How to undef calloc() function from stdlib to use our own function

Question

I have a header file calloc.h, where I defined calloc() function using stdlib's malloc() function. But I am getting multiple definition of 'calloc' error.

calloc.h:

#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

void *calloc(size_t num, size_t size)
{
    /* num * size unsigned integer wrapping check */
    if ((num >= MUL_NO_OVERFLOW || size >= MUL_NO_OVERFLOW) && num > 0 && SIZE_MAX / num < size)
    {
        return NULL;
    }

    size_t total_size = num * size;
    void *ptr = malloc(total_size);

    if (ptr)
    {
        memset(ptr, 0, total_size);
    }

    return ptr;
}

Is there any way to resolve this issue?

I tried this:

#define calloc _calloc_

#include <stdbool.h>
#include <stdlib.h>
#include <string.h>

#undef calloc

But I am getting the following error:

cstdlib:144:11: error: 'calloc' has not been declared in '::'

stdlib.h:59:12: error: 'calloc' has not been declared in 'std'

Answer 1

The link time error you get seems to indicate the module in the C library that defines calloc() is linked into your program, not because you redefoine calloc, but because you use at least another symbol defined in the same module. It is possible that both calloc and malloc are defined in the same module in your C library, which means you cannot redefine calloc() with a call to malloc().

Also note that you seem to be compiling your C source file as C++.

On your system, you may have to redefine both malloc(), calloc(), realloc(), free() and possibly other library functions such as strdup(), aligned_alloc(), so none of the original allocation modules get pulled into your executable. This is a non trivial task.

If you want to divert the calls to calloc() in a program, you can add a command line definition such as -Dcalloc=my_calloc so all source files end up calling your function instead of the standard library function.