Question

I’m stuck with a weird problem.
I have two files a.c and b.c as follows:
b.c:

#include <stdlib.h>

int *foo() {
  int *x;
  x = (int *) malloc(sizeof(int));
  *x = 4;
  return x;
}

I compile b.c to b.so using gcc:
$ gcc -o b.so -shared -fpic

a.c:

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

int main() {
  void *hdl;
  hdl = dlopen("./b.so", RTLD_LAZY);
  int *((*fn)(void));
  int *x;
  x = (*fn)();
  fn = dlsym(hdl, "foo");
  printf("%d", *x);
}

I compile a.c using gcc:

$ gcc -fpic -ldl a.c

Now when I run it:

$ ./a.out
Segmentation fault

Where I’m I going wrong?
This works when the function in b.c doesn’t return a pointer.

And moreover, I tried checking for errors using dlerror(), but it reports none.

Answer 1

By inspection, you are using fn before you have initialized it. It doesn’t yet point to foo, it doesn’t yet point to anything in particular, and I suspect the resultant behavior is undefined.