그냥... 생각나서 적어봅니다만... C 에서...함수호출에관해..

#include <stdio.h>
#include <stdarg.h>
 
struct libio {
  int (*print)(const char *str, ...);
};
 
static int
my_print(const char *fmt, ...)
{
  va_list args;
  int ret;
  va_start(args, fmt);
  ret = printf(fmt, args);
  va_end(args);
  return ret;
}
 
struct libio lib1 = {
  .print = my_print,
};
 
struct libio lib2 = {
  .print = my_print,
};
 
int main(void)
{
  lib1.print("lib1\n");
  lib2.print("lib2\n");
  return 0;
}

#include <stdio.h>
 
void hello(void)
{
	printf("hello foo~!\n");
}

#include <stdio.h>
 
void hello(void)
{
	printf("hello bar~!\n");
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
 
struct libs
{
	const char *path;
	void *handle;
	char *error;
 
	void (*hello)(void);
};
 
extern void hello(void);
 
static int dl_init(struct libs *lib)
{
	void *handle;
 
	handle = dlopen(lib->path, RTLD_LAZY);
	if (handle == NULL)
	{
		lib->error = dlerror();
		return -1;
	}
 
	dlerror();    /* Clear any existing error */
 
	*(void **) (&lib->hello) = dlsym(handle, "hello");
	if ((lib->error = dlerror()) != NULL)
		return -1;
 
	lib->handle = handle;
 
	return 0;
}
 
static void dl_fini(struct libs *lib)
{
	if (lib->handle)
	{
		dlclose(lib->handle);
		lib->handle = NULL;
	}
}
 
int main(void)
{
	struct libs foo, bar;
 
	memset(&foo, 0x00, sizeof(struct libs));
	memset(&bar, 0x00, sizeof(struct libs));
 
	foo.path = "./libfoo.so";
	if (dl_init(&foo) < 0)
	{
		fprintf(stderr, "dl_init for libfoo is failed : %s\n", foo.error);
		return (EXIT_FAILURE);
	}
 
	bar.path = "./libbar.so";
	if (dl_init(&bar) < 0)
	{
		fprintf(stderr, "dl_init for libbar is failed : %s\n", bar.error);
		return (EXIT_FAILURE);
	}
 
	printf("by calling foo : "); fflush(stdout);
	foo.hello();
	printf("by calling bar : "); fflush(stdout);
	bar.hello();
 
	dl_fini(&foo);
	dl_fini(&bar);
 
	printf("by link order : "); fflush(stdout);
	hello();
 
	return (EXIT_SUCCESS);
}

$ gcc -g -W -Wall -fPIC -shared -o libfoo.so libfoo.c 
$ gcc -g -W -Wall -fPIC -shared -o libbar.so libbar.c 
 
$ gcc -g -W -Wall -L . -o foobar foobar.c -l foo -l bar -ldl
$ ldd foobar
	linux-gate.so.1 =>  (0x00df3000)
	libfoo.so => not found
	libbar.so => not found
	libdl.so.2 => /lib/libdl.so.2 (0x007b6000)
	libc.so.6 => /lib/libc.so.6 (0x005e0000)
	/lib/ld-linux.so.2 (0x005be000)
$ LD_LIBRARY_PATH=. ./foobar
by calling foo : hello foo~!
by calling bar : hello bar~!
by link order : hello foo~!
 
$ gcc -g -W -Wall -L . -o foobar foobar.c -l bar -l foo -ldl
$ ldd foobar
	linux-gate.so.1 =>  (0x00a9a000)
	libbar.so => not found
	libfoo.so => not found
	libdl.so.2 => /lib/libdl.so.2 (0x007b6000)
	libc.so.6 => /lib/libc.so.6 (0x005e0000)
	/lib/ld-linux.so.2 (0x005be000)
$ LD_LIBRARY_PATH=. ./foobar
by calling foo : hello foo~!
by calling bar : hello bar~!
by link order : hello bar~!
$