Hide symbol of dynamic library

os_api.c 

#include <stdio.h>

void func_os_api(void)
{
        printf("I am OS API\n");
}

 os_api.h

#ifndef __OS_API_H__
#define __OS_API_H__
extern void func_os_api(void);
#endif

 call_os_api.c

#include <stdio.h>
#include "os_api.h"

void call_os_api(void)
{
        printf("call_os_api start\n");
        func_os_api();
        printf("call_os_api end\n");
}

call_os_api.h

#ifndef __CALL_OS_API_H__
#define __CALL_OS_API_H__
extern void call_os_api(void);
#endif

native_os_api.c

#include <stdio.h>

void __attribute__((visibility("hidden")))func_os_api(void)
{
        printf("I am native os API\n");
}

app.c

#include <stdio.h>
#include "os_api.h"
#include "call_os_api.h"

int main(int argc,char** argv)
{
        func_os_api();
        call_os_api();
        return 0;
}

Makefile

CC = gcc

all: libosapi.so libcallosapi.so app

libosapi.so: os_api.c
        $(CC) -o [email protected] $^ -I. -fPIC -shared

libcallosapi.so: call_os_api.c libosapi.so
        $(CC) -o [email protected] $< -I. -fPIC -shared -L. -l:libosapi.so

app: app.c native_os_api.c
        $(CC) -o [email protected] $^ -I. -L. -l:libcallosapi.so -l:libosapi.so

%.o: %.c
        $(CC) -I. -c $^

clean:
        rm *.so
        rm app

>make

>export LD_LIBRARY_PATH=/home/chunxie/test_symb_hidden/

>./app

 We can see two func_os_api() calls to different implementations. 

We can use this feature when we need implement an OS API but avoid override OS API when some one really want it.