10;
      int &ref = val;

  then applying `Value.dereference` to the `gdb.Value` object corresponding to `ref` will result in an error, while applying `Value.referenced_value` will result in a `gdb.Value` object identical to that corresponding to `val`.

      py_ref = gdb.parse_and_eval ("ref")
      er_ref = py_ref.dereference ()       # Results in error
      py_val = py_ref.referenced_value ()  # Returns the referenced value

  The `gdb.Value` object `py_val` is identical to that corresponding to `val`.

Function: Value.dynamic_cast (type)
  Like `Value.cast`, but works as if the C`++` `dynamic_cast` operator were used. Consult a C`++` reference for details.

Function: Value.reinterpret_cast (type)
  Like `Value.cast`, but works as if the C`++` `reinterpret_cast` operator were used. Consult a C`++` reference for details.

Function: Value.string ([encoding[, errors[, length]]])
  If this `gdb.Value` represents a string, then this method converts the contents to a Python string. Otherwise, this method will throw an exception.

  Strings are recognized in a language-specific way; whether a given `gdb.Value` represents a string is determined by the current language.

  For C-like languages, a value is a string if it is a pointer to or an array of characters or ints. The string is assumed to be terminated by a zero of the appropriate width. However if the optional length argument is given, the string will be converted to that given length, ignoring any embedded zeros that the string may contain.

  If the optional encoding argument is given, it must be a string naming the encoding of the string in the `gdb.Value`, such as `"ascii"`, `"iso-8859-6"` or `"utf-8"`. It accepts the same encodings as the corresponding argument to Python's `string.decode` method, and the Python codec machinery will be used to convert the string. If encoding is not given, or if encoding is the empty string, then either the `target-charset` (see section 10.20 Character Sets) will be used, or a language-specific encoding will be used, if the current language is able to supply one.

  The optional errors argument is the same as the corresponding argument to Python's `string.decode` method.

  If the optional length argument is given, the string will be fetched and converted to the given length.

Function: Value.lazy_string ([encoding [, length]])
  If this `gdb.Value` represents a string, then this method converts the contents to a `gdb.LazyString` (see section 23.2.2.27 Python representation of lazy strings.). Otherwise, this method will throw an exception.

  If the optional encoding argument is given, it must be a string naming the encoding of the `gdb.LazyString`. Some examples are: ``ascii'`, ``iso-8859-6'` or ``utf-8'`. If the encoding argument is an encoding that GDB does recognize, GDB will raise an error.

  When a lazy string is printed, the GDB encoding machinery is used to convert the string during printing. If the optional encoding argument is not provided, or is an empty string, GDB will automatically select the encoding most suitable for the string type. For further information on encoding in GDB please see 10.20 Character Sets.

  If the optional length argument is given, the string will be fetched and encoded to the length of characters specified. If the length argument is not provided, the string will be fetched and encoded until a null of appropriate width is found.

Function: Value.fetch_lazy ()
  If the `gdb.Value` object is currently a lazy value (`gdb.Value.is_lazy` is `True`), then the value is fetched from the inferior. Any errors that occur in the process will produce a Python exception.

  If the `gdb.Value` object is not a lazy value, this method has no effect.

  This method does not return a value.

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#### 23.2.2.4 Types In Python

GDB represents types from the inferior using the class `gdb.Type`.

The following type-related functions are available in the `gdb` module:

Function: gdb.lookup_type (name [, block])
  This function looks up a type by name. name is the name of the type to look up. It must be a string.

  If block is given, then name is looked up in that scope. Otherwise, it is searched for globally.

  Ordinarily, this function will return an instance of `gdb.Type`. If the named type cannot be found, it will throw an exception.

If the type is a structure or class type, or an enum type, the fields of that type can be accessed using the Python dictionary syntax. For example, if `some_type` is a `gdb.Type` instance holding a structure type, you can access its `foo` field with:

    bar = some_type['foo']

`bar` will be a `gdb.Field` object; see below under the description of the `Type.fields` method for a description of the `gdb.Field` class.

An instance of `Type` has the following attributes:

Variable: Type.code
  The type code for this type. The type code will be one of the `TYPE_CODE_` constants defined below.

Variable: Type.name
  The name of this type. If this type has no name, then `None` is returned.

Variable: Type.sizeof
  The size of this type, in target `char` units. Usually, a target's `char` type will be an 8-bit byte. However, on some unusual platforms, this type may have a different size.

Variable: Type.tag
  The tag name for this type. The tag name is the name after `struct`, `union`, or `enum` in C and C`++`; not all languages have this concept. If this type has no tag name, then `None` is returned.

The following methods are provided:

Function: Type.fields ()
  For structure and union types, this method returns the fields. Range types have two fields, the minimum and maximum values. Enum types have one field per enum constant. Function and method types have one field per parameter. The base types of C`++` classes are also represented as fields. If the type has no fields, or does not fit into one of these categories, an empty sequence will be returned.

  Each field is a `gdb.Field` object, with some pre-defined attributes:

  `bitpos`
    This attribute is not available for `enum` or `static` (as in C`++` or Java) fields. The value is the position, counting in bits, from the start of the containing type.

  `enumval`
    This attribute is only available for `enum` fields, and its value is the enumeration member's integer representation.

  `name`
    The name of the field, or `None` for anonymous fields.

  `artificial`
    This is `True` if the field is artificial, usually meaning that it was provided by the compiler and not the user. This attribute is always provided, and is `False` if the field is not artificial.

  `is_base_class`
    This is `True` if the field represents a base class of a C`++` structure. This attribute is always provided, and is `False` if the field is not a base class of the type that is the argument of `fields`, or if that type was not a C`++` class.

  `bitsize`
    If the field is packed, or is a bitfield, then this will have a non-zero value, which is the size of the field in bits. Otherwise, this will be zero; in this case the field's size is given by its type.

  `type`
    The type of the field. This is usually an instance of `Type`, but it can be `None` in some situations.

  `parent_type`
    The type which contains this field. This is an instance of `gdb.Type`.

Function: Type.array (n1 [, n2])
  Return a new `gdb.Type` object which represents an array of this type. If one argument is given, it is the inclusive upper bound of the array; in this case the lower bound is zero. If two arguments are given, the first argument is the lower bound of the array, and the second argument is the upper bound of the array. An array's length must not be negative, but the bounds can be.

Function: Type.vector (n1 [, n2])
  Return a new `gdb.Type` object which represents a vector of this type. If one argument is given, it is the inclusive upper bound of the vector; in this case the lower bound is zero. If two arguments are given, the first argument is the lower bound of the vector, and the second argument is the upper bound of the vector. A vector's length must not be negative, but the bounds can be.

  The difference between an `array` and a `vector` is that arrays behave like in C: when used in expressions they decay to a pointer to the first element whereas vectors are treated as first class values.

Function: Type.const ()
  Return a new `gdb.Type` object which represents a `const`-qualified variant of this type.

Function: Type.volatile ()
  Return a new `gdb.Type` object which represents a `volatile`-qualified variant of this type.

Function: Type.unqualified ()
  Return a new `gdb.Type` object which represents an unqualified variant of this type. That is, the result is neither `const` nor `volatile`.

Function: Type.range ()
  Return a Python `Tuple` object that contains two elements: the low bound of the argument type and the high bound of that type. If the type does not have a range, GDB will raise a `gdb.error` exception (see section 23.2.2.2 Exception Handling).

Function: Type.reference ()
  Return a new `gdb.Type` object which represents a reference to this type.

Function: Type.pointer ()
  Return a new `gdb.Type` object which represents a pointer to this type.

Function: Type.strip_typedefs ()
  Return a new `gdb.Type` that represents the real type, after removing all layers of typedefs.

Function: Type.target ()
  Return a new `gdb.Type` object which represents the target type of this type.

  For a pointer type, the target type is the type of the pointed-to object. For an array type (meaning C-like arrays), the target type is the type of the elements of the array. For a function or method type, the target type is the type of the return value. For a complex type, the target type is the type of the elements. For a typedef, the target type is the aliased type.

  If the type does not have a target, this method will throw an exception.

Function: Type.template_argument (n [, block])
  If this `gdb.Type` is an instantiation of a template, this will return a new `gdb.Type` which represents the type of the nth template argument.

  If this `gdb.Type` is not a template type, this will throw an exception. Ordinarily, only C`++` code will have template types.

  If block is given, then name is looked up in that scope. Otherwise, it is searched for globally.

Each type has a code, which indicates what category this type falls into. The available type categories are represented by constants defined in the `gdb` module:

`gdb.TYPE_CODE_PTR`
  The type is a pointer.

`gdb.TYPE_CODE_ARRAY`
  The type is an array.

`gdb.TYPE_CODE_STRUCT`
  The type is a structure.

`gdb.TYPE_CODE_UNION`
  The type is a union.

`gdb.TYPE_CODE_ENUM`
  The type is an enum.

`gdb.TYPE_CODE_FLAGS`
  A bit flags type, used for things such as status registers.

`gdb.TYPE_CODE_FUNC`
  The type is a function.

`gdb.TYPE_CODE_INT`
  The type is an integer type.

`gdb.TYPE_CODE_FLT`
  A floating point type.

`gdb.TYPE_CODE_VOID`
  The special type `void`.

`gdb.TYPE_CODE_SET`
  A Pascal set type.

`gdb.TYPE_CODE_RANGE`
  A range type, that is, an integer type with bounds.

`gdb.TYPE_CODE_STRING`
  A string type. Note that this is only used for certain languages with language-defined string types; C strings are not represented this way.

`gdb.TYPE_CODE_BITSTRING`
  A string of bits. It is deprecated.

`gdb.TYPE_CODE_ERROR`
  An unknown or erroneous type.

`gdb.TYPE_CODE_METHOD`
  A method type, as found in C`++` or Java.

`gdb.TYPE_CODE_METHODPTR`
  A pointer-to-member-function.

`gdb.TYPE_CODE_MEMBERPTR`
  A pointer-to-member.

`gdb.TYPE_CODE_REF`
  A reference type.

`gdb.TYPE_CODE_CHAR`
  A character type.

`gdb.TYPE_CODE_BOOL`
  A boolean type.

`gdb.TYPE_CODE_COMPLEX`
  A complex float type.

`gdb.TYPE_CODE_TYPEDEF`
  A typedef to some other type.

`gdb.TYPE_CODE_NAMESPACE`
  A C`++` namespace.

`gdb.TYPE_CODE_DECFLOAT`
  A decimal floating point type.

`gdb.TYPE_CODE_INTERNAL_FUNCTION`
  A function internal to GDB. This is the type used to represent convenience functions.

Further support for types is provided in the `gdb.types` Python module (see section 23.2.4.2 gdb.types).

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#### 23.2.2.5 Pretty Printing API

An example output is provided (see section 10.9 Pretty Printing).

A pretty-printer is just an object that holds a value and implements a specific interface, defined here.

Function: pretty_printer.children (self)
  GDB will call this method on a pretty-printer to compute the children of the pretty-printer's value.

  This method must return an object conforming to the Python iterator protocol. Each item returned by the iterator must be a tuple holding two elements. The first element is the "name" of the child; the second element is the child's value. The value can be any Python object which is convertible to a GDB value.

  This method is optional. If it does not exist, GDB will act as though the value has no children.

Function: pretty_printer.display_hint (self)
  The CLI may call this method and use its result to change the formatting of a value. The result will also be supplied to an MI consumer as a ``displayhint'` attribute of the variable being printed.

  This method is optional. If it does exist, this method must return a string.

  Some display hints are predefined by GDB:

  ``array'`
    Indicate that the object being printed is "array-like". The CLI uses this to respect parameters such as `set print elements` and `set print array`.

  ``map'`
    Indicate that the object being printed is "map-like", and that the children of this value can be assumed to alternate between keys and values.

  ``string'`
    Indicate that the object being printed is "string-like". If the printer's `to_string` method returns a Python string of some kind, then GDB will call its internal language-specific string-printing function to format the string. For the CLI this means adding quotation marks, possibly escaping some characters, respecting `set print elements`, and the like.

Function: pretty_printer.to_string (self)
  GDB will call this method to display the string representation of the value passed to the object's constructor.

  When printing from the CLI, if the `to_string` method exists, then GDB will prepend its result to the values returned by `children`. Exactly how this formatting is done is dependent on the display hint, and may change as more hints are added. Also, depending on the print settings (see section 10.8 Print Settings), the CLI may print just the result of `to_string` in a stack trace, omitting the result of `children`.

  If this method returns a string, it is printed verbatim.

  Otherwise, if this method returns an instance of `gdb.Value`, then GDB prints this value. This may result in a call to another pretty-printer.

  If instead the method returns a Python value which is convertible to a `gdb.Value`, then GDB performs the conversion and prints the resulting value. Again, this may result in a call to another pretty-printer. Python scalars (integers, floats, and booleans) and strings are convertible to `gdb.Value`; other types are not.

  Finally, if this method returns `None` then no further operations are peformed in this method and nothing is printed.

  If the result is not one of these types, an exception is raised.

GDB provides a function which can be used to look up the default pretty-printer for a `gdb.Value`:

Function: gdb.default_visualizer (value)
  This function takes a `gdb.Value` object as an argument. If a pretty-printer for this value exists, then it is returned. If no such printer exists, then this returns `None`.

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#### 23.2.2.6 Selecting Pretty-Printers

The Python list `gdb.pretty_printers` contains an array of functions or callable objects that have been registered via addition as a pretty-printer. Printers in this list are called `global` printers, they're available when debugging all inferiors. Each `gdb.Progspace` contains a `pretty_printers` attribute. Each `gdb.Objfile` also contains a `pretty_printers` attribute.

Each function on these lists is passed a single `gdb.Value` argument and should return a pretty-printer object conforming to the interface definition above (see section 23.2.2.5 Pretty Printing API). If a function cannot create a pretty-printer for the value, it should return `None`.

GDB first checks the `pretty_printers` attribute of each `gdb.Objfile` in the current program space and iteratively calls each enabled lookup routine in the list for that `gdb.Objfile` until it receives a pretty-printer object. If no pretty-printer is found in the objfile lists, GDB then searches the pretty-printer list of the current program space, calling each enabled function until an object is returned. After these lists have been exhausted, it tries the global `gdb.pretty_printers` list, again calling each enabled function until an object is returned.

The order in which the objfiles are searched is not specified. For a given list, functions are always invoked from the head of the list, and iterated over sequentially until the end of the list, or a printer object is returned.

For various reasons a pretty-printer may not work. For example, the underlying data structure may have changed and the pretty-printer is out of date.

The consequences of a broken pretty-printer are severe enough that GDB provides support for enabling and disabling individual printers. For example, if `print frame-arguments` is on, a backtrace can become highly illegible if any argument is printed with a broken printer.

Pretty-printers are enabled and disabled by attaching an `enabled` attribute to the registered function or callable object. If this attribute is present and its value is `False`, the printer is disabled, otherwise the printer is enabled.

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#### 23.2.2.7 Writing a Pretty-Printer

A pretty-printer consists of two parts: a lookup function to detect if the type is supported, and the printer itself.

Here is an example showing how a `std::string` printer might be written. See section 23.2.2.5 Pretty Printing API, for details on the API this class must provide.

    class StdStringPrinter(object):
        "Print a std::string"

        def __init__(self, val):
            self.val = val

        def to_string(self):
            return self.val['_M_dataplus']['_M_p']

        def display_hint(self):
            return 'string'

And here is an example showing how a lookup function for the printer example above might be written.

    def str_lookup_function(val):
        lookup_tag = val.type.tag
        if lookup_tag == None:
            return None
        regex = re.compile("^std::basic_string$")
        if regex.match(lookup_tag):
            return StdStringPrinter(val)
        return None

The example lookup function extracts the value's type, and attempts to match it to a type that it can pretty-print. If it is a type the printer can pretty-print, it will return a printer object. If not, it returns `None`.

We recommend that you put your core pretty-printers into a Python package. If your pretty-printers are for use with a library, we further recommend embedding a version number into the package name. This practice will enable GDB to load multiple versions of your pretty-printers at the same time, because they will have different names.

You should write auto-loaded code (see section 23.2.3 Python Auto-loading) such that it can be evaluated multiple times without changing its meaning. An ideal auto-load file will consist solely of `import`s of your printer modules, followed by a call to a register pretty-printers with the current objfile.

Taken as a whole, this approach will scale nicely to multiple inferiors, each potentially using a different library version. Embedding a version number in the Python package name will ensure that GDB is able to load both sets of printers simultaneously. Then, because the search for pretty-printers is done by objfile, and because your auto-loaded code took care to register your library's printers with a specific objfile, GDB will find the correct printers for the specific version of the library used by each inferior.

To continue the `std::string` example (see section 23.2.2.5 Pretty Printing API), this code might appear in `gdb.libstdcxx.v6`:

    def register_printers(objfile):
        objfile.pretty_printers.append(str_lookup_function)

And then the corresponding contents of the auto-load file would be:

    import gdb.libstdcxx.v6
    gdb.libstdcxx.v6.register_printers(gdb.current_objfile())

The previous example illustrates a basic pretty-printer. There are a few things that can be improved on. The printer doesn't have a name, making it hard to identify in a list of installed printers. The lookup function has a name, but lookup functions can have arbitrary, even identical, names.

Second, the printer only handles one type, whereas a library typically has several types. One could install a lookup function for each desired type in the library, but one could also have a single lookup function recognize several types. The latter is the conventional way this is handled. If a pretty-printer can handle multiple data types, then its subprinters are the printers for the individual data types.

The `gdb.printing` module provides a formal way of solving these problems (see section 23.2.4.1 gdb.printing). Here is another example that handles multiple types.

These are the types we are going to pretty-print:

    struct foo { int a, b; };
    struct bar { struct foo x, y; };

Here are the printers:

    class fooPrinter:
        """Print a foo object."""

        def __init__(self, val):
            self.val = val

        def to_string(self):
            return ("a=<" + str(self.val["a"]) +
                    "> b=<" + str(self.val["b"]) + ">")

    class barPrinter:
        """Print a bar object."""

        def __init__(self, val):
            self.val = val

        def to_string(self):
            return ("x=<" + str(self.val["x"]) +
                    "> y=<" + str(self.val["y"]) + ">")

This example doesn't need a lookup function, that is handled by the `gdb.printing` module. Instead a function is provided to build up the object that handles the lookup.

    import gdb.printing

    def build_pretty_printer():
        pp = gdb.printing.RegexpCollectionPrettyPrinter(
            "my_library")
        pp.add_printer('foo', '^foo$', fooPrinter)
        pp.add_printer('bar', '^bar$', barPrinter)
        return pp

And here is the autoload support:

    import gdb.printing
    import my_library
    gdb.printing.register_pretty_printer(
        gdb.current_objfile(),
        my_library.build_pretty_printer())

Finally, when this printer is loaded into GDB, here is the corresponding output of ``info pretty-printer'`:

    (gdb) info pretty-printer
    my_library.so:
      my_library
        foo
        bar

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#### 23.2.2.8 Type Printing API

GDB provides a way for Python code to customize type display. This is mainly useful for substituting canonical typedef names for types.

A type printer is just a Python object conforming to a certain protocol. A simple base class implementing the protocol is provided; see 23.2.4.2 gdb.types. A type printer must supply at least:

Instance Variable: type_printer enabled
  A boolean which is True if the printer is enabled, and False otherwise. This is manipulated by the `enable type-printer` and `disable type-printer` commands.

Instance Variable: type_printer name
  The name of the type printer. This must be a string. This is used by the `enable type-printer` and `disable type-printer` commands.

Method: type_printer instantiate (self)
  This is called by GDB at the start of type-printing. It is only called if the type printer is enabled. This method must return a new object that supplies a `recognize` method, as described below.

When displaying a type, say via the `ptype` command, GDB will compute a list of type recognizers. This is done by iterating first over the per-objfile type printers (see section 23.2.2.19 Objfiles In Python), followed by the per-progspace type printers (see section 23.2.2.18 Program Spaces In Python), and finally the global type printers.

GDB will call the `instantiate` method of each enabled type printer. If this method returns `None`, then the result is ignored; otherwise, it is appended to the list of recognizers.

Then, when GDB is going to display a type name, it iterates over the list of recognizers. For each one, it calls the recognition function, stopping if the function returns a non-`None` value. The recognition function is defined as:

Method: type_recognizer recognize (self, type)
  If type is not recognized, return `None`. Otherwise, return a string which is to be printed as the name of type. type will be an instance of `gdb.Type` (see section 23.2.2.4 Types In Python).

GDB uses this two-pass approach so that type printers can efficiently cache information without holding on to it too long. For example, it can be convenient to look up type information in a type printer and hold it for a recognizer's lifetime; if a single pass were done then type printers would have to make use of the event system in order to avoid holding information that could become stale as the inferior changed.

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#### 23.2.2.9 Filtering Frames.

Frame filters are Python objects that manipulate the visibility of a frame or frames when a backtrace (see section 8.2 Backtraces) is printed by GDB.

Only commands that print a backtrace, or, in the case of GDB/MI commands (see section 27. The GDB/MI Interface), those that return a collection of frames are affected. The commands that work with frame filters are:

`backtrace` (see section The backtrace command), `-stack-list-frames` (see section The -stack-list-frames command), `-stack-list-variables` (see section The -stack-list-variables command), `-stack-list-arguments` see section The -stack-list-arguments command) and `-stack-list-locals` (see section The -stack-list-locals command).

A frame filter works by taking an iterator as an argument, applying actions to the contents of that iterator, and returning another iterator (or, possibly, the same iterator it was provided in the case where the filter does not perform any operations). Typically, frame filters utilize tools such as the Python's `itertools` module to work with and create new iterators from the source iterator. Regardless of how a filter chooses to apply actions, it must not alter the underlying GDB frame or frames, or attempt to alter the call-stack within GDB. This preserves data integrity within GDB. Frame filters are executed on a priority basis and care should be taken that some frame filters may have been executed before, and that some frame filters will be executed after.

An important consideration when designing frame filters, and well worth reflecting upon, is that frame filters should avoid unwinding the call stack if possible. Some stacks can run very deep, into the tens of thousands in some cases. To search every frame when a frame filter executes may be too expensive at that step. The frame filter cannot know how many frames it has to iterate over, and it may have to iterate through them all. This ends up duplicating effort as GDB performs this iteration when it prints the frames. If the filter can defer unwinding frames until frame decorators are executed, after the last filter has executed, it should. See section 23.2.2.10 Decorating Frames., for more information on decorators. Also, there are examples for both frame decorators and filters in later chapters. See section 23.2.2.11 Writing a Frame Filter, for more information.

The Python dictionary `gdb.frame_filters` contains key/object pairings that comprise a frame filter. Frame filters in this dictionary are called `global` frame filters, and they are available when debugging all inferiors. These frame filters must register with the dictionary directly. In addition to the `global` dictionary, there are other dictionaries that are loaded with different inferiors via auto-loading (see section 23.2.3 Python Auto-loading). The two other areas where frame filter dictionaries can be found are: `gdb.Progspace` which contains a `frame_filters` dictionary attribute, and each `gdb.Objfile` object which also contains a `frame_filters` dictionary attribute.

When a command is executed from GDB that is compatible with frame filters, GDB combines the `global`, `gdb.Progspace` and all `gdb.Objfile` dictionaries currently loaded. All of the `gdb.Objfile` dictionaries are combined, as several frames, and thus several object files, might be in use. GDB then prunes any frame filter whose `enabled` attribute is `False`. This pruned list is then sorted according to the `priority` attribute in each filter.

Once the dictionaries are combined, pruned and sorted, GDB creates an iterator which wraps each frame in the call stack in a `FrameDecorator` object, and calls each filter in order. The output from the previous filter will always be the input to the next filter, and so on.

Frame filters have a mandatory interface which each frame filter must implement, defined here:

Function: FrameFilter.filter (iterator)
  GDB will call this method on a frame filter when it has reached the order in the priority list for that filter.

  For example, if there are four frame filters:

      Name         Priority

      Filter1      5
      Filter2      10
      Filter3      100
      Filter4      1

  The order that the frame filters will be called is:

      Filter3 -> Filter2 -> Filter1 -> Filter4

  Note that the output from `Filter3` is passed to the input of `Filter2`, and so on.

  This `filter` method is passed a Python iterator. This iterator contains a sequence of frame decorators that wrap each `gdb.Frame`, or a frame decorator that wraps another frame decorator. The first filter that is executed in the sequence of frame filters will receive an iterator entirely comprised of default `FrameDecorator` objects. However, after each frame filter is executed, the previous frame filter may have wrapped some or all of the frame decorators with their own frame decorator. As frame decorators must also conform to a mandatory interface, these decorators can be assumed to act in a uniform manner (see section 23.2.2.10 Decorating Frames.).

  This method must return an object conforming to the Python iterator protocol. Each item in the iterator must be an object conforming to the frame decorator interface. If a frame filter does not wish to perform any operations on this iterator, it should return that iterator untouched.

  This method is not optional. If it does not exist, GDB will raise and print an error.

Variable: FrameFilter.name
  The `name` attribute must be Python string which contains the name of the filter displayed by GDB (see section 8.3 Management of Frame Filters.). This attribute may contain any combination of letters or numbers. Care should be taken to ensure that it is unique. This attribute is mandatory.

Variable: FrameFilter.enabled
  The `enabled` attribute must be Python boolean. This attribute indicates to GDB whether the frame filter is enabled, and should be considered when frame filters are executed. If `enabled` is `True`, then the frame filter will be executed when any of the backtrace commands detailed earlier in this chapter are executed. If `enabled` is `False`, then the frame filter will not be executed. This attribute is mandatory.

Variable: FrameFilter.priority
  The `priority` attribute must be Python integer. This attribute controls the order of execution in relation to other frame filters. There are no imposed limits on the range of `priority` other than it must be a valid integer. The higher the `priority` attribute, the sooner the frame filter will be executed in relation to other frame filters. Although `priority` can be negative, it is recommended practice to assume zero is the lowest priority that a frame filter can be assigned. Frame filters that have the same priority are executed in unsorted order in that priority slot. This attribute is mandatory.

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#### 23.2.2.10 Decorating Frames.

Frame decorators are sister objects to frame filters (see section 23.2.2.9 Filtering Frames.). Frame decorators are applied by a frame filter and can only be used in conjunction with frame filters.

The purpose of a frame decorator is to customize the printed content of each `gdb.Frame` in commands where frame filters are executed. This concept is called decorating a frame. Frame decorators decorate a `gdb.Frame` with Python code contained within each API call. This separates the actual data contained in a `gdb.Frame` from the decorated data produced by a frame decorator. This abstraction is necessary to maintain integrity of the data contained in each `gdb.Frame`.

Frame decorators have a mandatory interface, defined below.

