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

POSIX defines PTHREAD_MUTEX_NORMAL locks to have the following undefined behavior \[[Open Group 4|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04] \]:

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.

The DEFAULT mutex typepthread of pthreads is also generally mapped to PTHREAD_MUTEX_NORMAL, but is known to vary from platform to platform \[[SOL 10|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view] \].  UsageConsequently, of NORMAL locks should thusnot be avoidedused, and ERRORCHECK or RECURSIVE locks should be defined explicitly defined when using mutex locks are used.

Noncompliant Code Example

The non compliant This noncompliant code example shows a simple mutex being created using PTHREAD_MUTEX_NORMAL. It should be noted that no return code is expected by the caller when NORMAL mutexes 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(){
     int rc =0;

     rc = pthread_mutexattr_settype(&mutex, PTHREAD_MUTEX_NORMAL);
     if(rc != 0){
       /* Handle Error */
     }

     rc = pthread_mutex_init(&mutex, &attr);
     if(rc != 0){
       /* Handle Error */
     }


     pthread_mutex_lock(&mutex);

     /* Critical Region*/

     pthread_mutex_unlock(&mutex);

     return 0;
 }

Compliant Solution

The This compliant solution shows a pthread an ERRORCHECK mutex lock being created with type PTHREAD_MUTEX_ERRORCHECK where return codes will be available during locking and unlocking.

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Risk Assessment

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

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\[[Open Group 4|https://www.securecoding.cert.org/confluence/display/seccode/AA.+C+References#AA.CReferences-OpenGroup04] \]
\[[SOL 10|http://docs.sun.com/app/docs/doc/816-5137/sync-28983?a=view] \]