More Examples

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# 6. More Examples

The following are more examples of all three approaches (static, shared, and dynamically loaded libraries). File libhello.c is a trivial library, with libhello.h as its header. File demo_use.c is a trivial caller of the library. This is followed by commented scripts (script_static and script_dynamic) showing how to use the library as a static and shared library. This is followed by demo_dynamic.c and script_dynamic, which show how to use the shared library as a dynamically loaded library.

## 6.1. File libhello.c

    /* libhello.c - demonstrate library use. */

    #include <stdio.h>

    void hello(void) {
      printf("Hello, library world.\n");
    }

## 6.2. File libhello.h

    /* libhello.h - demonstrate library use. */

    void hello(void);

## 6.3. File demo_use.c

    /* demo_use.c -- demonstrate direct use of the "hello" routine */

    #include "libhello.h"

    int main(void) {
     hello();
     return 0;
    }

## 6.4. File script_static

    #!/bin/sh
    # Static library demo

    # Create static library's object file, libhello-static.o.
    # I'm using the name libhello-static to clearly
    # differentiate the static library from the
    # dynamic library examples, but you don't need to use
    # "-static" in the names of your
    # object files or static libraries.

    gcc -Wall -g -c -o libhello-static.o libhello.c

    # Create static library.

    ar rcs libhello-static.a libhello-static.o

    # At this point we could just copy libhello-static.a
    # somewhere else to use it.
    # For demo purposes, we'll just keep the library
    # in the current directory.

    # Compile demo_use program file.

    gcc -Wall -g -c demo_use.c -o demo_use.o

    # Create demo_use program; -L. causes "." to be searched during
    # creation of the program.  Note that this command causes
    # the relevant object file in libhello-static.a to be
    # incorporated into file demo_use_static.

    gcc -g -o demo_use_static demo_use.o -L. -lhello-static

    # Execute the program.

    ./demo_use_static

## 6.5. File script_shared

    #!/bin/sh
    # Shared library demo

    # Create shared library's object file, libhello.o.

    gcc -fPIC -Wall -g -c libhello.c

    # Create shared library.
    # Use -lc to link it against C library, since libhello
    # depends on the C library.

    gcc -g -shared -Wl,-soname,libhello.so.0 \
        -o libhello.so.0.0 libhello.o -lc

    # At this point we could just copy libhello.so.0.0 into
    # some directory, say /usr/local/lib.

    # Now we need to call ldconfig to fix up the symbolic links.

    # Set up the soname.  We could just execute:
    # ln -sf libhello.so.0.0 libhello.so.0
    # but let's let ldconfig figure it out.

    /sbin/ldconfig -n .

    # Set up the linker name.
    # In a more sophisticated setting, we'd need to make
    # sure that if there was an existing linker name,
    # and if so, check if it should stay or not.

    ln -sf libhello.so.0 libhello.so

    # Compile demo_use program file.

    gcc -Wall -g -c demo_use.c -o demo_use.o

    # Create program demo_use.
    # The -L. causes "." to be searched during creation
    # of the program; note that this does NOT mean that "."
    # will be searched when the program is executed.

    gcc -g -o demo_use demo_use.o -L. -lhello

    # Execute the program.  Note that we need to tell the program
    # where the shared library is, using LD_LIBRARY_PATH.

    LD_LIBRARY_PATH="." ./demo_use

## 6.6. File demo_dynamic.c

    /* demo_dynamic.c -- demonstrate dynamic loading and
       use of the "hello" routine */

    /* Need dlfcn.h for the routines to
       dynamically load libraries */
    #include <dlfcn.h>

    #include <stdlib.h>
    #include <stdio.h>

    /* Note that we don't have to include "libhello.h".
       However, we do need to specify something related;
       we need to specify a type that will hold the value
       we're going to get from dlsym(). */

    /* The type "simple_demo_function" describes a function that
       takes no arguments, and returns no value: */

    typedef void (*simple_demo_function)(void);

    int main(void) {
     const char *error;
     void *module;
     simple_demo_function demo_function;

     /* Load dynamically loaded library */
     module = dlopen("libhello.so", RTLD_LAZY);
     if (!module) {
       fprintf(stderr, "Couldn't open libhello.so: %s\n",
               dlerror());
       exit(1);
     }

     /* Get symbol */
     dlerror();
     demo_function = dlsym(module, "hello");
     if ((error = dlerror())) {
       fprintf(stderr, "Couldn't find hello: %s\n", error);
       exit(1);
     }

     /* Now call the function in the DL library */
     (*demo_function)();

     /* All done, close things cleanly */
     dlclose(module);
     return 0;
    }

## 6.7. File script_dynamic

    #!/bin/sh
    # Dynamically loaded library demo

    # Presume that libhello.so and friends have
    # been created (see dynamic example).

    # Compile demo_dynamic program file into an object file.

    gcc -Wall -g -c demo_dynamic.c

    # Create program demo_use.
    # Note that we don't have to tell it where to search for DL libraries,
    # since the only special library this program uses won't be
    # loaded until after the program starts up.
    # However, we DO need the option -ldl to include the library
    # that loads the DL libraries.

    gcc -g -o demo_dynamic demo_dynamic.o -ldl

    # Execute the program.  Note that we need to tell the
    # program where get the dynamically loaded library,
    # using LD_LIBRARY_PATH.

    LD_LIBRARY_PATH="." ./demo_dynamic

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