GDB already contains a frame decorator called `FrameDecorator`. This contains substantial amounts of boilerplate code to decorate the content of a `gdb.Frame`. It is recommended that other frame decorators inherit and extend this object, and only to override the methods needed.

Function: FrameDecorator.elided (self)

  The `elided` method groups frames together in a hierarchical system. An example would be an interpreter, where multiple low-level frames make up a single call in the interpreted language. In this example, the frame filter would elide the low-level frames and present a single high-level frame, representing the call in the interpreted language, to the user.

  The `elided` function must return an iterable and this iterable must contain the frames that are being elided wrapped in a suitable frame decorator. If no frames are being elided this function may return an empty iterable, or `None`. Elided frames are indented from normal frames in a `CLI` backtrace, or in the case of `GDB/MI`, are placed in the `children` field of the eliding frame.

  It is the frame filter's task to also filter out the elided frames from the source iterator. This will avoid printing the frame twice.

Function: FrameDecorator.function (self)

  This method returns the name of the function in the frame that is to be printed.

  This method must return a Python string describing the function, or `None`.

  If this function returns `None`, GDB will not print any data for this field.

Function: FrameDecorator.address (self)

  This method returns the address of the frame that is to be printed.

  This method must return a Python numeric integer type of sufficient size to describe the address of the frame, or `None`.

  If this function returns a `None`, GDB will not print any data for this field.

Function: FrameDecorator.filename (self)

  This method returns the filename and path associated with this frame.

  This method must return a Python string containing the filename and the path to the object file backing the frame, or `None`.

  If this function returns a `None`, GDB will not print any data for this field.

Function: FrameDecorator.line (self):

  This method returns the line number associated with the current position within the function addressed by this frame.

  This method must return a Python integer type, or `None`.

  If this function returns a `None`, GDB will not print any data for this field.

Function: FrameDecorator.frame_args (self)

  This method must return an iterable, or `None`. Returning an empty iterable, or `None` means frame arguments will not be printed for this frame. This iterable must contain objects that implement two methods, described here.

  This object must implement a `argument` method which takes a single `self` parameter and must return a `gdb.Symbol` (see section 23.2.2.22 Python representation of Symbols.), or a Python string. The object must also implement a `value` method which takes a single `self` parameter and must return a `gdb.Value` (see section 23.2.2.3 Values From Inferior), a Python value, or `None`. If the `value` method returns `None`, and the `argument` method returns a `gdb.Symbol`, GDB will look-up and print the value of the `gdb.Symbol` automatically.

  A brief example:

      class SymValueWrapper():

          def __init__(self, symbol, value):
              self.sym = symbol
              self.val = value

          def value(self):
              return self.val

          def symbol(self):
              return self.sym

      class SomeFrameDecorator()
      ...
      ...
          def frame_args(self):
              args = []
              try:
                  block = self.inferior_frame.block()
              except:
                  return None

              # Iterate over all symbols in a block.  Only add
              # symbols that are arguments.
              for sym in block:
                  if not sym.is_argument:
                      continue
                  args.append(SymValueWrapper(sym,None))

              # Add example synthetic argument.
              args.append(SymValueWrapper(``foo'', 42))

              return args

Function: FrameDecorator.frame_locals (self)

  This method must return an iterable or `None`. Returning an empty iterable, or `None` means frame local arguments will not be printed for this frame.

  The object interface, the description of the various strategies for reading frame locals, and the example are largely similar to those described in the `frame_args` function, (see section The frame filter frame_args function). Below is a modified example:

      class SomeFrameDecorator()
      ...
      ...
          def frame_locals(self):
              vars = []
              try:
                  block = self.inferior_frame.block()
              except:
                  return None

              # Iterate over all symbols in a block.  Add all
              # symbols, except arguments.
              for sym in block:
                  if sym.is_argument:
                      continue
                  vars.append(SymValueWrapper(sym,None))

              # Add an example of a synthetic local variable.
              vars.append(SymValueWrapper(``bar'', 99))

              return vars

Function: FrameDecorator.inferior_frame (self):

  This method must return the underlying `gdb.Frame` that this frame decorator is decorating. GDB requires the underlying frame for internal frame information to determine how to print certain values when printing a frame.

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#### 23.2.2.11 Writing a Frame Filter

There are three basic elements that a frame filter must implement: it must correctly implement the documented interface (see section 23.2.2.9 Filtering Frames.), it must register itself with GDB, and finally, it must decide if it is to work on the data provided by GDB. In all cases, whether it works on the iterator or not, each frame filter must return an iterator. A bare-bones frame filter follows the pattern in the following example.

    import gdb

    class FrameFilter():

        def __init__(self):
            # Frame filter attribute creation.
            #
            # 'name' is the name of the filter that GDB will display.
            #
            # 'priority' is the priority of the filter relative to other
            # filters.
            #
            # 'enabled' is a boolean that indicates whether this filter is
            # enabled and should be executed.

            self.name = "Foo"
            self.priority = 100
            self.enabled = True

            # Register this frame filter with the global frame_filters
            # dictionary.
            gdb.frame_filters[self.name] = self

        def filter(self, frame_iter):
            # Just return the iterator.
            return frame_iter

The frame filter in the example above implements the three requirements for all frame filters. It implements the API, self registers, and makes a decision on the iterator (in this case, it just returns the iterator untouched).

The first step is attribute creation and assignment, and as shown in the comments the filter assigns the following attributes: `name`, `priority` and whether the filter should be enabled with the `enabled` attribute.

The second step is registering the frame filter with the dictionary or dictionaries that the frame filter has interest in. As shown in the comments, this filter just registers itself with the global dictionary `gdb.frame_filters`. As noted earlier, `gdb.frame_filters` is a dictionary that is initialized in the `gdb` module when GDB starts. What dictionary a filter registers with is an important consideration. Generally, if a filter is specific to a set of code, it should be registered either in the `objfile` or `progspace` dictionaries as they are specific to the program currently loaded in GDB. The global dictionary is always present in GDB and is never unloaded. Any filters registered with the global dictionary will exist until GDB exits. To avoid filters that may conflict, it is generally better to register frame filters against the dictionaries that more closely align with the usage of the filter currently in question. See section 23.2.3 Python Auto-loading, for further information on auto-loading Python scripts.

GDB takes a hands-off approach to frame filter registration, therefore it is the frame filter's responsibility to ensure registration has occurred, and that any exceptions are handled appropriately. In particular, you may wish to handle exceptions relating to Python dictionary key uniqueness. It is mandatory that the dictionary key is the same as frame filter's `name` attribute. When a user manages frame filters (see section 8.3 Management of Frame Filters.), the names GDB will display are those contained in the `name` attribute.

The final step of this example is the implementation of the `filter` method. As shown in the example comments, we define the `filter` method and note that the method must take an iterator, and also must return an iterator. In this bare-bones example, the frame filter is not very useful as it just returns the iterator untouched. However this is a valid operation for frame filters that have the `enabled` attribute set, but decide not to operate on any frames.

In the next example, the frame filter operates on all frames and utilizes a frame decorator to perform some work on the frames. See section 23.2.2.10 Decorating Frames., for further information on the frame decorator interface.

This example works on inlined frames. It highlights frames which are inlined by tagging them with an "[inlined]" tag. By applying a frame decorator to all frames with the Python `itertools imap` method, the example defers actions to the frame decorator. Frame decorators are only processed when GDB prints the backtrace.

This introduces a new decision making topic: whether to perform decision making operations at the filtering step, or at the printing step. In this example's approach, it does not perform any filtering decisions at the filtering step beyond mapping a frame decorator to each frame. This allows the actual decision making to be performed when each frame is printed. This is an important consideration, and well worth reflecting upon when designing a frame filter. An issue that frame filters should avoid is unwinding the stack if possible. Some stacks can run very deep, into the tens of thousands in some cases. To search every frame to determine if it is inlined ahead of time may be too expensive at the filtering step. The frame filter cannot know how many frames it has to iterate over, and it would have to iterate through them all. This ends up duplicating effort as GDB performs this iteration when it prints the frames.

In this example decision making can be deferred to the printing step. As each frame is printed, the frame decorator can examine each frame in turn when GDB iterates. From a performance viewpoint, this is the most appropriate decision to make as it avoids duplicating the effort that the printing step would undertake anyway. Also, if there are many frame filters unwinding the stack during filtering, it can substantially delay the printing of the backtrace which will result in large memory usage, and a poor user experience.

    class InlineFilter():

        def __init__(self):
            self.name = "InlinedFrameFilter"
            self.priority = 100
            self.enabled = True
            gdb.frame_filters[self.name] = self

        def filter(self, frame_iter):
            frame_iter = itertools.imap(InlinedFrameDecorator,
                                        frame_iter)
            return frame_iter

This frame filter is somewhat similar to the earlier example, except that the `filter` method applies a frame decorator object called `InlinedFrameDecorator` to each element in the iterator. The `imap` Python method is light-weight. It does not proactively iterate over the iterator, but rather creates a new iterator which wraps the existing one.

Below is the frame decorator for this example.

    class InlinedFrameDecorator(FrameDecorator):

        def __init__(self, fobj):
            super(InlinedFrameDecorator, self).__init__(fobj)

        def function(self):
            frame = fobj.inferior_frame()
            name = str(frame.name())

            if frame.type() == gdb.INLINE_FRAME:
                name = name + " [inlined]"

            return name

This frame decorator only defines and overrides the `function` method. It lets the supplied `FrameDecorator`, which is shipped with GDB, perform the other work associated with printing this frame.

The combination of these two objects create this output from a backtrace:

    #0  0x004004e0 in bar () at inline.c:11
    #1  0x00400566 in max [inlined] (b=6, a=12) at inline.c:21
    #2  0x00400566 in main () at inline.c:31

So in the case of this example, a frame decorator is applied to all frames, regardless of whether they may be inlined or not. As GDB iterates over the iterator produced by the frame filters, GDB executes each frame decorator which then makes a decision on what to print in the `function` callback. Using a strategy like this is a way to defer decisions on the frame content to printing time.

### Eliding Frames

It might be that the above example is not desirable for representing inlined frames, and a hierarchical approach may be preferred. If we want to hierarchically represent frames, the `elided` frame decorator interface might be preferable.

This example approaches the issue with the `elided` method. This example is quite long, but very simplistic. It is out-of-scope for this section to write a complete example that comprehensively covers all approaches of finding and printing inlined frames. However, this example illustrates the approach an author might use.

This example comprises of three sections.

    class InlineFrameFilter():

        def __init__(self):
            self.name = "InlinedFrameFilter"
            self.priority = 100
            self.enabled = True
            gdb.frame_filters[self.name] = self

        def filter(self, frame_iter):
            return ElidingInlineIterator(frame_iter)

This frame filter is very similar to the other examples. The only difference is this frame filter is wrapping the iterator provided to it (`frame_iter`) with a custom iterator called `ElidingInlineIterator`. This again defers actions to when GDB prints the backtrace, as the iterator is not traversed until printing.

The iterator for this example is as follows. It is in this section of the example where decisions are made on the content of the backtrace.

    class ElidingInlineIterator:
        def __init__(self, ii):
            self.input_iterator = ii

        def __iter__(self):
            return self

        def next(self):
            frame = next(self.input_iterator)

            if frame.inferior_frame().type() != gdb.INLINE_FRAME:
                return frame

            try:
                eliding_frame = next(self.input_iterator)
            except StopIteration:
                return frame
            return ElidingFrameDecorator(eliding_frame, [frame])

This iterator implements the Python iterator protocol. When the `next` function is called (when GDB prints each frame), the iterator checks if this frame decorator, `frame`, is wrapping an inlined frame. If it is not, it returns the existing frame decorator untouched. If it is wrapping an inlined frame, it assumes that the inlined frame was contained within the next oldest frame, `eliding_frame`, which it fetches. It then creates and returns a frame decorator, `ElidingFrameDecorator`, which contains both the elided frame, and the eliding frame.

    class ElidingInlineDecorator(FrameDecorator):

        def __init__(self, frame, elided_frames):
            super(ElidingInlineDecorator, self).__init__(frame)
            self.frame = frame
            self.elided_frames = elided_frames

        def elided(self):
            return iter(self.elided_frames)

This frame decorator overrides one function and returns the inlined frame in the `elided` method. As before it lets `FrameDecorator` do the rest of the work involved in printing this frame. This produces the following output.

    #0  0x004004e0 in bar () at inline.c:11
    #2  0x00400529 in main () at inline.c:25
        #1  0x00400529 in max (b=6, a=12) at inline.c:15

In that output, `max` which has been inlined into `main` is printed hierarchically. Another approach would be to combine the `function` method, and the `elided` method to both print a marker in the inlined frame, and also show the hierarchical relationship.

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#### 23.2.2.12 Inferiors In Python

Programs which are being run under GDB are called inferiors (see section 4.9 Debugging Multiple Inferiors and Programs). Python scripts can access information about and manipulate inferiors controlled by GDB via objects of the `gdb.Inferior` class.

The following inferior-related functions are available in the `gdb` module:

Function: gdb.inferiors ()
  Return a tuple containing all inferior objects.

Function: gdb.selected_inferior ()
  Return an object representing the current inferior.

A `gdb.Inferior` object has the following attributes:

Variable: Inferior.num
  ID of inferior, as assigned by GDB.

Variable: Inferior.pid
  Process ID of the inferior, as assigned by the underlying operating system.

Variable: Inferior.was_attached
  Boolean signaling whether the inferior was created using `attach', or started by GDB itself.

