CON05-C. Do not perform operations that can block while holding a lock

If a lock is being held and an operation that can block is performed, any other thread that needs to acquire that lock may also block. This condition can degrade system performance or cause a deadlock to occur.  Blocking calls include, but are not limited to, network, file, and console I/O.

Using a blocking operation while holding a lock may be unavoidable for a portable solution.  For instance, file access could be protected via a lock to prevent multiple threads from mutating the contents of the file.  Or, a thread may be required to block while holding one or more locks and waiting to acquire another lock.  In these cases, attempt to hold the lock for the least time required.  Additionally, if acquiring multiple locks, the order of locking must avoid deadlock, as specified in CON35-C. Avoid deadlock by locking in predefined order.

Noncompliant Code Example

This noncompliant example calls fopen() while a mutex is locked.  The calls to fopen() and fclose() are blocking, and may block for an extended period of time if the file resides on a network drive.  While the call is blocked, other threads that are waiting for the lock are also blocked.

#include <stdio.h>
#include <threads.h>
 
mtx_t mutex;

int thread_foo(void *ptr) {
  int result;
  FILE *fp;

  if ((result = mtx_lock(&mutex)) != thrd_success) {
    /* Handle error */
  }

 
  fp = fopen("SomeNetworkFile", "r");
  if (fp != NULL) {
    /* Work with the file. */
    fclose(fp);
  }
 
  if ((result = mtx_unlock(&mutex)) != thrd_success) {
    /* Handle error */
  }

  return 0;
}

int main(void) {
  thrd_t thread;
  int result;

  if ((result = mtx_init(&mutex, mtx_plain)) != thrd_success) {
    /* Handle error */
  }

  if (thrd_create(&thread, thread_foo, NULL) != thrd_success) {
    /* Handle error */
  }

  /* ... */

  if (thrd_join(thread, NULL) != thrd_success) {
    /* Handle error */
  }

  mtx_destroy(&mutex);

  return 0;
}

Compliant Solution (Block while Not Locked)

This compliant solution performs the file operations when the lock has not been acquired. The blocking behavior consequently affects only the thread that called the blocking function.

#include <stdio.h>
#include <threads.h>
 
mtx_t mutex;
 
int thread_foo(void *ptr) {
  int result;
  FILE *fp = fopen("SomeNetworkFile", "r");
 
  if (fp != NULL) {
    /* Work with the file. */
    fclose(fp);
  }


  if ((result = mtx_lock(&mutex)) != thrd_success) {
    /* Handle error */
  }

  /* ... */

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

  return 0;
}

Risk Assessment

Blocking or lengthy operations performed within synchronized regions could result in a deadlocked or an unresponsive system.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines