helper.c

#include <stdio.h>

int foo(void) {
    printf("hello from helper\n");
    return 1;
}

hijack.c

#include <stdio.h>

int foo(void) {
    printf("hello from hijack\n");
    return 0;
}

main.c

#include <stdlib.h>
#define _GNU_SOURCE 1
#include <dlfcn.h>

// int foo(void);

int main(void) {
    void *libhandle = dlopen("libhelper.so", RTLD_LAZY | RTLD_GLOBAL);
    int (*foo)(void) = dlsym(NULL, "foo");
    return foo();
}

Makefile

a.out: main.c
	gcc $< -o $@ -ldl

libhijack.so: hijack.c
	gcc $< -shared -o $@

libhelper.so: helper.c
	gcc $< -shared -o $@

run: a.out libhijack.so libhelper.so
	LD_DEBUG=libs,symbols LD_LIBRARY_PATH=`pwd` $(if $(HIJACK),LD_PRELOAD=libhijack.so) ./$<