A `gdb.Inferior` object has the following methods:

Function: Inferior.is_valid ()
  Returns `True` if the `gdb.Inferior` object is valid, `False` if not. A `gdb.Inferior` object will become invalid if the inferior no longer exists within GDB. All other `gdb.Inferior` methods will throw an exception if it is invalid at the time the method is called.

Function: Inferior.threads ()
  This method returns a tuple holding all the threads which are valid when it is called. If there are no valid threads, the method will return an empty tuple.

Function: Inferior.read_memory (address, length)
  Read length bytes of memory from the inferior, starting at address. Returns a buffer object, which behaves much like an array or a string. It can be modified and given to the `Inferior.write_memory` function. In `Python` 3, the return value is a `memoryview` object.

Function: Inferior.write_memory (address, buffer [, length])
  Write the contents of buffer to the inferior, starting at address. The buffer parameter must be a Python object which supports the buffer protocol, i.e., a string, an array or the object returned from `Inferior.read_memory`. If given, length determines the number of bytes from buffer to be written.

Function: Inferior.search_memory (address, length, pattern)
  Search a region of the inferior memory starting at address with the given length using the search pattern supplied in pattern. The pattern parameter must be a Python object which supports the buffer protocol, i.e., a string, an array or the object returned from `gdb.read_memory`. Returns a Python `Long` containing the address where the pattern was found, or `None` if the pattern could not be found.

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#### 23.2.2.13 Events In Python

GDB provides a general event facility so that Python code can be notified of various state changes, particularly changes that occur in the inferior.

An event is just an object that describes some state change. The type of the object and its attributes will vary depending on the details of the change. All the existing events are described below.

In order to be notified of an event, you must register an event handler with an event registry. An event registry is an object in the `gdb.events` module which dispatches particular events. A registry provides methods to register and unregister event handlers:

Function: EventRegistry.connect (object)
  Add the given callable object to the registry. This object will be called when an event corresponding to this registry occurs.

Function: EventRegistry.disconnect (object)
  Remove the given object from the registry. Once removed, the object will no longer receive notifications of events.

Here is an example:

    def exit_handler (event):
        print "event type: exit"
        print "exit code: %d" % (event.exit_code)

    gdb.events.exited.connect (exit_handler)

In the above example we connect our handler `exit_handler` to the registry `events.exited`. Once connected, `exit_handler` gets called when the inferior exits. The argument event in this example is of type `gdb.ExitedEvent`. As you can see in the example the `ExitedEvent` object has an attribute which indicates the exit code of the inferior.

The following is a listing of the event registries that are available and details of the events they emit:

`events.cont`
  Emits `gdb.ThreadEvent`.

  Some events can be thread specific when GDB is running in non-stop mode. When represented in Python, these events all extend `gdb.ThreadEvent`. Note, this event is not emitted directly; instead, events which are emitted by this or other modules might extend this event. Examples of these events are `gdb.BreakpointEvent` and `gdb.ContinueEvent`.

  Variable: ThreadEvent.inferior_thread
    In non-stop mode this attribute will be set to the specific thread which was involved in the emitted event. Otherwise, it will be set to `None`.

  Emits `gdb.ContinueEvent` which extends `gdb.ThreadEvent`.

  This event indicates that the inferior has been continued after a stop. For inherited attribute refer to `gdb.ThreadEvent` above.

`events.exited`
  Emits `events.ExitedEvent` which indicates that the inferior has exited. `events.ExitedEvent` has two attributes:
  Variable: ExitedEvent.exit_code
    An integer representing the exit code, if available, which the inferior has returned. (The exit code could be unavailable if, for example, GDB detaches from the inferior.) If the exit code is unavailable, the attribute does not exist.
  Variable: ExitedEvent inferior
    A reference to the inferior which triggered the `exited` event.

`events.stop`
  Emits `gdb.StopEvent` which extends `gdb.ThreadEvent`.

  Indicates that the inferior has stopped. All events emitted by this registry extend StopEvent. As a child of `gdb.ThreadEvent`, `gdb.StopEvent` will indicate the stopped thread when GDB is running in non-stop mode. Refer to `gdb.ThreadEvent` above for more details.

  Emits `gdb.SignalEvent` which extends `gdb.StopEvent`.

  This event indicates that the inferior or one of its threads has received as signal. `gdb.SignalEvent` has the following attributes:

  Variable: SignalEvent.stop_signal
    A string representing the signal received by the inferior. A list of possible signal values can be obtained by running the command `info signals` in the GDB command prompt.

  Also emits `gdb.BreakpointEvent` which extends `gdb.StopEvent`.

  `gdb.BreakpointEvent` event indicates that one or more breakpoints have been hit, and has the following attributes:

  Variable: BreakpointEvent.breakpoints
    A sequence containing references to all the breakpoints (type `gdb.Breakpoint`) that were hit. See section 23.2.2.25 Manipulating breakpoints using Python, for details of the `gdb.Breakpoint` object.
  Variable: BreakpointEvent.breakpoint
    A reference to the first breakpoint that was hit. This function is maintained for backward compatibility and is now deprecated in favor of the `gdb.BreakpointEvent.breakpoints` attribute.

`events.new_objfile`
  Emits `gdb.NewObjFileEvent` which indicates that a new object file has been loaded by GDB. `gdb.NewObjFileEvent` has one attribute:

  Variable: NewObjFileEvent.new_objfile
    A reference to the object file (`gdb.Objfile`) which has been loaded. See section 23.2.2.19 Objfiles In Python, for details of the `gdb.Objfile` object.

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#### 23.2.2.14 Threads In Python

Python scripts can access information about, and manipulate inferior threads controlled by GDB, via objects of the `gdb.InferiorThread` class.

The following thread-related functions are available in the `gdb` module:

Function: gdb.selected_thread ()
  This function returns the thread object for the selected thread. If there is no selected thread, this will return `None`.

A `gdb.InferiorThread` object has the following attributes:

Variable: InferiorThread.name
  The name of the thread. If the user specified a name using `thread name`, then this returns that name. Otherwise, if an OS-supplied name is available, then it is returned. Otherwise, this returns `None`.

  This attribute can be assigned to. The new value must be a string object, which sets the new name, or `None`, which removes any user-specified thread name.

Variable: InferiorThread.num
  ID of the thread, as assigned by GDB.

Variable: InferiorThread.ptid
  ID of the thread, as assigned by the operating system. This attribute is a tuple containing three integers. The first is the Process ID (PID); the second is the Lightweight Process ID (LWPID), and the third is the Thread ID (TID). Either the LWPID or TID may be 0, which indicates that the operating system does not use that identifier.

A `gdb.InferiorThread` object has the following methods:

Function: InferiorThread.is_valid ()
  Returns `True` if the `gdb.InferiorThread` object is valid, `False` if not. A `gdb.InferiorThread` object will become invalid if the thread exits, or the inferior that the thread belongs is deleted. All other `gdb.InferiorThread` methods will throw an exception if it is invalid at the time the method is called.

Function: InferiorThread.switch ()
  This changes GDB's currently selected thread to the one represented by this object.

Function: InferiorThread.is_stopped ()
  Return a Boolean indicating whether the thread is stopped.

Function: InferiorThread.is_running ()
  Return a Boolean indicating whether the thread is running.

Function: InferiorThread.is_exited ()
  Return a Boolean indicating whether the thread is exited.

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#### 23.2.2.15 Commands In Python

You can implement new GDB CLI commands in Python. A CLI command is implemented using an instance of the `gdb.Command` class, most commonly using a subclass.

Function: Command.__init__ (name, command_class [, completer_class [, prefix]])
  The object initializer for `Command` registers the new command with GDB. This initializer is normally invoked from the subclass' own `__init__` method.

  name is the name of the command. If name consists of multiple words, then the initial words are looked for as prefix commands. In this case, if one of the prefix commands does not exist, an exception is raised.

  There is no support for multi-line commands.

  command_class should be one of the ``COMMAND_'` constants defined below. This argument tells GDB how to categorize the new command in the help system.

  completer_class is an optional argument. If given, it should be one of the ``COMPLETE_'` constants defined below. This argument tells GDB how to perform completion for this command. If not given, GDB will attempt to complete using the object's `complete` method (see below); if no such method is found, an error will occur when completion is attempted.

  prefix is an optional argument. If `True`, then the new command is a prefix command; sub-commands of this command may be registered.

  The help text for the new command is taken from the Python documentation string for the command's class, if there is one. If no documentation string is provided, the default value "This command is not documented." is used.

Function: Command.dont_repeat ()
  By default, a GDB command is repeated when the user enters a blank line at the command prompt. A command can suppress this behavior by invoking the `dont_repeat` method. This is similar to the user command `dont-repeat`, see dont-repeat.

Function: Command.invoke (argument, from_tty)
  This method is called by GDB when this command is invoked.

  argument is a string. It is the argument to the command, after leading and trailing whitespace has been stripped.

  from_tty is a boolean argument. When true, this means that the command was entered by the user at the terminal; when false it means that the command came from elsewhere.

  If this method throws an exception, it is turned into a GDB `error` call. Otherwise, the return value is ignored.

  To break argument up into an argv-like string use `gdb.string_to_argv`. This function behaves identically to GDB's internal argument lexer `buildargv`. It is recommended to use this for consistency. Arguments are separated by spaces and may be quoted. Example:

      print gdb.string_to_argv ("1 2\ \\\"3 '4 \"5' \"6 '7\"")
      ['1', '2 "3', '4 "5', "6 '7"]

Function: Command.complete (text, word)
  This method is called by GDB when the user attempts completion on this command. All forms of completion are handled by this method, that is, the `TAB` and `M-?` key bindings (see section 3.2 Command Completion), and the `complete` command (see section complete).

  The arguments text and word are both strings. text holds the complete command line up to the cursor's location. word holds the last word of the command line; this is computed using a word-breaking heuristic.

  The `complete` method can return several values:

  - If the return value is a sequence, the contents of the sequence are used as the completions. It is up to `complete` to ensure that the contents actually do complete the word. A zero-length sequence is allowed, it means that there were no completions available. Only string elements of the sequence are used; other elements in the sequence are ignored.

  - If the return value is one of the ``COMPLETE_'` constants defined below, then the corresponding GDB-internal completion function is invoked, and its result is used.

  - All other results are treated as though there were no available completions.

When a new command is registered, it must be declared as a member of some general class of commands. This is used to classify top-level commands in the on-line help system; note that prefix commands are not listed under their own category but rather that of their top-level command. The available classifications are represented by constants defined in the `gdb` module:

`gdb.COMMAND_NONE`
  The command does not belong to any particular class. A command in this category will not be displayed in any of the help categories.

`gdb.COMMAND_RUNNING`
  The command is related to running the inferior. For example, `start`, `step`, and `continue` are in this category. Type `help running` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_DATA`
  The command is related to data or variables. For example, `call`, `find`, and `print` are in this category. Type `help data` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_STACK`
  The command has to do with manipulation of the stack. For example, `backtrace`, `frame`, and `return` are in this category. Type `help stack` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_FILES`
  This class is used for file-related commands. For example, `file`, `list` and `section` are in this category. Type `help files` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_SUPPORT`
  This should be used for "support facilities", generally meaning things that are useful to the user when interacting with GDB, but not related to the state of the inferior. For example, `help`, `make`, and `shell` are in this category. Type `help support` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_STATUS`
  The command is an ``info'`-related command, that is, related to the state of GDB itself. For example, `info`, `macro`, and `show` are in this category. Type `help status` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_BREAKPOINTS`
  The command has to do with breakpoints. For example, `break`, `clear`, and `delete` are in this category. Type `help breakpoints` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_TRACEPOINTS`
  The command has to do with tracepoints. For example, `trace`, `actions`, and `tfind` are in this category. Type `help tracepoints` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_USER`
  The command is a general purpose command for the user, and typically does not fit in one of the other categories. Type `help user-defined` at the GDB prompt to see a list of commands in this category, as well as the list of gdb macros (see section 23.1 Canned Sequences of Commands).

`gdb.COMMAND_OBSCURE`
  The command is only used in unusual circumstances, or is not of general interest to users. For example, `checkpoint`, `fork`, and `stop` are in this category. Type `help obscure` at the GDB prompt to see a list of commands in this category.

`gdb.COMMAND_MAINTENANCE`
  The command is only useful to GDB maintainers. The `maintenance` and `flushregs` commands are in this category. Type `help internals` at the GDB prompt to see a list of commands in this category.

A new command can use a predefined completion function, either by specifying it via an argument at initialization, or by returning it from the `complete` method. These predefined completion constants are all defined in the `gdb` module:

`gdb.COMPLETE_NONE`
  This constant means that no completion should be done.

`gdb.COMPLETE_FILENAME`
  This constant means that filename completion should be performed.

`gdb.COMPLETE_LOCATION`
  This constant means that location completion should be done. See section 9.2 Specifying a Location.

`gdb.COMPLETE_COMMAND`
  This constant means that completion should examine GDB command names.

`gdb.COMPLETE_SYMBOL`
  This constant means that completion should be done using symbol names as the source.

`gdb.COMPLETE_EXPRESSION`
  This constant means that completion should be done on expressions. Often this means completing on symbol names, but some language parsers also have support for completing on field names.

The following code snippet shows how a trivial CLI command can be implemented in Python:

    class HelloWorld (gdb.Command):
      """Greet the whole world."""

      def __init__ (self):
        super (HelloWorld, self).__init__ ("hello-world", gdb.COMMAND_USER)

      def invoke (self, arg, from_tty):
        print "Hello, World!"

