libpreload

Simple header that helps you to do LD_PRELOAD tricks. It provides two macros: HIDE_START and HIDE_END. Use HIDE_START to hide library function and put HIDE_END at the end of your own implementation. Just below macro HIDE_START use <FUNC>_orig function to call original library implementation. Inside your implementation use first_run variable to detect first call.

USE_START(return_value, function_name, arguments)

Example (trace open system call and call original function):

#include "libpreload.h"
#include <stdio.h>

HIDE_START(int, open, (const char *pathname, int flags))
{
	if(first_run)
		puts("Hello from open!");
	else
		puts("open is called");
	return open_orig(pathname, flags);
}
HIDE_END

To build that just run:

gcc -shared -fPIC -O2 example.c -ldl -o libexample.so

And use:

LD_PRELOAD=./libexample.so cat example.c

scriptum/libpreload

libpreload