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 the performance of a system or cause a deadlock to occur.

Blocking calls include, but are not limited to: network, file, and console I/O. This rule is a specific instance of CON05-C. Do not perform operations that can block while holding a lock using POSIX threads.

Noncompliant Code Example

This noncompliant code example demonstrates an occurrence of a blocking call that waits to receive data on a socket while a mutex is locked. The recv() call blocks until data arrives on the socket. While it is blocked, other threads that are waiting for the lock are also blocked.

Although this example is specific to network I/O, the recv() call could be replaced with any blocking call, and the same behavior would occur.

pthread_mutexattr_t attr;
pthread_mutex_t mutex;

void thread_foo(void *ptr) {
  uint32_t num;
  int result;
  int sock;

  /* sock is a connected TCP socket */

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

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), 0)) < 0) {
    /* Handle Error */
  }

  /* ... */

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

int main() {
  pthread_t thread;
  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 (pthread_create(&thread, NULL,(void *)& thread_foo, NULL) != 0) {
    /* Handle Error */
  }

  /* ... */

  pthread_join(thread, NULL);

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

  return 0;
}

Compliant Solution (Block while Not Locked)

This compliant solution performs the recv() call when the lock has not been acquired. The blocking behavior consequently affects only the thread that called the blocking function.

void thread_foo(void *ptr) {
  uint32_t num;
  int result;
  int sock;

  /* sock is a connected TCP socket */

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), 0)) < 0) {
    /* Handle Error */
  }

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

  /* ... */

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

Compliant Solution (Use a Nonblocking Call)

This compliant solution performs the recv() call with the parameter o_nonblock, which causes the call to fail if no messages are available on the socket:

void thread_foo(void *ptr) {
  uint32_t num;
  int result;

  /* sock is a connected TCP socket */

  if ((result = recv(sock, (void *)&num, sizeof(uint32_t), O_NONBLOCK)) < 0) {
    /* Handle Error */
  }

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

  /* ... */

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

Exceptions

POS52-C-EX1: A thread may block while holding one or more locks and waiting to acquire another lock. When acquiring multiple locks, the order of locking must avoid deadlock, as specified in CON35-C. Avoid deadlock by locking in a predefined order.

Risk Assessment

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

Automated Detection

Tool	Version	Checker	Description
CodeSonar		CONCURRENCY.STARVE.BLOCKING	Blocking in Critical Section
Helix QAC		DF4966, DF4967
Klocwork		CONC.SLEEP
Parasoft C/C++test		CERT_C-POS52-a	Do not use blocking functions while holding a lock
Polyspace Bug Finder		CERT C: Rule POS52-C	Checks for blocking operation while holding lock (rule fully covered)

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-557 and POS52-C

CWE-557 = Union( POS52-C, list) where list =

• Concurrency issues besides blocking while holding a POSIX lock

Bibliography