    HelloWorld ()

The last line instantiates the class, and is necessary to trigger the registration of the command with GDB. Depending on how the Python code is read into GDB, you may need to import the `gdb` module explicitly.

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#### 23.2.2.16 Parameters In Python

You can implement new GDB parameters using Python. A new parameter is implemented as an instance of the `gdb.Parameter` class.

Parameters are exposed to the user via the `set` and `show` commands. See section 3.3 Getting Help.

There are many parameters that already exist and can be set in GDB. Two examples are: `set follow fork` and `set charset`. Setting these parameters influences certain behavior in GDB. Similarly, you can define parameters that can be used to influence behavior in custom Python scripts and commands.

Function: Parameter.__init__ (name, command-class, parameter-class [, enum-sequence])
  The object initializer for `Parameter` registers the new parameter with GDB. This initializer is normally invoked from the subclass' own `__init__` method.

  name is the name of the new parameter. If name consists of multiple words, then the initial words are looked for as prefix parameters. An example of this can be illustrated with the `set print` set of parameters. If name is `print foo`, then `print` will be searched as the prefix parameter. In this case the parameter can subsequently be accessed in GDB as `set print foo`.

  If name consists of multiple words, and no prefix parameter group can be found, an exception is raised.

  command-class should be one of the ``COMMAND_'` constants (see section 23.2.2.15 Commands In Python). This argument tells GDB how to categorize the new parameter in the help system.

  parameter-class should be one of the ``PARAM_'` constants defined below. This argument tells GDB the type of the new parameter; this information is used for input validation and completion.

  If parameter-class is `PARAM_ENUM`, then enum-sequence must be a sequence of strings. These strings represent the possible values for the parameter.

  If parameter-class is not `PARAM_ENUM`, then the presence of a fourth argument will cause an exception to be thrown.

  The help text for the new parameter is taken from the Python documentation string for the parameter's class, if there is one. If there is no documentation string, a default value is used.

Variable: Parameter.set_doc
  If this attribute exists, and is a string, then its value is used as the help text for this parameter's `set` command. The value is examined when `Parameter.__init__` is invoked; subsequent changes have no effect.

Variable: Parameter.show_doc
  If this attribute exists, and is a string, then its value is used as the help text for this parameter's `show` command. The value is examined when `Parameter.__init__` is invoked; subsequent changes have no effect.

Variable: Parameter.value
  The `value` attribute holds the underlying value of the parameter. It can be read and assigned to just as any other attribute. GDB does validation when assignments are made.

There are two methods that should be implemented in any `Parameter` class. These are:

Function: Parameter.get_set_string (self)
  GDB will call this method when a parameter's value has been changed via the `set` API (for example, `set foo off`). The `value` attribute has already been populated with the new value and may be used in output. This method must return a string.

Function: Parameter.get_show_string (self, svalue)
  GDB will call this method when a parameter's `show` API has been invoked (for example, `show foo`). The argument `svalue` receives the string representation of the current value. This method must return a string.

When a new parameter is defined, its type must be specified. The available types are represented by constants defined in the `gdb` module:

`gdb.PARAM_BOOLEAN`
  The value is a plain boolean. The Python boolean values, `True` and `False` are the only valid values.

`gdb.PARAM_AUTO_BOOLEAN`
  The value has three possible states: true, false, and ``auto'`. In Python, true and false are represented using boolean constants, and ``auto'` is represented using `None`.

`gdb.PARAM_UINTEGER`
  The value is an unsigned integer. The value of 0 should be interpreted to mean "unlimited".

`gdb.PARAM_INTEGER`
  The value is a signed integer. The value of 0 should be interpreted to mean "unlimited".

`gdb.PARAM_STRING`
  The value is a string. When the user modifies the string, any escape sequences, such as ``\t'`, ``\f'`, and octal escapes, are translated into corresponding characters and encoded into the current host charset.

`gdb.PARAM_STRING_NOESCAPE`
  The value is a string. When the user modifies the string, escapes are passed through untranslated.

`gdb.PARAM_OPTIONAL_FILENAME`
  The value is a either a filename (a string), or `None`.

`gdb.PARAM_FILENAME`
  The value is a filename. This is just like `PARAM_STRING_NOESCAPE`, but uses file names for completion.

`gdb.PARAM_ZINTEGER`
  The value is an integer. This is like `PARAM_INTEGER`, except 0 is interpreted as itself.

`gdb.PARAM_ENUM`
  The value is a string, which must be one of a collection string constants provided when the parameter is created.

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#### 23.2.2.17 Writing new convenience functions

You can implement new convenience functions (see section 10.11 Convenience Variables) in Python. A convenience function is an instance of a subclass of the class `gdb.Function`.

Function: Function.__init__ (name)
  The initializer for `Function` registers the new function with GDB. The argument name is the name of the function, a string. The function will be visible to the user as a convenience variable of type `internal function`, whose name is the same as the given name.

  The documentation for the new function is taken from the documentation string for the new class.

Function: Function.invoke (*args)
  When a convenience function is evaluated, its arguments are converted to instances of `gdb.Value`, and then the function's `invoke` method is called. Note that GDB does not predetermine the arity of convenience functions. Instead, all available arguments are passed to `invoke`, following the standard Python calling convention. In particular, a convenience function can have default values for parameters without ill effect.

  The return value of this method is used as its value in the enclosing expression. If an ordinary Python value is returned, it is converted to a `gdb.Value` following the usual rules.

The following code snippet shows how a trivial convenience function can be implemented in Python:

    class Greet (gdb.Function):
      """Return string to greet someone.
    Takes a name as argument."""

      def __init__ (self):
        super (Greet, self).__init__ ("greet")

      def invoke (self, name):
        return "Hello, %s!" % name.string ()

    Greet ()

The last line instantiates the class, and is necessary to trigger the registration of the function with GDB. Depending on how the Python code is read into GDB, you may need to import the `gdb` module explicitly.

Now you can use the function in an expression:

    (gdb) print $greet("Bob")
    $1 = "Hello, Bob!"

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#### 23.2.2.18 Program Spaces In Python

A program space, or progspace, represents a symbolic view of an address space. It consists of all of the objfiles of the program. See section 23.2.2.19 Objfiles In Python. See section program spaces, for more details about program spaces.

The following progspace-related functions are available in the `gdb` module:

Function: gdb.current_progspace ()
  This function returns the program space of the currently selected inferior. See section 4.9 Debugging Multiple Inferiors and Programs.

Function: gdb.progspaces ()
  Return a sequence of all the progspaces currently known to GDB.

Each progspace is represented by an instance of the `gdb.Progspace` class.

Variable: Progspace.filename
  The file name of the progspace as a string.

Variable: Progspace.pretty_printers
  The `pretty_printers` attribute is a list of functions. It is used to look up pretty-printers. A `Value` is passed to each function in order; if the function returns `None`, then the search continues. Otherwise, the return value should be an object which is used to format the value. See section 23.2.2.5 Pretty Printing API, for more information.

Variable: Progspace.type_printers
  The `type_printers` attribute is a list of type printer objects. See section 23.2.2.8 Type Printing API, for more information.

Variable: Progspace.frame_filters
  The `frame_filters` attribute is a dictionary of frame filter objects. See section 23.2.2.9 Filtering Frames., for more information.

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#### 23.2.2.19 Objfiles In Python

GDB loads symbols for an inferior from various symbol-containing files (see section 18.1 Commands to Specify Files). These include the primary executable file, any shared libraries used by the inferior, and any separate debug info files (see section 18.2 Debugging Information in Separate Files). GDB calls these symbol-containing files objfiles.

The following objfile-related functions are available in the `gdb` module:

Function: gdb.current_objfile ()
  When auto-loading a Python script (see section 23.2.3 Python Auto-loading), GDB sets the "current objfile" to the corresponding objfile. This function returns the current objfile. If there is no current objfile, this function returns `None`.

Function: gdb.objfiles ()
  Return a sequence of all the objfiles current known to GDB. See section 23.2.2.19 Objfiles In Python.

Each objfile is represented by an instance of the `gdb.Objfile` class.

Variable: Objfile.filename
  The file name of the objfile as a string.

Variable: Objfile.pretty_printers
  The `pretty_printers` attribute is a list of functions. It is used to look up pretty-printers. A `Value` is passed to each function in order; if the function returns `None`, then the search continues. Otherwise, the return value should be an object which is used to format the value. See section 23.2.2.5 Pretty Printing API, for more information.

Variable: Objfile.type_printers
  The `type_printers` attribute is a list of type printer objects. See section 23.2.2.8 Type Printing API, for more information.

Variable: Objfile.frame_filters
  The `frame_filters` attribute is a dictionary of frame filter objects. See section 23.2.2.9 Filtering Frames., for more information.

A `gdb.Objfile` object has the following methods:

Function: Objfile.is_valid ()
  Returns `True` if the `gdb.Objfile` object is valid, `False` if not. A `gdb.Objfile` object can become invalid if the object file it refers to is not loaded in GDB any longer. All other `gdb.Objfile` methods will throw an exception if it is invalid at the time the method is called.

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#### 23.2.2.20 Accessing inferior stack frames from Python.

When the debugged program stops, GDB is able to analyze its call stack (see section Stack frames). The `gdb.Frame` class represents a frame in the stack. A `gdb.Frame` object is only valid while its corresponding frame exists in the inferior's stack. If you try to use an invalid frame object, GDB will throw a `gdb.error` exception (see section 23.2.2.2 Exception Handling).

Two `gdb.Frame` objects can be compared for equality with the `==` operator, like:

    (gdb) python print gdb.newest_frame() == gdb.selected_frame ()
    True

The following frame-related functions are available in the `gdb` module:

Function: gdb.selected_frame ()
  Return the selected frame object. (see section Selecting a Frame).

Function: gdb.newest_frame ()
  Return the newest frame object for the selected thread.

Function: gdb.frame_stop_reason_string (reason)
  Return a string explaining the reason why GDB stopped unwinding frames, as expressed by the given reason code (an integer, see the `unwind_stop_reason` method further down in this section).

A `gdb.Frame` object has the following methods:

Function: Frame.is_valid ()
  Returns true if the `gdb.Frame` object is valid, false if not. A frame object can become invalid if the frame it refers to doesn't exist anymore in the inferior. All `gdb.Frame` methods will throw an exception if it is invalid at the time the method is called.

Function: Frame.name ()
  Returns the function name of the frame, or `None` if it can't be obtained.

Function: Frame.architecture ()
  Returns the `gdb.Architecture` object corresponding to the frame's architecture. See section 23.2.2.28 Python representation of architectures.

Function: Frame.type ()
  Returns the type of the frame. The value can be one of:
  `gdb.NORMAL_FRAME`
    An ordinary stack frame.

  `gdb.DUMMY_FRAME`
    A fake stack frame that was created by GDB when performing an inferior function call.

  `gdb.INLINE_FRAME`
    A frame representing an inlined function. The function was inlined into a `gdb.NORMAL_FRAME` that is older than this one.

  `gdb.TAILCALL_FRAME`
    A frame representing a tail call. See section 11.2 Tail Call Frames.

  `gdb.SIGTRAMP_FRAME`
    A signal trampoline frame. This is the frame created by the OS when it calls into a signal handler.

  `gdb.ARCH_FRAME`
    A fake stack frame representing a cross-architecture call.

  `gdb.SENTINEL_FRAME`
    This is like `gdb.NORMAL_FRAME`, but it is only used for the newest frame.

Function: Frame.unwind_stop_reason ()
  Return an integer representing the reason why it's not possible to find more frames toward the outermost frame. Use `gdb.frame_stop_reason_string` to convert the value returned by this function to a string. The value can be one of:

  `gdb.FRAME_UNWIND_NO_REASON`
    No particular reason (older frames should be available).

  `gdb.FRAME_UNWIND_NULL_ID`
    The previous frame's analyzer returns an invalid result. This is no longer used by GDB, and is kept only for backward compatibility.

  `gdb.FRAME_UNWIND_OUTERMOST`
    This frame is the outermost.

  `gdb.FRAME_UNWIND_UNAVAILABLE`
    Cannot unwind further, because that would require knowing the values of registers or memory that have not been collected.

  `gdb.FRAME_UNWIND_INNER_ID`
    This frame ID looks like it ought to belong to a NEXT frame, but we got it for a PREV frame. Normally, this is a sign of unwinder failure. It could also indicate stack corruption.

  `gdb.FRAME_UNWIND_SAME_ID`
    This frame has the same ID as the previous one. That means that unwinding further would almost certainly give us another frame with exactly the same ID, so break the chain. Normally, this is a sign of unwinder failure. It could also indicate stack corruption.

  `gdb.FRAME_UNWIND_NO_SAVED_PC`
    The frame unwinder did not find any saved PC, but we needed one to unwind further.

  `gdb.FRAME_UNWIND_FIRST_ERROR`
    Any stop reason greater or equal to this value indicates some kind of error. This special value facilitates writing code that tests for errors in unwinding in a way that will work correctly even if the list of the other values is modified in future GDB versions. Using it, you could write:

        reason = gdb.selected_frame().unwind_stop_reason ()
        reason_str =  gdb.frame_stop_reason_string (reason)
        if reason >=  gdb.FRAME_UNWIND_FIRST_ERROR:
            print "An error occured: %s" % reason_str

Function: Frame.pc ()
  Returns the frame's resume address.

Function: Frame.block ()
  Return the frame's code block. See section 23.2.2.21 Accessing blocks from Python..

