Using the GNU Compiler Collection (GCC): C Implementation

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# 4. C Implementation-defined behavior

A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated "implementation defined". The following lists all such areas, along with the section numbers from the ISO/IEC 9899:1990 and ISO/IEC 9899:1999 standards. Some areas are only implementation-defined in one version of the standard.

Some choices depend on the externally determined ABI for the platform (including standard character encodings) which GCC follows; these are listed as "determined by ABI" below. See section Binary Compatibility, and http://gcc.gnu.org/readings.html. Some choices are documented in the preprocessor manual. See section `Implementation-defined behavior' in The C Preprocessor. Some choices are made by the library and operating system (or other environment when compiling for a freestanding environment); refer to their documentation for details.

> 4.1 Translation |  |
> 4.2 Environment |  |
> 4.3 Identifiers |  |
> 4.4 Characters |  |
> 4.5 Integers |  |
> 4.6 Floating point |  |
> 4.7 Arrays and pointers |  |
> 4.8 Hints |  |
> 4.9 Structures, unions, enumerations, and bit-fields |  |
> 4.10 Qualifiers |  |
> 4.11 Declarators |  |
> 4.12 Statements |  |
> 4.13 Preprocessing directives |  |
> 4.14 Library functions |  |
> 4.15 Architecture |  |
> 4.16 Locale-specific behavior |  |

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## 4.1 Translation

- How a diagnostic is identified (C90 3.7, C99 3.10, C90 and C99 5.1.1.3).

  Diagnostics consist of all the output sent to stderr by GCC.

- Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character in translation phase 3 (C90 and C99 5.1.1.2).

  See section `Implementation-defined behavior' in The C Preprocessor.

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## 4.2 Environment

The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

- The mapping between physical source file multibyte characters and the source character set in translation phase 1 (C90 and C99 5.1.1.2).

  See section `Implementation-defined behavior' in The C Preprocessor.

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## 4.3 Identifiers

- Which additional multibyte characters may appear in identifiers and their correspondence to universal character names (C99 6.4.2).

  See section `Implementation-defined behavior' in The C Preprocessor.

- The number of significant initial characters in an identifier (C90 6.1.2, C90 and C99 5.2.4.1, C99 6.4.2).

  For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.

- Whether case distinctions are significant in an identifier with external linkage (C90 6.1.2).

  This is a property of the linker. C99 requires that case distinctions are always significant in identifiers with external linkage and systems without this property are not supported by GCC.

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## 4.4 Characters

- The number of bits in a byte (C90 3.4, C99 3.6).

  Determined by ABI.

- The values of the members of the execution character set (C90 and C99 5.2.1).

  Determined by ABI.

- The unique value of the member of the execution character set produced for each of the standard alphabetic escape sequences (C90 and C99 5.2.2).

  Determined by ABI.

- The value of a `char` object into which has been stored any character other than a member of the basic execution character set (C90 6.1.2.5, C99 6.2.5).

  Determined by ABI.

- Which of `signed char` or `unsigned char` has the same range, representation, and behavior as "plain" `char` (C90 6.1.2.5, C90 6.2.1.1, C99 6.2.5, C99 6.3.1.1).

  Determined by ABI. The options ``-funsigned-char'` and ``-fsigned-char'` change the default. See section Options Controlling C Dialect.

- The mapping of members of the source character set (in character constants and string literals) to members of the execution character set (C90 6.1.3.4, C99 6.4.4.4, C90 and C99 5.1.1.2).

  Determined by ABI.

- The value of an integer character constant containing more than one character or containing a character or escape sequence that does not map to a single-byte execution character (C90 6.1.3.4, C99 6.4.4.4).

  See section `Implementation-defined behavior' in The C Preprocessor.

- The value of a wide character constant containing more than one multibyte character, or containing a multibyte character or escape sequence not represented in the extended execution character set (C90 6.1.3.4, C99 6.4.4.4).

  See section `Implementation-defined behavior' in The C Preprocessor.

- The current locale used to convert a wide character constant consisting of a single multibyte character that maps to a member of the extended execution character set into a corresponding wide character code (C90 6.1.3.4, C99 6.4.4.4).

  See section `Implementation-defined behavior' in The C Preprocessor.

- The current locale used to convert a wide string literal into corresponding wide character codes (C90 6.1.4, C99 6.4.5).

  See section `Implementation-defined behavior' in The C Preprocessor.

- The value of a string literal containing a multibyte character or escape sequence not represented in the execution character set (C90 6.1.4, C99 6.4.5).

  See section `Implementation-defined behavior' in The C Preprocessor.

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## 4.5 Integers

- Any extended integer types that exist in the implementation (C99 6.2.5).

  GCC does not support any extended integer types.

- Whether signed integer types are represented using sign and magnitude, two's complement, or one's complement, and whether the extraordinary value is a trap representation or an ordinary value (C99 6.2.6.2).

  GCC supports only two's complement integer types, and all bit patterns are ordinary values.

