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In this example, each thread has its own distinct condition predicate because each thread requires current_step to have a different value before proceeding. Upon the signal operation (pthreadcnd_cond_signal()), any of the waiting threads can wake up. If, by chance, it the notified thread is not the thread with the next step value, that thread will wait again (pthreadcnd_cond_wait()), resulting in a deadlock situation because no more signal operations will notifications can occur.

Consider the following example:

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This noncompliant code example violates the liveness property.

Compliant Solution (

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cnd_broadcast())

This compliant solution uses the cnd_broadcast() method function to signal all waiting threads instead of a single random onethread. Only the run_step() thread code from the noncompliant code example is modified, as follows:

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The fact that all threads will be awake solves the problem because each one ends up executing its condition predicate test; , and exactly one will find its test to be true succeed and will continue the execution until the end.

Compliant Solution (Windows, Condition Variables)

This compliant solution uses  a CONDITION_VARIABLE object, available on Microsoft Windows (Vista and later).

#include <Windows.h>
#include <stdio.h>
 
CRITICAL_SECTION lock;
CONDITION_VARIABLE cond;
 
DWORD WINAPI run_step(LPVOID t) {
  static int current_step = 0;
  int my_step = (int)t;

  EnterCriticalSection(&lock);  
  printf("Thread %d has the lock\n", my_step);

  while (current_step != my_step) {
    printf("Thread %d is sleeping...\n", my_step);
 
    if (!SleepConditionVariableCS(&cond, &lock, INFINITE)) {
      /* Handle error condition. */
    }

    printf("Thread %d woke up\n", my_step);
  }

  /* Do processing... */
  printf("Thread %d is processing...\n", my_step);

  current_step++;
 
  LeaveCriticalSection(&lock);
 
  /* Signal ALL waiting tasks. */
  WakeAllConditionVariable(&cond);
 
  printf("Thread %d is exiting...\n", my_step);
  return 0;
}
 
enum { NTHREADS = 5 };
int main(int argc, char** argv) {
  HANDLE threads[NTHREADS];
  
  InitializeCriticalSection(&lock);
  InitializeConditionVariable(&cond);
 
  /* Create threads. */
  for (int i = 0; i < NTHREADS; ++i) {
    threads[i] = CreateThread(NULL, 0, run_step, (LPVOID)i, 0, NULL);
  }
 
  /* Wait for all threads to complete. */
  WaitForMultipleObjects(NTHREADS, threads, TRUE, INFINITE);
 
  DeleteCriticalSection(&lock);
 
  return 0;
}

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