Hack the standard function in library and call the native library function afterwards

Question

I am trying to hack the malloc function to call my malloc function first.Once my malloc function is executed within that, I want to invoke the standard malloc. But, I am getting

Answer 1

To override shared functions you need to compile your own shared library and preload it via the LD_PRELOAD environment variable.

#define _GNU_SOURCE

#include 
#include 
#include 

void *malloc(size_t size) {
    printf("called..my malloc\r\n");

    void *(*original_malloc)(size_t size);
    // Find original malloc function
    original_malloc = dlsym(RTLD_NEXT, "malloc");

    if ( dlerror() != NULL)
    {
        puts("malloc symbol not found..");
        exit(1);
    }

    printf("This should call actual malloc now..\r\n");
    return (*original_malloc)(size);
}

$ gcc -Wall -fPIC -shared -o mymalloc.so mymalloc.c -ldl
$ LD_PRELOAD=./mymalloc.so ./prog

Now your program will use malloc from preloaded library.