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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.

Wiki MarkupPOSIX describes {{PTHREAD_MUTEX_NORMAL}} locks as having the following undefined behavior \ [[Open Group 2004|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04]\]:

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.

Wiki MarkupThe {{DEFAULT}} mutex pthread is also generally mapped to {{PTHREAD_MUTEX_NORMAL}} but is known to vary from platform to platform \ [[SOL 2010|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view]\]. Consequently, {{NORMAL}} locks should not be used, and {{ERRORCHECK}} or {{RECURSIVE}} locks should be defined explicitly when mutex locks are used.

Noncompliant Code Example

...

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

int main(void) {
  int result;

  if ((result = pthread_mutexattr_init(&attr)) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_mutexattr_settype(&mutexattr, {{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;
}

...

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_init(&attr)) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_mutexattr_settype(&mutexattr, 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;
}

...

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

Bibliography

\[[Open Group 2004|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04]\]
\[[SOL 2010|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view]\]

Automated Detection

Bibliography

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Image AddedImage AddedPOS03-C. Do not use volatile as a synchronization primitive      50. POSIX (POS)      Image Modified