Pthread mutual exclusion (mutex) locks are used to avoid simultaneous usage of common resources. Several types of mutex locks are defined by pthreads: NORMAL, ERRORCHECK, RECURSIVE, and DEFAULT.

POSIX describes PTHREAD_MUTEX_NORMAL locks as having the following undefined behavior [Open Group 2004]:

This type of mutex does not provide deadlock detection. A thread attempting to relock this mutex without first unlocking it shall deadlock. An error is not returned to the caller. Attempting to unlock a mutex locked by a different thread results in undefined behavior. Attempting to unlock an unlocked mutex results in undefined behavior.

The DEFAULT mutex pthread is also generally mapped to PTHREAD_MUTEX_NORMAL but is known to vary from platform to platform [SOL 2010]. Consequently, NORMAL locks should not be used, and ERRORCHECK or RECURSIVE locks should be defined explicitly when mutex locks are used.

Noncompliant Code Example

This noncompliant code example shows a simple mutex being created using PTHREAD_MUTEX_NORMAL. Note that the caller does not expect a return code when NORMAL mutex locks are used.

pthread_mutexattr_t attr;
pthread_mutex_t mutex;
size_t const shared_var = 0;

int main(void) {
  int result;

  if ((result = pthread_mutexattr_settype(&mutex, {{PTHREAD_MUTEX_NORMAL}})) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_mutex_init(&mutex, &attr)) != 0) {
    /* Handle Error */
  }

  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }

  /* Critical Region*/

  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }

  return 0;
}

Compliant Solution

This compliant solution shows an ERRORCHECK mutex lock being created so that return codes will be available during locking and unlocking.

pthread_mutexattr_t attr;
pthread_mutex_t mutex;
size_t const shared_var = 0;

int main(void) {
  int result;

  if ((result = pthread_mutexattr_settype(&mutex, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_mutex_init(&mutex, &attr)) != 0) {
    /* Handle Error */
  }


  if ((result = pthread_mutex_lock(&mutex)) != 0) {
    /* Handle Error */
  }

  /* Critical Region*/

  if ((result = pthread_mutex_unlock(&mutex)) != 0) {
    /* Handle Error */
  }

  return 0;
}

Risk Assessment

Using NORMAL mutex locks can lead to deadlocks or abnormal program termination.

Bibliography

[Open Group 2004]
[SOL 2010]

POS03-C. Do not use volatile as a synchronization primitive      50. POSIX (POS)