The difference between getting function pointers from a shared library

The question is, how can I get the address of a function from a shared library (UNIX / LINUX)?

I wrote several test files in C (see below), compiled and run on Ubuntu 10.04 (amd64) and FreeBSD-8.2 (amd64). I did not feel any difference, but I want to know more about possible troubles.

Here they are:

• Test 1

lib.c

char* f0(void) {
    return "Hello, World!";
}

main.c

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

void *hlib, *addr;
char* (*foo)(void);
char* s;

int main(int argc, char** argv) {
    if ( !(hlib = dlopen("./lib.so", RTLD_LAZY)) )
      return 1;
    if ( !(addr = foo = dlsym(hlib, "f0")) )
      return 2;
    s = foo();
    printf("%p => %s\n", addr, s);
    return 0;
}

Now build it:

gcc -o lib.o -c lib.c -Wall -Werror -O3 -fPIC
gcc -o lib.so -shared -nostartfiles lib.o
gcc -o main.o -c main.c -Wall -Werror -O3
gcc -o prog main.o -ldl

Sends the address of the library function f0 () and the result of execution.

• Test 2

lib.h (define the standard interface of dynamically linking libraries here)

#ifndef __LIB_H__
#define __LIB_H__

typedef struct __syminfo {
  char* name; // function name
  void* addr; // function address
} syminfo_t;

typedef struct __libinfo {
  int       num;    // number of exported functions
  syminfo_t sym[1]; // vector of exported function information
} libinfo_t;

extern int (*__getinfo)(libinfo_t**);

#endif
/* __LIB_H__
*/

lib.c (the library itself)

#include <stdlib.h>
#include <lib.h>

static libinfo_t* li;

char* foo(void);

__attribute__((constructor)) void __init() {
  if ( (li = calloc(1, sizeof(libinfo_t))) ) {
    li->num = 1;
    li->sym[0].name = "foo";
    li->sym[0].addr = &foo;
  }
}

__attribute__((destructor)) void __free() {
  if (li)
    free(li);
}

int getinfo(libinfo_t** inf) {
  if (!inf)
    return -1;
  *inf = li;
  return 0;
}

char* foo(void) {
  return "Hello, World!";
}

main.c

#include <stdio.h>
#include <dlfcn.h>
#include <lib.h>

libinfo_t* inf;

void* hlib;

int (*__getinfo)(libinfo_t**);

char* (*foo)(void);

char* s;

int main(int argc, char** argv) {
  if ( !(hlib = dlopen("./lib.so", RTLD_LAZY)) ) 
    return 1;
  if ( !(__getinfo = dlsym(hlib, "getinfo")) )
    return 2;
  if (__getinfo(&inf))
    return 3;
  if ( !(foo = inf->sym[0].addr) )
    return 4;
  s = foo();
  printf("%p => %s\n", inf->sym[0].addr, s);      
  return 0;
}    

Compile it (without -nostartfiles):

gcc -I. -o lib.o -c lib.c -Wall -Werror -O3 -fPIC
gcc -o lib.so lib.o -shared
gcc -I. -o main.o -c main.c -Wall -Werror -O3
gcc -o prog main.o -ldl

This printf is similar to test 1: the address of the library function foo () and the result of its execution.

, , ? ?

. FreeBSD-8.2 -ldl, dlfcn.h libc.

, - .