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In this noncompliant code example, mutex1 protects count1 and mutex2 protects count2. A race condition exists between the waiter1 and waiter2 threads because they use the same condition variable with different mutexes. If both threads attempt to call pthread_cond_wait() at the same time, one thread will succeed and the other thread will invoke undefined behavior.

Code Block

bgColor	#FFcccc
lang	c

#include <stdio.h>
#include <string.h>
#include <pthread.h>
#include <assert.h>
#include <unistd.h>
#include <errno.h>

pthread_mutex_t mutex1;
pthread_mutex_t mutex2;
pthread_mutexattr_t attr;
pthread_cond_t cv;

void *waiter1();
void *waiter2();
void *signaler();

int count1 = 0, count2 = 0;
#define COUNT_LIMIT 5

int main() {
  int ret;
  pthread_t thread1, thread2, thread3;

  if ((ret = pthread_mutexattr_init( &attr)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_mutexattr_settype( &attr, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_mutex_init( &mutex1, &attr)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_mutex_init( &mutex2, &attr)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_cond_init( &cv, NULL)) != 0) {
    /* handle error */
  }

  if ((ret = pthread_create( &thread1, NULL, &waiter1, NULL))) {
    /* handleHandle error */
  }

  if ((ret = pthread_create( &thread2, NULL, &waiter2, NULL))) {
    /* handle error */
  }

  if ((ret = pthread_create( &thread3, NULL, &signaler, NULL))) {
    /* handleHandle error */
  }

  if ((ret = pthread_join( thread1, NULL)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_join( thread2, NULL)) != 0) {
    /* handleHandle error */
  }

  if ((ret = pthread_join( thread3, NULL)) != 0) {
    /* handleHandle error */
  }

  return 0;
}


void *waiter1() {
  int ret;
  while (count1 < COUNT_LIMIT) {
    if ((ret = pthread_mutex_lock(&mutex1)) != 0) {
      /* handleHandle error */
    }

    if ((ret = pthread_cond_wait(&cv, &mutex1)) != 0) {
      /* handleHandle error */
    }

    printf("count1 = %d\n", ++count1);

    if ((ret = pthread_mutex_unlock(&mutex1)) != 0) {
      /* handleHandle error */
    }
  }

  return NULL;
}

void *waiter2() {
  int ret;
  while (count2 < COUNT_LIMIT) {
    if ((ret = pthread_mutex_lock(&mutex2)) != 0) {
      /* handleHandle error */
    }

    if ((ret = pthread_cond_wait(&cv, &mutex2)) != 0) {
      /* handleHandle error */
    }

    printf("count2 = %d\n", ++count2);

    if ((ret = pthread_mutex_unlock(&mutex2)) != 0) {
      /* handleHandle error */
    }
  }

  return NULL;
}

void *signaler() {
  int ret;
  while ((count1 < COUNT_LIMIT) || (count2 < COUNT_LIMIT)) {
    sleep(1);
    printf("signaling\n");
    if ((ret = pthread_cond_signal(&cv)) != 0) {
      /* handleHandle error */
    }
  }

  return NULL;
}

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pthread_cond_wait() returns EINVAL if it is called when another thread is waiting on the condition variable with a different mutex. This approach is arguably better because it forces the coder to fix the problem instead of allowing reliance on undefined behavior.

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This problem can be solved either by always using the same mutex whenever a particular condition variable is used or by using separate condition variables, depending on how the code is expected to work. Here we use This compliant code uses the same-mutex solution:

Code Block

bgColor	#ccccff
lang	c

pthread_mutex_t mutex1; /* initializedInitialized as PTHREAD_MUTEX_ERRORCHECK */
pthread_cond_t cv;
int count1 = 0, count2 = 0;

void *waiter1() {
  int ret;
  while (count1 < COUNT_LIMIT) {
    if ((ret = pthread_mutex_lock(&mutex1)) != 0) {
      /* handleHandle error */
    }

    if ((ret = pthread_cond_wait(&cv, &mutex1)) != 0) {
      /* handleHandle error */
    }

    printf("count1 = %d\n", ++count1);

    if ((ret = pthread_mutex_unlock(&mutex1)) != 0) {
      /* handleHandle error */
    }
  }

  return NULL;
}

void *waiter2() {
  int ret;
  while (count2 < COUNT_LIMIT) {
    if ((ret = pthread_mutex_lock(&mutex1)) != 0) {
      /* handleHandle error */
    }

    if ((ret = pthread_cond_wait(&cv, &mutex1)) != 0) {
      /* handleHandle error */
    }

    printf("count2 = %d\n", ++count2);

    if ((ret = pthread_mutex_unlock(&mutex1)) != 0) {
      /* handleHandle error */
    }
  }

  return NULL;
}

...

Rule	Severity	Likelihood	Remediation Cost	Priority	Level
POS53-C	mediumMedium	probableProbable	highHigh	P4	L3

Bibliography

[Open Group 2004] pthread_cond_timedwait()/pthread_cond_wait()

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