Function: Frame.function ()
  Return the symbol for the function corresponding to this frame. See section 23.2.2.22 Python representation of Symbols..

Function: Frame.older ()
  Return the frame that called this frame.

Function: Frame.newer ()
  Return the frame called by this frame.

Function: Frame.find_sal ()
  Return the frame's symtab and line object. See section 23.2.2.23 Symbol table representation in Python..

Function: Frame.read_var (variable [, block])
  Return the value of variable in this frame. If the optional argument block is provided, search for the variable from that block; otherwise start at the frame's current block (which is determined by the frame's current program counter). variable must be a string or a `gdb.Symbol` object. block must be a `gdb.Block` object.

Function: Frame.select ()
  Set this frame to be the selected frame. See section Examining the Stack.

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#### 23.2.2.21 Accessing blocks from Python.

In GDB, symbols are stored in blocks. A block corresponds roughly to a scope in the source code. Blocks are organized hierarchically, and are represented individually in Python as a `gdb.Block`. Blocks rely on debugging information being available.

A frame has a block. Please see 23.2.2.20 Accessing inferior stack frames from Python., for a more in-depth discussion of frames.

The outermost block is known as the global block. The global block typically holds public global variables and functions.

The block nested just inside the global block is the static block. The static block typically holds file-scoped variables and functions.

GDB provides a method to get a block's superblock, but there is currently no way to examine the sub-blocks of a block, or to iterate over all the blocks in a symbol table (see section 23.2.2.23 Symbol table representation in Python.).

Here is a short example that should help explain blocks:

    /* This is in the global block.  */
    int global;

    /* This is in the static block.  */
    static int file_scope;

    /* 'function' is in the global block, and 'argument' is
       in a block nested inside of 'function'.  */
    int function (int argument)
    {
      /* 'local' is in a block inside 'function'.  It may or may
         not be in the same block as 'argument'.  */
      int local;

      {
         /* 'inner' is in a block whose superblock is the one holding
            'local'.  */
         int inner;

         /* If this call is expanded by the compiler, you may see
            a nested block here whose function is 'inline_function'
            and whose superblock is the one holding 'inner'.  */
         inline_function ();
      }
    }

A `gdb.Block` is iterable. The iterator returns the symbols (see section 23.2.2.22 Python representation of Symbols.) local to the block. Python programs should not assume that a specific block object will always contain a given symbol, since changes in GDB features and infrastructure may cause symbols move across blocks in a symbol table.

The following block-related functions are available in the `gdb` module:

Function: gdb.block_for_pc (pc)
  Return the innermost `gdb.Block` containing the given pc value. If the block cannot be found for the pc value specified, the function will return `None`.

A `gdb.Block` object has the following methods:

Function: Block.is_valid ()
  Returns `True` if the `gdb.Block` object is valid, `False` if not. A block object can become invalid if the block it refers to doesn't exist anymore in the inferior. All other `gdb.Block` methods will throw an exception if it is invalid at the time the method is called. The block's validity is also checked during iteration over symbols of the block.

A `gdb.Block` object has the following attributes:

Variable: Block.start
  The start address of the block. This attribute is not writable.

Variable: Block.end
  The end address of the block. This attribute is not writable.

Variable: Block.function
  The name of the block represented as a `gdb.Symbol`. If the block is not named, then this attribute holds `None`. This attribute is not writable.

  For ordinary function blocks, the superblock is the static block. However, you should note that it is possible for a function block to have a superblock that is not the static block -- for instance this happens for an inlined function.

Variable: Block.superblock
  The block containing this block. If this parent block does not exist, this attribute holds `None`. This attribute is not writable.

Variable: Block.global_block
  The global block associated with this block. This attribute is not writable.

Variable: Block.static_block
  The static block associated with this block. This attribute is not writable.

Variable: Block.is_global
  `True` if the `gdb.Block` object is a global block, `False` if not. This attribute is not writable.

Variable: Block.is_static
  `True` if the `gdb.Block` object is a static block, `False` if not. This attribute is not writable.

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#### 23.2.2.22 Python representation of Symbols.

GDB represents every variable, function and type as an entry in a symbol table. See section Examining the Symbol Table. Similarly, Python represents these symbols in GDB with the `gdb.Symbol` object.

The following symbol-related functions are available in the `gdb` module:

Function: gdb.lookup_symbol (name [, block [, domain]])
  This function searches for a symbol by name. The search scope can be restricted to the parameters defined in the optional domain and block arguments.

  name is the name of the symbol. It must be a string. The optional block argument restricts the search to symbols visible in that block. The block argument must be a `gdb.Block` object. If omitted, the block for the current frame is used. The optional domain argument restricts the search to the domain type. The domain argument must be a domain constant defined in the `gdb` module and described later in this chapter.

  The result is a tuple of two elements. The first element is a `gdb.Symbol` object or `None` if the symbol is not found. If the symbol is found, the second element is `True` if the symbol is a field of a method's object (e.g., `this` in C`++`), otherwise it is `False`. If the symbol is not found, the second element is `False`.

Function: gdb.lookup_global_symbol (name [, domain])
  This function searches for a global symbol by name. The search scope can be restricted to by the domain argument.

  name is the name of the symbol. It must be a string. The optional domain argument restricts the search to the domain type. The domain argument must be a domain constant defined in the `gdb` module and described later in this chapter.

  The result is a `gdb.Symbol` object or `None` if the symbol is not found.

A `gdb.Symbol` object has the following attributes:

Variable: Symbol.type
  The type of the symbol or `None` if no type is recorded. This attribute is represented as a `gdb.Type` object. See section 23.2.2.4 Types In Python. This attribute is not writable.

Variable: Symbol.symtab
  The symbol table in which the symbol appears. This attribute is represented as a `gdb.Symtab` object. See section 23.2.2.23 Symbol table representation in Python.. This attribute is not writable.

Variable: Symbol.line
  The line number in the source code at which the symbol was defined. This is an integer.

Variable: Symbol.name
  The name of the symbol as a string. This attribute is not writable.

Variable: Symbol.linkage_name
  The name of the symbol, as used by the linker (i.e., may be mangled). This attribute is not writable.

Variable: Symbol.print_name
  The name of the symbol in a form suitable for output. This is either `name` or `linkage_name`, depending on whether the user asked GDB to display demangled or mangled names.

Variable: Symbol.addr_class
  The address class of the symbol. This classifies how to find the value of a symbol. Each address class is a constant defined in the `gdb` module and described later in this chapter.

Variable: Symbol.needs_frame
  This is `True` if evaluating this symbol's value requires a frame (see section 23.2.2.20 Accessing inferior stack frames from Python.) and `False` otherwise. Typically, local variables will require a frame, but other symbols will not.

Variable: Symbol.is_argument
  `True` if the symbol is an argument of a function.

Variable: Symbol.is_constant
  `True` if the symbol is a constant.

Variable: Symbol.is_function
  `True` if the symbol is a function or a method.

Variable: Symbol.is_variable
  `True` if the symbol is a variable.

A `gdb.Symbol` object has the following methods:

Function: Symbol.is_valid ()
  Returns `True` if the `gdb.Symbol` object is valid, `False` if not. A `gdb.Symbol` object can become invalid if the symbol it refers to does not exist in GDB any longer. All other `gdb.Symbol` methods will throw an exception if it is invalid at the time the method is called.

Function: Symbol.value ([frame])
  Compute the value of the symbol, as a `gdb.Value`. For functions, this computes the address of the function, cast to the appropriate type. If the symbol requires a frame in order to compute its value, then frame must be given. If frame is not given, or if frame is invalid, then this method will throw an exception.

The available domain categories in `gdb.Symbol` are represented as constants in the `gdb` module:

`gdb.SYMBOL_UNDEF_DOMAIN`
  This is used when a domain has not been discovered or none of the following domains apply. This usually indicates an error either in the symbol information or in GDB's handling of symbols.
`gdb.SYMBOL_VAR_DOMAIN`
  This domain contains variables, function names, typedef names and enum type values.
`gdb.SYMBOL_STRUCT_DOMAIN`
  This domain holds struct, union and enum type names.
`gdb.SYMBOL_LABEL_DOMAIN`
  This domain contains names of labels (for gotos).
`gdb.SYMBOL_VARIABLES_DOMAIN`
  This domain holds a subset of the `SYMBOLS_VAR_DOMAIN`; it contains everything minus functions and types.
`gdb.SYMBOL_FUNCTION_DOMAIN`
  This domain contains all functions.
`gdb.SYMBOL_TYPES_DOMAIN`
  This domain contains all types.

The available address class categories in `gdb.Symbol` are represented as constants in the `gdb` module:

`gdb.SYMBOL_LOC_UNDEF`
  If this is returned by address class, it indicates an error either in the symbol information or in GDB's handling of symbols.
`gdb.SYMBOL_LOC_CONST`
  Value is constant int.
`gdb.SYMBOL_LOC_STATIC`
  Value is at a fixed address.
`gdb.SYMBOL_LOC_REGISTER`
  Value is in a register.
`gdb.SYMBOL_LOC_ARG`
  Value is an argument. This value is at the offset stored within the symbol inside the frame's argument list.
`gdb.SYMBOL_LOC_REF_ARG`
  Value address is stored in the frame's argument list. Just like `LOC_ARG` except that the value's address is stored at the offset, not the value itself.
`gdb.SYMBOL_LOC_REGPARM_ADDR`
  Value is a specified register. Just like `LOC_REGISTER` except the register holds the address of the argument instead of the argument itself.
`gdb.SYMBOL_LOC_LOCAL`
  Value is a local variable.
`gdb.SYMBOL_LOC_TYPEDEF`
  Value not used. Symbols in the domain `SYMBOL_STRUCT_DOMAIN` all have this class.
`gdb.SYMBOL_LOC_BLOCK`
  Value is a block.
`gdb.SYMBOL_LOC_CONST_BYTES`
  Value is a byte-sequence.
`gdb.SYMBOL_LOC_UNRESOLVED`
  Value is at a fixed address, but the address of the variable has to be determined from the minimal symbol table whenever the variable is referenced.
`gdb.SYMBOL_LOC_OPTIMIZED_OUT`
  The value does not actually exist in the program.
`gdb.SYMBOL_LOC_COMPUTED`
  The value's address is a computed location.

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#### 23.2.2.23 Symbol table representation in Python.

Access to symbol table data maintained by GDB on the inferior is exposed to Python via two objects: `gdb.Symtab_and_line` and `gdb.Symtab`. Symbol table and line data for a frame is returned from the `find_sal` method in `gdb.Frame` object. See section 23.2.2.20 Accessing inferior stack frames from Python..

For more information on GDB's symbol table management, see Examining the Symbol Table, for more information.

A `gdb.Symtab_and_line` object has the following attributes:

Variable: Symtab_and_line.symtab
  The symbol table object (`gdb.Symtab`) for this frame. This attribute is not writable.

Variable: Symtab_and_line.pc
  Indicates the start of the address range occupied by code for the current source line. This attribute is not writable.

Variable: Symtab_and_line.last
  Indicates the end of the address range occupied by code for the current source line. This attribute is not writable.

Variable: Symtab_and_line.line
  Indicates the current line number for this object. This attribute is not writable.

A `gdb.Symtab_and_line` object has the following methods:

Function: Symtab_and_line.is_valid ()
  Returns `True` if the `gdb.Symtab_and_line` object is valid, `False` if not. A `gdb.Symtab_and_line` object can become invalid if the Symbol table and line object it refers to does not exist in GDB any longer. All other `gdb.Symtab_and_line` methods will throw an exception if it is invalid at the time the method is called.

A `gdb.Symtab` object has the following attributes:

Variable: Symtab.filename
  The symbol table's source filename. This attribute is not writable.

Variable: Symtab.objfile
  The symbol table's backing object file. See section 23.2.2.19 Objfiles In Python. This attribute is not writable.

A `gdb.Symtab` object has the following methods:

Function: Symtab.is_valid ()
  Returns `True` if the `gdb.Symtab` object is valid, `False` if not. A `gdb.Symtab` object can become invalid if the symbol table it refers to does not exist in GDB any longer. All other `gdb.Symtab` methods will throw an exception if it is invalid at the time the method is called.

Function: Symtab.fullname ()
  Return the symbol table's source absolute file name.

Function: Symtab.global_block ()
  Return the global block of the underlying symbol table. See section 23.2.2.21 Accessing blocks from Python..

Function: Symtab.static_block ()
  Return the static block of the underlying symbol table. See section 23.2.2.21 Accessing blocks from Python..

Function: Symtab.linetable ()
  Return the line table associated with the symbol table. See section 23.2.2.24 Manipulating line tables using Python.

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#### 23.2.2.24 Manipulating line tables using Python

Python code can request and inspect line table information from a symbol table that is loaded in GDB. A line table is a mapping of source lines to their executable locations in memory. To acquire the line table information for a particular symbol table, use the `linetable` function (see section 23.2.2.23 Symbol table representation in Python.).

A `gdb.LineTable` is iterable. The iterator returns `LineTableEntry` objects that correspond to the source line and address for each line table entry. `LineTableEntry` objects have the following attributes:

Variable: LineTableEntry.line
  The source line number for this line table entry. This number corresponds to the actual line of source. This attribute is not writable.

Variable: LineTableEntry.pc
  The address that is associated with the line table entry where the executable code for that source line resides in memory. This attribute is not writable.

