Question

How can we replace a C standard library function with our own implementation of that function ?

For example, how can I replace strcpy() with my own implementation of strcpy() and have all calls link to the new implementations instead?

Answer 1

At least with GCC and glibc, the symbols for the standard C functions are weak and thus you can override them. For example,

strcpy.c:

#include <string.h>
#include <stdio.h>

char * strcpy(char *dst, const char *src)
{
  char *d = dst;
  while (*src) {
    *d = *src;
    d++;
    src++;
  }
  printf("Called my strcpy()\n");

  return (dst);
}

int main(void)
{
  char foo[10];
  strcpy(foo, "hello");

  puts(foo);

  return 0;
}

And build it like this:

gcc -fno-builtin -o strcpy strcpy.c

and then:

$ ./strcpy 
Called my strcpy()
hello

Note the importance of -fno-builtin here. If you don’t use this, GCC will replace the strcpy() call to a builtin function, of which GCC has a number.

I’m not sure if this works with other compilers/platforms.