extern void __VERIFIER_error() __attribute__ ((__noreturn__));

/* Testcase from Threader's distribution. For details see:
   http://www.model.in.tum.de/~popeea/research/threader

   This file is adapted from the example introduced in the paper:
   Thread-Modular Verification for Shared-Memory Programs 
   by Cormac Flanagan, Stephen Freund, Shaz Qadeer.
*/

#include <pthread.h>
#define assert(e) if (!(e)) ERROR: __VERIFIER_error();

int w=0, r=0, x, y;

void __VERIFIER_atomic_take_write_lock() {
  __VERIFIER_assume(w==0 && r==0);
  w = 1;
} 

void __VERIFIER_atomic_take_read_lock() {
  __VERIFIER_assume(w==0);
  r = r+1;
}

void *writer() { //writer
  __VERIFIER_atomic_take_write_lock();  
  x = 3;
  w = 0;
}

void *reader() { //reader
  int l;
  __VERIFIER_atomic_take_read_lock();
  l = x;
  y = l;
  assert(y == x);
  l = r-1;
  r = l;
}

int main() {
  pthread_t t1, t2, t3, t4;
  pthread_create(&t1, 0, writer, 0);
  pthread_create(&t2, 0, reader, 0);
  pthread_create(&t3, 0, writer, 0);
  pthread_create(&t4, 0, reader, 0);
  pthread_join(t1, 0);
  pthread_join(t2, 0);
  pthread_join(t3, 0);
  pthread_join(t4, 0);
  return 0;
}