thing.c

#include <stdio.h>

void lib1f(void (*f)()) {
  printf("lib1 now calling function:\n");
  f();
  printf("lib1 done!\n");
}

void lib2f(void (*f)()) {
  printf("lib2 now calling function:\n");
  f();
  printf("lib2 done!\n");
}

void udf() {
  printf("Hello World!\n");
}

void *g = NULL;

void tf () {
  lib2f(g);
}

void mergelibf(void (*f)()) {
  g = f;
  lib1f(tf);
}

int main() {
  mergelibf(udf);
  return 0;
}