How to replace C standard library function ?

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 imp

Answer 1

I'm not sure how hard it will be to get the linker to do what you want, but here's a solution that doesn't involve changing any linker settings and uses preprocessor macros instead so that any code that tries to call strcpy actually calls a function called my_strcpy:

mystuff.h:

#define strcpy my_strcpy
char * my_strcpy(char * dst, const char * src);

my_strcpy.c:

#include <mystuff.h>
char * my_strcpy(char * dst, const char * src);
{
    ...
}

my_code.c:

#include <mystuff.h>

int main()
{
   /* Any call to strcpy will look like a normal call to strcpy
      but will actually call my_strcpy. */
}

Answer 2

You can try playing with LD_PRELOAD if you are on Linux.

Answer 3

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.