Is there a way to overwrite the malloc/free function in C?

Question

Is there a way to hook the malloc/free function call from a C application it self?

Answer 1

malloc() and free() are defined in the standard library; when linking code, the linker will search the library only for symbols that are not already resolved by eailier encountered object code, and object files generated from compilation are always linked before any libraries.

So you can override any library function simply by defining it in your own code, ensuring that it has the correct signature (same name, same number and types of parameters and same return type).

Answer 2

Yes you can. Here's an example program. It compiles and builds with gcc 4.8.2 but does not do anything useful since the implementations are not functional.

#include <stdlib.h>

int main()
{
   int* ip = malloc(sizeof(int));
   double* dp = malloc(sizeof(double));

   free(ip);
   free(dp);
}

void* malloc(size_t s)
{
   return NULL;
}

void free(void* p)
{
}

Answer 3

Not sure if this counts as "overwriting', but you can effectively change the behavior of code that calls malloc and free by using a macro:

#define malloc(x) my_malloc(x)
#define free(x) my_free(x)

void * my_malloc(size_t nbytes)
{
    /* Do your magic here! */
}

void my_free(void *p)
{
    /* Do your magic here! */
}

int main(void)
{
   int *p = malloc(sizeof(int) * 4); /* calls my_malloc */
   free(p);                          /* calls my_free   */
}