- The rank of any extended integer type relative to another extended integer type with the same precision (C99 6.3.1.1).

  GCC does not support any extended integer types.

- The result of, or the signal raised by, converting an integer to a signed integer type when the value cannot be represented in an object of that type (C90 6.2.1.2, C99 6.3.1.3).

  For conversion to a type of width N, the value is reduced modulo 2^N to be within range of the type; no signal is raised.

- The results of some bitwise operations on signed integers (C90 6.3, C99 6.5).

  Bitwise operators act on the representation of the value including both the sign and value bits, where the sign bit is considered immediately above the highest-value value bit. Signed ``>>'` acts on negative numbers by sign extension.

  GCC does not use the latitude given in C99 only to treat certain aspects of signed ``<<'` as undefined, but this is subject to change.

- The sign of the remainder on integer division (C90 6.3.5).

  GCC always follows the C99 requirement that the result of division is truncated towards zero.

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## 4.6 Floating point

- The accuracy of the floating-point operations and of the library functions in `<math.h>` and `<complex.h>` that return floating-point results (C90 and C99 5.2.4.2.2).

  The accuracy is unknown.

- The rounding behaviors characterized by non-standard values of `FLT_ROUNDS` (C90 and C99 5.2.4.2.2).

  GCC does not use such values.

- The evaluation methods characterized by non-standard negative values of `FLT_EVAL_METHOD` (C99 5.2.4.2.2).

  GCC does not use such values.

- The direction of rounding when an integer is converted to a floating-point number that cannot exactly represent the original value (C90 6.2.1.3, C99 6.3.1.4).

  C99 Annex F is followed.

- The direction of rounding when a floating-point number is converted to a narrower floating-point number (C90 6.2.1.4, C99 6.3.1.5).

  C99 Annex F is followed.

- How the nearest representable value or the larger or smaller representable value immediately adjacent to the nearest representable value is chosen for certain floating constants (C90 6.1.3.1, C99 6.4.4.2).

  C99 Annex F is followed.

- Whether and how floating expressions are contracted when not disallowed by the `FP_CONTRACT` pragma (C99 6.5).

  Expressions are currently only contracted if ``-funsafe-math-optimizations'` or ``-ffast-math'` are used. This is subject to change.

- The default state for the `FENV_ACCESS` pragma (C99 7.6.1).

  This pragma is not implemented, but the default is to "off" unless ``-frounding-math'` is used in which case it is "on".

- Additional floating-point exceptions, rounding modes, environments, and classifications, and their macro names (C99 7.6, C99 7.12).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

- The default state for the `FP_CONTRACT` pragma (C99 7.12.2).

  This pragma is not implemented. Expressions are currently only contracted if ``-funsafe-math-optimizations'` or ``-ffast-math'` are used. This is subject to change.

- Whether the "inexact" floating-point exception can be raised when the rounded result actually does equal the mathematical result in an IEC 60559 conformant implementation (C99 F.9).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

- Whether the "underflow" (and "inexact") floating-point exception can be raised when a result is tiny but not inexact in an IEC 60559 conformant implementation (C99 F.9).

  This is dependent on the implementation of the C library, and is not defined by GCC itself.

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## 4.7 Arrays and pointers

- The result of converting a pointer to an integer or vice versa (C90 6.3.4, C99 6.3.2.3).

  A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends(2) if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.

  A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.

  When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in C99 6.5.6/8.

- The size of the result of subtracting two pointers to elements of the same array (C90 6.3.6, C99 6.5.6).

  The value is as specified in the standard and the type is determined by the ABI.

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## 4.8 Hints

- The extent to which suggestions made by using the `register` storage-class specifier are effective (C90 6.5.1, C99 6.7.1).

  The `register` specifier affects code generation only in these ways:

  - When used as part of the register variable extension, see 6.44 Variables in Specified Registers.

  - When ``-O0'` is in use, the compiler allocates distinct stack memory for all variables that do not have the `register` storage-class specifier; if `register` is specified, the variable may have a shorter lifespan than the code would indicate and may never be placed in memory.

  - On some rare x86 targets, `setjmp` doesn't save the registers in all circumstances. In those cases, GCC doesn't allocate any variables in registers unless they are marked `register`.

- The extent to which suggestions made by using the inline function specifier are effective (C99 6.7.4).

  GCC will not inline any functions if the ``-fno-inline'` option is used or if ``-O0'` is used. Otherwise, GCC may still be unable to inline a function for many reasons; the ``-Winline'` option may be used to determine if a function has not been inlined and why not.

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## 4.9 Structures, unions, enumerations, and bit-fields

- A member of a union object is accessed using a member of a different type (C90 6.3.2.3).

  The relevant bytes of the representation of the object are treated as an object of the type used for the access. See Type-punning. This may be a trap representation.

- Whether a "plain" `int` bit-field is treated as a `signed int` bit-field or as an `unsigned int` bit-field (C90 6.5.2, C90 6.5.2.1, C99 6.7.2, C99 6.7.2.1).

