A sample to hook an applocation.

hook.c

#include <stdio.h>
#include <unistd.h>

void hook_start() __attribute__((constructor));
void hook_end() __attribute__((destructor));

void hook_start()
{
    printf("hook_start. PID=%d, PPID=%d\n", getpid(), getppid());
}

void hook_end()
{
    printf("hook_end. PID=%d, PPID=%d\n", getpid(), getppid());
}

link_and_run

#!/bin/sh

gcc main.c
gcc -shared -fPIC -o hook.so hook.c

./a.out
# It gets "Hello world".

LD_PRELOAD=hook.so ./a.out
# It gets "Hello world" between hook functions.

main.c

#include <stdio.h>

int main()
{
    printf("Hello world.\n");
    return 0;
}