GCC – Using –wrap flag on assert() function

May 12, 2023

I have a module, lets call it fut:

fut.h

int fut(int n);

fut.c

#include <assert.h>

int fut(int n)
{
    if (n > 0)
    {
        assert(0);
    }

    return n;
}

I want to intercept the call of assert with --wrap linker flag, but the linking fails with the message undefined reference to 'assert'.
Here is the main function:

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

static int useRealAssert = 1;
static int assertCalled;

void __real_assert(int expression);

void __wrap_assert(int expression)
{
    if (1 == useRealAssert)
    {
        __real_assert(expression);
    }
    else
    {
        assertCalled = 1;
    }
}

int main()
{
    useRealAssert = 0;

    printf("Calling fut...\n");
    fut(1);
    printf("assertCalled = %d\n", assertCalled);
}

and here is the build command:
gcc.exe -Wl,--wrap=assert -o my_program main.c fut.c

Any ideas why this doesn’t work for assert function? I am doing the same for malloc and some other functions, and it works as I expect.

>Solution :

why this doesn’t work for assert function?

Because it’s a macro, not a function.

On glibc https://sourceware.org/git/?p=glibc.git;a=blob;f=assert/assert.h;hb=HEAD __assert_fail function is called when assertion fails. On newlib https://github.com/eblot/newlib/blob/master/newlib/libc/include/assert.h it’s __assert_func. Depending on the standard C library implementation you are working it can be a different function, but it’s easy to check it – just in your IDE jump to definition of assert and inspect the macro definition.