Linker error with intercepting function calls in GCC

Question

I am using the standard __wrap_function and __real_function to intercept function calls with -Wl,--wrap=function. This works successfully for most functions like malloc, fopen, etc. However, I am unable to wrap these two functions:

1. int connect(int, const struct sockaddr*, socklen_t)
2. int stat(const char*, struct stat*)

With these functions, the linker complains with undefined reference to __real_connect and __real_stat.

Is there any particular reason for this? (Note: I can also wrap socket functions for example)

Answer 1

It is likely you forgot to add -Wl,--wrap=connect and -Wl,--wrap=stat to your link line.

This works for me:

#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/stat.h>
#include <unistd.h>

int __wrap_connect (int s, const struct sockaddr *addr, socklen_t len)
{
    puts(__func__);
    return __real_connect(s, addr, len);
}

int __wrap_stat (const char *path, struct stat *buf)
{
    puts(__func__);
    return __real_stat(path, buf);
}

int main(void) {
    connect(0, NULL, 0);
    stat("/", 0);
    return 0;
}

When compiled on my system.

$ uname -s -r
Linux 2.6.32-696.16.1.el6.x86_64
$ gcc --version | grep gcc
gcc (GCC) 4.4.7 20120313 (Red Hat 4.4.7-18)
$ gcc c.c -Wl,--wrap=connect -Wl,--wrap=stat
$

However, when leaving off -Wl,--wrap=stat, for example, I get:

$ gcc c.c -Wl,--wrap=connect
/tmp/cchVzvsE.o: In function `__wrap_stat':
c.c:(.text+0x65): undefined reference to `__real_stat'
collect2: ld returned 1 exit status
$