As there can be multiple addresses for a single source line, you may receive multiple `LineTableEntry` objects with matching `line` attributes, but with different `pc` attributes. The iterator is sorted in ascending `pc` order. Here is a small example illustrating iterating over a line table.

    symtab = gdb.selected_frame().find_sal().symtab
    linetable = symtab.linetable()
    for line in linetable:
       print "Line: "+str(line.line)+" Address: "+hex(line.pc)

This will have the following output:

    Line: 33 Address: 0x4005c8L
    Line: 37 Address: 0x4005caL
    Line: 39 Address: 0x4005d2L
    Line: 40 Address: 0x4005f8L
    Line: 42 Address: 0x4005ffL
    Line: 44 Address: 0x400608L
    Line: 42 Address: 0x40060cL
    Line: 45 Address: 0x400615L

In addition to being able to iterate over a `LineTable`, it also has the following direct access methods:

Function: LineTable.line (line)
  Return a Python `Tuple` of `LineTableEntry` objects for any entries in the line table for the given line. line refers to the source code line. If there are no entries for that source code line, the Python `None` is returned.

Function: LineTable.has_line (line)
  Return a Python `Boolean` indicating whether there is an entry in the line table for this source line. Return `True` if an entry is found, or `False` if not.

Function: LineTable.source_lines ()
  Return a Python `List` of the source line numbers in the symbol table. Only lines with executable code locations are returned. The contents of the `List` will just be the source line entries represented as Python `Long` values.

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#### 23.2.2.25 Manipulating breakpoints using Python

Python code can manipulate breakpoints via the `gdb.Breakpoint` class.

Function: Breakpoint.__init__ (spec [, type [, wp_class [,internal [,temporary]]]])
  Create a new breakpoint. spec is a string naming the location of the breakpoint, or an expression that defines a watchpoint. The contents can be any location recognized by the `break` command, or in the case of a watchpoint, by the `watch` command. The optional type denotes the breakpoint to create from the types defined later in this chapter. This argument can be either: `gdb.BP_BREAKPOINT` or `gdb.BP_WATCHPOINT`. type defaults to `gdb.BP_BREAKPOINT`. The optional internal argument allows the breakpoint to become invisible to the user. The breakpoint will neither be reported when created, nor will it be listed in the output from `info breakpoints` (but will be listed with the `maint info breakpoints` command). The optional temporary argument makes the breakpoint a temporary breakpoint. Temporary breakpoints are deleted after they have been hit. Any further access to the Python breakpoint after it has been hit will result in a runtime error (as that breakpoint has now been automatically deleted). The optional wp_class argument defines the class of watchpoint to create, if type is `gdb.BP_WATCHPOINT`. If a watchpoint class is not provided, it is assumed to be a `gdb.WP_WRITE` class.

Function: Breakpoint.stop (self)
  The `gdb.Breakpoint` class can be sub-classed and, in particular, you may choose to implement the `stop` method. If this method is defined in a sub-class of `gdb.Breakpoint`, it will be called when the inferior reaches any location of a breakpoint which instantiates that sub-class. If the method returns `True`, the inferior will be stopped at the location of the breakpoint, otherwise the inferior will continue.

  If there are multiple breakpoints at the same location with a `stop` method, each one will be called regardless of the return status of the previous. This ensures that all `stop` methods have a chance to execute at that location. In this scenario if one of the methods returns `True` but the others return `False`, the inferior will still be stopped.

  You should not alter the execution state of the inferior (i.e., step, next, etc.), alter the current frame context (i.e., change the current active frame), or alter, add or delete any breakpoint. As a general rule, you should not alter any data within GDB or the inferior at this time.

  Example `stop` implementation:

      class MyBreakpoint (gdb.Breakpoint):
            def stop (self):
              inf_val = gdb.parse_and_eval("foo")
              if inf_val == 3:
                return True
              return False

The available watchpoint types represented by constants are defined in the `gdb` module:

`gdb.WP_READ`
  Read only watchpoint.

`gdb.WP_WRITE`
  Write only watchpoint.

`gdb.WP_ACCESS`
  Read/Write watchpoint.

Function: Breakpoint.is_valid ()
  Return `True` if this `Breakpoint` object is valid, `False` otherwise. A `Breakpoint` object can become invalid if the user deletes the breakpoint. In this case, the object still exists, but the underlying breakpoint does not. In the cases of watchpoint scope, the watchpoint remains valid even if execution of the inferior leaves the scope of that watchpoint.

Function: Breakpoint.delete
  Permanently deletes the GDB breakpoint. This also invalidates the Python `Breakpoint` object. Any further access to this object's attributes or methods will raise an error.

Variable: Breakpoint.enabled
  This attribute is `True` if the breakpoint is enabled, and `False` otherwise. This attribute is writable.

Variable: Breakpoint.silent
  This attribute is `True` if the breakpoint is silent, and `False` otherwise. This attribute is writable.

  Note that a breakpoint can also be silent if it has commands and the first command is `silent`. This is not reported by the `silent` attribute.

Variable: Breakpoint.thread
  If the breakpoint is thread-specific, this attribute holds the thread id. If the breakpoint is not thread-specific, this attribute is `None`. This attribute is writable.

Variable: Breakpoint.task
  If the breakpoint is Ada task-specific, this attribute holds the Ada task id. If the breakpoint is not task-specific (or the underlying language is not Ada), this attribute is `None`. This attribute is writable.

Variable: Breakpoint.ignore_count
  This attribute holds the ignore count for the breakpoint, an integer. This attribute is writable.

Variable: Breakpoint.number
  This attribute holds the breakpoint's number -- the identifier used by the user to manipulate the breakpoint. This attribute is not writable.

Variable: Breakpoint.type
  This attribute holds the breakpoint's type -- the identifier used to determine the actual breakpoint type or use-case. This attribute is not writable.

Variable: Breakpoint.visible
  This attribute tells whether the breakpoint is visible to the user when set, or when the ``info breakpoints'` command is run. This attribute is not writable.

Variable: Breakpoint.temporary
  This attribute indicates whether the breakpoint was created as a temporary breakpoint. Temporary breakpoints are automatically deleted after that breakpoint has been hit. Access to this attribute, and all other attributes and functions other than the `is_valid` function, will result in an error after the breakpoint has been hit (as it has been automatically deleted). This attribute is not writable.

The available types are represented by constants defined in the `gdb` module:

`gdb.BP_BREAKPOINT`
  Normal code breakpoint.

`gdb.BP_WATCHPOINT`
  Watchpoint breakpoint.

`gdb.BP_HARDWARE_WATCHPOINT`
  Hardware assisted watchpoint.

`gdb.BP_READ_WATCHPOINT`
  Hardware assisted read watchpoint.

`gdb.BP_ACCESS_WATCHPOINT`
  Hardware assisted access watchpoint.

Variable: Breakpoint.hit_count
  This attribute holds the hit count for the breakpoint, an integer. This attribute is writable, but currently it can only be set to zero.

Variable: Breakpoint.location
  This attribute holds the location of the breakpoint, as specified by the user. It is a string. If the breakpoint does not have a location (that is, it is a watchpoint) the attribute's value is `None`. This attribute is not writable.

Variable: Breakpoint.expression
  This attribute holds a breakpoint expression, as specified by the user. It is a string. If the breakpoint does not have an expression (the breakpoint is not a watchpoint) the attribute's value is `None`. This attribute is not writable.

Variable: Breakpoint.condition
  This attribute holds the condition of the breakpoint, as specified by the user. It is a string. If there is no condition, this attribute's value is `None`. This attribute is writable.

Variable: Breakpoint.commands
  This attribute holds the commands attached to the breakpoint. If there are commands, this attribute's value is a string holding all the commands, separated by newlines. If there are no commands, this attribute is `None`. This attribute is not writable.

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#### 23.2.2.26 Finish Breakpoints

A finish breakpoint is a temporary breakpoint set at the return address of a frame, based on the `finish` command. `gdb.FinishBreakpoint` extends `gdb.Breakpoint`. The underlying breakpoint will be disabled and deleted when the execution will run out of the breakpoint scope (i.e. `Breakpoint.stop` or `FinishBreakpoint.out_of_scope` triggered). Finish breakpoints are thread specific and must be create with the right thread selected.

Function: FinishBreakpoint.__init__ ([frame] [, internal])
  Create a finish breakpoint at the return address of the `gdb.Frame` object frame. If frame is not provided, this defaults to the newest frame. The optional internal argument allows the breakpoint to become invisible to the user. See section 23.2.2.25 Manipulating breakpoints using Python, for further details about this argument.

Function: FinishBreakpoint.out_of_scope (self)
  In some circumstances (e.g. `longjmp`, C`++` exceptions, GDB `return` command, ...), a function may not properly terminate, and thus never hit the finish breakpoint. When GDB notices such a situation, the `out_of_scope` callback will be triggered.

  You may want to sub-class `gdb.FinishBreakpoint` and override this method:

      class MyFinishBreakpoint (gdb.FinishBreakpoint)
          def stop (self):
              print "normal finish"
              return True

          def out_of_scope ():
              print "abnormal finish"

Variable: FinishBreakpoint.return_value
  When GDB is stopped at a finish breakpoint and the frame used to build the `gdb.FinishBreakpoint` object had debug symbols, this attribute will contain a `gdb.Value` object corresponding to the return value of the function. The value will be `None` if the function return type is `void` or if the return value was not computable. This attribute is not writable.

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#### 23.2.2.27 Python representation of lazy strings.

A lazy string is a string whose contents is not retrieved or encoded until it is needed.

A `gdb.LazyString` is represented in GDB as an `address` that points to a region of memory, an `encoding` that will be used to encode that region of memory, and a `length` to delimit the region of memory that represents the string. The difference between a `gdb.LazyString` and a string wrapped within a `gdb.Value` is that a `gdb.LazyString` will be treated differently by GDB when printing. A `gdb.LazyString` is retrieved and encoded during printing, while a `gdb.Value` wrapping a string is immediately retrieved and encoded on creation.

A `gdb.LazyString` object has the following functions:

Function: LazyString.value ()
  Convert the `gdb.LazyString` to a `gdb.Value`. This value will point to the string in memory, but will lose all the delayed retrieval, encoding and handling that GDB applies to a `gdb.LazyString`.

Variable: LazyString.address
  This attribute holds the address of the string. This attribute is not writable.

Variable: LazyString.length
  This attribute holds the length of the string in characters. If the length is -1, then the string will be fetched and encoded up to the first null of appropriate width. This attribute is not writable.

Variable: LazyString.encoding
  This attribute holds the encoding that will be applied to the string when the string is printed by GDB. If the encoding is not set, or contains an empty string, then GDB will select the most appropriate encoding when the string is printed. This attribute is not writable.

Variable: LazyString.type
  This attribute holds the type that is represented by the lazy string's type. For a lazy string this will always be a pointer type. To resolve this to the lazy string's character type, use the type's `target` method. See section 23.2.2.4 Types In Python. This attribute is not writable.

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#### 23.2.2.28 Python representation of architectures

GDB uses architecture specific parameters and artifacts in a number of its various computations. An architecture is represented by an instance of the `gdb.Architecture` class.

A `gdb.Architecture` class has the following methods:

Function: Architecture.name ()
  Return the name (string value) of the architecture.

Function: Architecture.disassemble (start_pc [, end_pc [, count]])
  Return a list of disassembled instructions starting from the memory address start_pc. The optional arguments end_pc and count determine the number of instructions in the returned list. If both the optional arguments end_pc and count are specified, then a list of at most count disassembled instructions whose start address falls in the closed memory address interval from start_pc to end_pc are returned. If end_pc is not specified, but count is specified, then count number of instructions starting from the address start_pc are returned. If count is not specified but end_pc is specified, then all instructions whose start address falls in the closed memory address interval from start_pc to end_pc are returned. If neither end_pc nor count are specified, then a single instruction at start_pc is returned. For all of these cases, each element of the returned list is a Python `dict` with the following string keys:

  `addr`
    The value corresponding to this key is a Python long integer capturing the memory address of the instruction.

  `asm`
    The value corresponding to this key is a string value which represents the instruction with assembly language mnemonics. The assembly language flavor used is the same as that specified by the current CLI variable `disassembly-flavor`. See section 9.6 Source and Machine Code.

  `length`
    The value corresponding to this key is the length (integer value) of the instruction in bytes.

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### 23.2.3 Python Auto-loading

When a new object file is read (for example, due to the `file` command, or because the inferior has loaded a shared library), GDB will look for Python support scripts in several ways: ``objfile-gdb.py'` and `.debug_gdb_scripts` section. See section 23.3 Auto-loading extensions.

The auto-loading feature is useful for supplying application-specific debugging commands and scripts.

Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be printed.

`set auto-load python-scripts [on|off]`
  Enable or disable the auto-loading of Python scripts.

`show auto-load python-scripts`
  Show whether auto-loading of Python scripts is enabled or disabled.

`info auto-load python-scripts [regexp]`
  Print the list of all Python scripts that GDB auto-loaded.

  Also printed is the list of Python scripts that were mentioned in the `.debug_gdb_scripts` section and were not found (see section 23.3.2 The `.debug_gdb_scripts` section). This is useful because their names are not printed when GDB tries to load them and fails. There may be many of them, and printing an error message for each one is problematic.

  If regexp is supplied only Python scripts with matching names are printed.

  Example:

      (gdb) info auto-load python-scripts
      Loaded Script
      Yes    py-section-script.py
             full name: /tmp/py-section-script.py
      No     my-foo-pretty-printers.py

When reading an auto-loaded file, GDB sets the current objfile. This is available via the `gdb.current_objfile` function (see section 23.2.2.19 Objfiles In Python). This can be useful for registering objfile-specific pretty-printers and frame-filters.

