LD_PRELOAD example

Substituting a function from dynamically linked library, under Linux:
Example:
/***
* LD_PRELOAD
*****************/

The main program:

main_test.c
gcc -o main_test main_test.c
//-----------
#include <stdio.h>

int main(int argc, char* argv[])
{
    if (argc < 2) {
        printf("Usage: %s <arg>\n", argv[0]);
        return -1;
    }
    int res = strcmp("this_is_the_pass", argv[1]);
    return res;
}

//-----------

We need to substitute strcmp, in order to display the password:

my_strcmp.c
gcc my_strcmp.c -shared -o my_strcmp.so -fPIC
//-----------
#include <stdio.h>

int strcmp(char *s1, char *s2)
{
    printf("s1: %s\n", s1);
    printf("s2: %s\n", s2);
    return 0;
}
//-----------

run:
LD_PRELOAD="./my_strcmp.so" ./main_test gimme_pass

result:
//-----------
s1: this_is_the_pass
s2: gimme_pass