  By default it is treated as `signed int` but this may be changed by the ``-funsigned-bitfields'` option.

- Allowable bit-field types other than `_Bool`, `signed int`, and `unsigned int` (C99 6.7.2.1).

  No other types are permitted in strictly conforming mode.

- Whether a bit-field can straddle a storage-unit boundary (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The order of allocation of bit-fields within a unit (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The alignment of non-bit-field members of structures (C90 6.5.2.1, C99 6.7.2.1).

  Determined by ABI.

- The integer type compatible with each enumerated type (C90 6.5.2.2, C99 6.7.2.2).

  Normally, the type is `unsigned int` if there are no negative values in the enumeration, otherwise `int`. If ``-fshort-enums'` is specified, then if there are negative values it is the first of `signed char`, `short` and `int` that can represent all the values, otherwise it is the first of `unsigned char`, `unsigned short` and `unsigned int` that can represent all the values.

  On some targets, ``-fshort-enums'` is the default; this is determined by the ABI.

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## 4.10 Qualifiers

- What constitutes an access to an object that has volatile-qualified type (C90 6.5.3, C99 6.7.3).

  Such an object is normally accessed by pointers and used for accessing hardware. In most expressions, it is intuitively obvious what is a read and what is a write. For example

      volatile int *dst = somevalue;
      volatile int *src = someothervalue;
      *dst = *src;

  will cause a read of the volatile object pointed to by src and store the value into the volatile object pointed to by dst. There is no guarantee that these reads and writes are atomic, especially for objects larger than `int`.

  However, if the volatile storage is not being modified, and the value of the volatile storage is not used, then the situation is less obvious. For example

      volatile int *src = somevalue;
      *src;

  According to the C standard, such an expression is an rvalue whose type is the unqualified version of its original type, i.e. `int`. Whether GCC interprets this as a read of the volatile object being pointed to or only as a request to evaluate the expression for its side-effects depends on this type.

  If it is a scalar type, or on most targets an aggregate type whose only member object is of a scalar type, or a union type whose member objects are of scalar types, the expression is interpreted by GCC as a read of the volatile object; in the other cases, the expression is only evaluated for its side-effects.

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## 4.11 Declarators

- The maximum number of declarators that may modify an arithmetic, structure or union type (C90 6.5.4).

  GCC is only limited by available memory.

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## 4.12 Statements

- The maximum number of `case` values in a `switch` statement (C90 6.6.4.2).

  GCC is only limited by available memory.

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## 4.13 Preprocessing directives

See section `Implementation-defined behavior' in The C Preprocessor, for details of these aspects of implementation-defined behavior.

- How sequences in both forms of header names are mapped to headers or external source file names (C90 6.1.7, C99 6.4.7).

- Whether the value of a character constant in a constant expression that controls conditional inclusion matches the value of the same character constant in the execution character set (C90 6.8.1, C99 6.10.1).

- Whether the value of a single-character character constant in a constant expression that controls conditional inclusion may have a negative value (C90 6.8.1, C99 6.10.1).

- The places that are searched for an included ``<>'` delimited header, and how the places are specified or the header is identified (C90 6.8.2, C99 6.10.2).

- How the named source file is searched for in an included ``""'` delimited header (C90 6.8.2, C99 6.10.2).

- The method by which preprocessing tokens (possibly resulting from macro expansion) in a `#include` directive are combined into a header name (C90 6.8.2, C99 6.10.2).

- The nesting limit for `#include` processing (C90 6.8.2, C99 6.10.2).

- Whether the ``#'` operator inserts a ``\'` character before the ``\'` character that begins a universal character name in a character constant or string literal (C99 6.10.3.2).

- The behavior on each recognized non-`STDC #pragma` directive (C90 6.8.6, C99 6.10.6).

  See section `Pragmas' in The C Preprocessor, for details of pragmas accepted by GCC on all targets. See section Pragmas Accepted by GCC, for details of target-specific pragmas.

- The definitions for `__DATE__` and `__TIME__` when respectively, the date and time of translation are not available (C90 6.8.8, C99 6.10.8).

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## 4.14 Library functions

The behavior of most of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

- The null pointer constant to which the macro `NULL` expands (C90 7.1.6, C99 7.17).

  In `<stddef.h>`, `NULL` expands to `((void *)0)`. GCC does not provide the other headers which define `NULL` and some library implementations may use other definitions in those headers.

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## 4.15 Architecture

- The values or expressions assigned to the macros specified in the headers `<float.h>`, `<limits.h>`, and `<stdint.h>` (C90 and C99 5.2.4.2, C99 7.18.2, C99 7.18.3).

  Determined by ABI.

- The number, order, and encoding of bytes in any object (when not explicitly specified in this International Standard) (C99 6.2.6.1).

  Determined by ABI.

- The value of the result of the `sizeof` operator (C90 6.3.3.4, C99 6.5.3.4).

  Determined by ABI.

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## 4.16 Locale-specific behavior

The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.

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