Void pointers

test.c

#include <stdio.h>

struct data {
  int fd;
};

void test(void *d) {
  struct data *x = (struct data *)d; // Cast the other way
  printf("fd = %d\n", x->fd); // Dereference
}

int main(int argc, char **argv) {
  struct data x; // Stack-local 'data' struct
  void *p = (void *)&x; // Cast and assign
  x.fd = 1; // Assign through struct
  test(p); // Pass to function
  return 0;
}