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### 23.2.4 Python modules

GDB comes with several modules to assist writing Python code.

> 23.2.4.1 gdb.printing |  | Building and registering pretty-printers.
> 23.2.4.2 gdb.types |  | Utilities for working with types.
> 23.2.4.3 gdb.prompt |  | Utilities for prompt value substitution.

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#### 23.2.4.1 gdb.printing

This module provides a collection of utilities for working with pretty-printers.

`PrettyPrinter (name, subprinters=None)`
  This class specifies the API that makes ``info pretty-printer'`, ``enable pretty-printer'` and ``disable pretty-printer'` work. Pretty-printers should generally inherit from this class.

`SubPrettyPrinter (name)`
  For printers that handle multiple types, this class specifies the corresponding API for the subprinters.

`RegexpCollectionPrettyPrinter (name)`
  Utility class for handling multiple printers, all recognized via regular expressions. See section 23.2.2.7 Writing a Pretty-Printer, for an example.

`FlagEnumerationPrinter (name)`
  A pretty-printer which handles printing of `enum` values. Unlike GDB's built-in `enum` printing, this printer attempts to work properly when there is some overlap between the enumeration constants. name is the name of the printer and also the name of the `enum` type to look up.

`register_pretty_printer (obj, printer, replace=False)`
  Register printer with the pretty-printer list of obj. If replace is `True` then any existing copy of the printer is replaced. Otherwise a `RuntimeError` exception is raised if a printer with the same name already exists.

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#### 23.2.4.2 gdb.types

This module provides a collection of utilities for working with `gdb.Type` objects.

`get_basic_type (type)`
  Return type with const and volatile qualifiers stripped, and with typedefs and C`++` references converted to the underlying type.

  C`++` example:

      typedef const int const_int;
      const_int foo (3);
      const_int& foo_ref (foo);
      int main () { return 0; }

  Then in gdb:

      (gdb) start
      (gdb) python import gdb.types
      (gdb) python foo_ref = gdb.parse_and_eval("foo_ref")
      (gdb) python print gdb.types.get_basic_type(foo_ref.type)
      int

`has_field (type, field)`
  Return `True` if type, assumed to be a type with fields (e.g., a structure or union), has field field.

`make_enum_dict (enum_type)`
  Return a Python `dictionary` type produced from enum_type.

`deep_items (type)`
  Returns a Python iterator similar to the standard `gdb.Type.iteritems` method, except that the iterator returned by `deep_items` will recursively traverse anonymous struct or union fields. For example:

      struct A
      {
          int a;
          union {
              int b0;
              int b1;
          };
      };

  Then in GDB:

      (gdb) python import gdb.types
      (gdb) python struct_a = gdb.lookup_type("struct A")
      (gdb) python print struct_a.keys ()
      {['a', '']}
      (gdb) python print [k for k,v in gdb.types.deep_items(struct_a)]
      {['a', 'b0', 'b1']}

`get_type_recognizers ()`
  Return a list of the enabled type recognizers for the current context. This is called by GDB during the type-printing process (see section 23.2.2.8 Type Printing API).

`apply_type_recognizers (recognizers, type_obj)`
  Apply the type recognizers, recognizers, to the type object type_obj. If any recognizer returns a string, return that string. Otherwise, return `None`. This is called by GDB during the type-printing process (see section 23.2.2.8 Type Printing API).

`register_type_printer (locus, printer)`
  This is a convenience function to register a type printer. printer is the type printer to register. It must implement the type printer protocol. locus is either a `gdb.Objfile`, in which case the printer is registered with that objfile; a `gdb.Progspace`, in which case the printer is registered with that progspace; or `None`, in which case the printer is registered globally.

`TypePrinter`
  This is a base class that implements the type printer protocol. Type printers are encouraged, but not required, to derive from this class. It defines a constructor:

  Method: TypePrinter __init__ (self, name)
    Initialize the type printer with the given name. The new printer starts in the enabled state.

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#### 23.2.4.3 gdb.prompt

This module provides a method for prompt value-substitution.

`substitute_prompt (string)`
  Return string with escape sequences substituted by values. Some escape sequences take arguments. You can specify arguments inside "{}" immediately following the escape sequence.

  The escape sequences you can pass to this function are:

  `\\`
    Substitute a backslash.
  `\e`
    Substitute an ESC character.
  `\f`
    Substitute the selected frame; an argument names a frame parameter.
  `\n`
    Substitute a newline.
  `\p`
    Substitute a parameter's value; the argument names the parameter.
  `\r`
    Substitute a carriage return.
  `\t`
    Substitute the selected thread; an argument names a thread parameter.
  `\v`
    Substitute the version of GDB.
  `\w`
    Substitute the current working directory.
  `\[`
    Begin a sequence of non-printing characters. These sequences are typically used with the ESC character, and are not counted in the string length. Example: "\[\e[0;34m\](gdb)\[\e[0m\]" will return a blue-colored "(gdb)" prompt where the length is five.
  `\]`
    End a sequence of non-printing characters.

  For example:

      substitute_prompt (``frame: \f,
                         print arguments: \p{print frame-arguments}'')

  will return the string:

      "frame: main, print arguments: scalars"

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## 23.3 Auto-loading extensions

GDB provides two mechanisms for automatically loading extensions when a new object file is read (for example, due to the `file` command, or because the inferior has loaded a shared library): ``objfile-gdb.ext'` and the `.debug_gdb_scripts` section of modern file formats like ELF.

> 23.3.1 The ``objfile-gdb.ext'` file |  |
> 23.3.2 The `.debug_gdb_scripts` section |  |
> 23.3.3 Which flavor to choose? |  |

The auto-loading feature is useful for supplying application-specific debugging commands and features.

Auto-loading can be enabled or disabled, and the list of auto-loaded scripts can be printed. See the ``auto-loading'` section of each extension language for more information. For GDB command files see 23.1.5 Controlling auto-loading native GDB scripts. For Python files see 23.2.3 Python Auto-loading.

Note that loading of this script file also requires accordingly configured `auto-load safe-path` (see section 22.7.3 Security restriction for auto-loading).

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### 23.3.1 The ``objfile-gdb.ext'` file

When a new object file is read, GDB looks for a file named ``objfile-gdb.ext'` (we call it script-name below), where objfile is the object file's name and where ext is the file extension for the extension language:

```objfile-gdb.gdb'``
  GDB's own command language
```objfile-gdb.py'``
  Python

script-name is formed by ensuring that the file name of objfile is absolute, following all symlinks, and resolving `.` and `..` components, and appending the ``-gdb.ext'` suffix. If this file exists and is readable, GDB will evaluate it as a script in the specified extension language.

If this file does not exist, then GDB will look for script-name file in all of the directories as specified below.

Note that loading of these files requires an accordingly configured `auto-load safe-path` (see section 22.7.3 Security restriction for auto-loading).

For object files using ``.exe'` suffix GDB tries to load first the scripts normally according to its ``.exe'` filename. But if no scripts are found GDB also tries script filenames matching the object file without its ``.exe'` suffix. This ``.exe'` stripping is case insensitive and it is attempted on any platform. This makes the script filenames compatible between Unix and MS-Windows hosts.

`set auto-load scripts-directory [directories]`
  Control GDB auto-loaded scripts location. Multiple directory entries may be delimited by the host platform path separator in use (``:'` on Unix, ``;'` on MS-Windows and MS-DOS).

  Each entry here needs to be covered also by the security setting `set auto-load safe-path` (see set auto-load safe-path).

  This variable defaults to ``$debugdir:$datadir/auto-load'`. The default `set auto-load safe-path` value can be also overriden by GDB configuration option ``--with-auto-load-dir'`.

  Any reference to ``$debugdir'` will get replaced by debug-file-directory value (see section 18.2 Debugging Information in Separate Files) and any reference to ``$datadir'` will get replaced by data-directory which is determined at GDB startup (see section 18.6 GDB Data Files). ``$debugdir'` and ``$datadir'` must be placed as a directory component -- either alone or delimited by ``/'` or ``\'` directory separators, depending on the host platform.

  The list of directories uses path separator (``:'` on GNU and Unix systems, ``;'` on MS-Windows and MS-DOS) to separate directories, similarly to the `PATH` environment variable.

`show auto-load scripts-directory`
  Show GDB auto-loaded scripts location.

GDB does not track which files it has already auto-loaded this way. GDB will load the associated script every time the corresponding objfile is opened. So your ``-gdb.ext'` file should be careful to avoid errors if it is evaluated more than once.

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### 23.3.2 The `.debug_gdb_scripts` section

For systems using file formats like ELF and COFF, when GDB loads a new object file it will look for a special section named `.debug_gdb_scripts`. If this section exists, its contents is a list of NUL-terminated names of scripts to load. Each entry begins with a non-NULL prefix byte that specifies the kind of entry, typically the extension language.

GDB will look for each specified script file first in the current directory and then along the source search path (see section Specifying Source Directories), except that ``$cdir'` is not searched, since the compilation directory is not relevant to scripts.

Entries can be placed in section `.debug_gdb_scripts` with, for example, this GCC macro for Python scripts.

    /* Note: The "MS" section flags are to remove duplicates.  */
    #define DEFINE_GDB_PY_SCRIPT(script_name) \
      asm("\
    .pushsection \".debug_gdb_scripts\", \"MS\",@progbits,1\n\
    .byte 1 /* Python */\n\
    .asciz \"" script_name "\"\n\
    .popsection \n\
    ");

Then one can reference the macro in a header or source file like this:

    DEFINE_GDB_PY_SCRIPT ("my-app-scripts.py")

The script name may include directories if desired.

Note that loading of this script file also requires accordingly configured `auto-load safe-path` (see section 22.7.3 Security restriction for auto-loading).

If the macro invocation is put in a header, any application or library using this header will get a reference to the specified script, and with the use of `"MS"` attributes on the section, the linker will remove duplicates.

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### 23.3.3 Which flavor to choose?

Given the multiple ways of auto-loading extensions, it might not always be clear which one to choose. This section provides some guidance.

Benefits of the ``-gdb.ext'` way:

- Can be used with file formats that don't support multiple sections.

- Ease of finding scripts for public libraries.

  Scripts specified in the `.debug_gdb_scripts` section are searched for in the source search path. For publicly installed libraries, e.g., ``libstdc++'`, there typically isn't a source directory in which to find the script.

- Doesn't require source code additions.

Benefits of the `.debug_gdb_scripts` way:

- Works with static linking.

  Scripts for libraries done the ``-gdb.ext'` way require an objfile to trigger their loading. When an application is statically linked the only objfile available is the executable, and it is cumbersome to attach all the scripts from all the input libraries to the executable's ``-gdb.ext'` script.

- Works with classes that are entirely inlined.

  Some classes can be entirely inlined, and thus there may not be an associated shared library to attach a ``-gdb.ext'` script to.

- Scripts needn't be copied out of the source tree.

  In some circumstances, apps can be built out of large collections of internal libraries, and the build infrastructure necessary to install the ``-gdb.ext'` scripts in a place where GDB can find them is cumbersome. It may be easier to specify the scripts in the `.debug_gdb_scripts` section as relative paths, and add a path to the top of the source tree to the source search path.

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## 23.4 Creating new spellings of existing commands

It is often useful to define alternate spellings of existing commands. For example, if a new GDB command defined in Python has a long name to type, it is handy to have an abbreviated version of it that involves less typing.

GDB itself uses aliases. For example ``s'` is an alias of the ``step'` command even though it is otherwise an ambiguous abbreviation of other commands like ``set'` and ``show'`.

Aliases are also used to provide shortened or more common versions of multi-word commands. For example, GDB provides the ``tty'` alias of the ``set inferior-tty'` command.

You can define a new alias with the ``alias'` command.

`alias [-a] [--] ALIAS = COMMAND`

ALIAS specifies the name of the new alias. Each word of ALIAS must consist of letters, numbers, dashes and underscores.

COMMAND specifies the name of an existing command that is being aliased.

The ``-a'` option specifies that the new alias is an abbreviation of the command. Abbreviations are not shown in command lists displayed by the ``help'` command.

The ``--'` option specifies the end of options, and is useful when ALIAS begins with a dash.

Here is a simple example showing how to make an abbreviation of a command so that there is less to type. Suppose you were tired of typing ``disas'`, the current shortest unambiguous abbreviation of the ``disassemble'` command and you wanted an even shorter version named ``di'`. The following will accomplish this.

    (gdb) alias -a di = disas

Note that aliases are different from user-defined commands. With a user-defined command, you also need to write documentation for it with the ``document'` command. An alias automatically picks up the documentation of the existing command.

Here is an example where we make ``elms'` an abbreviation of ``elements'` in the ``set print elements'` command. This is to show that you can make an abbreviation of any part of a command.

    (gdb) alias -a set print elms = set print elements
    (gdb) alias -a show print elms = show print elements
    (gdb) set p elms 20
    (gdb) show p elms
    Limit on string chars or array elements to print is 200.

Note that if you are defining an alias of a ``set'` command, and you want to have an alias for the corresponding ``show'` command, then you need to define the latter separately.

Unambiguously abbreviated commands are allowed in COMMAND and ALIAS, just as they are normally.

    (gdb) alias -a set pr elms = set p ele

Finally, here is an example showing the creation of a one word alias for a more complex command. This creates alias ``spe'` of the command ``set print elements'`.

    (gdb) alias spe = set print elements
    (gdb) spe 20

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