add.c

int add(int x, int y) { return x + y; }

int main() { return 0; }

call.c

#include <dlfcn.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

// works, when i compile with gcc -shared
// #define PATH "/home/v/wcc/add.so"
// does not work, dlopen() returns NULL
#define PATH "/home/v/wcc/add"

int main(int argc, char const* argv[]) {
  void    *handle;
  int     (*fptr)(int, int);

  handle = dlopen(PATH, RTLD_LOCAL | RTLD_NOW);
  if (handle == NULL) {
    printf("Failed to open " PATH ": %s\n", strerror(errno));
    return 1;
  }

  fptr = (int (*)(int, int))dlsym(handle, "add");
  if (fptr == NULL) {
    printf("Failed to find the symbol 'add': %s\n", strerror(errno));
    dlclose(handle);
    return 1;
  }

  int result = (*fptr)(10, 20);
  printf("%d\n", result);

  dlclose(handle);
  return 0;
}

Makefile

all: build run

build:
  # Pretend I have no control over how add.c is compiled
	gcc -Wall add.c -o add
  # I have control over how I compile ./call
	gcc -Wall -ldl call.c -o call

run:
	./call