How to debug a program executed by an operating system running inside an emulator (qemu) with gdb?

Question

I'm working on a project that consists of adding a system call to xv6. The code that I wrote is not working as I expected and I want to use gdb to know what is happening.

This is the test program.

#include "types.h"
#include "stat.h"
#include "user.h"

int
main(int argc, char *argv[]) {
  int x1 = getreadcount();
  int x2 = getreadcount();
  char buf[100];
  (void) read(4, buf, 1);
  int x3 = getreadcount();
  int i;
  for (i = 0; i < 1000; i++) {
    (void) read(4, buf, 1);
  }
  int x4 = getreadcount();
  printf(1, "XV6_TEST_OUTPUT %d %d %d\n", x2-x1, x3-x2, x4-x3);
  exit();
}

The problem is that this code is going to be executed in an operating system running on qemu and I don't know how to use gdb in this case. In other words: the function fork() from xv6 is going to generate a new process (the test program) and that is the process that I need to have access to from gdb.

How can I do it?