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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.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

POS52-C

Low

Probable

High

P2

L3

Automated Detection

ToolVersionCheckerDescription
CodeSonar
5.2p0
CONCURRENCY.STARVE.BLOCKINGBlocking in Critical Section
Klocwork
2018
CONC.SLEEP
Parasoft C/C++test
10.4.2

CERT_C-POS52-a

Do not use blocking functions while holding a lock

Polyspace Bug Finder

R2019b

CERT C: Rule POS52-CChecks for blocking operation while holding lock (rule partially covered)
PRQA QA-C

9.7

4966, 4967

Related Guidelines

Key here (explains table format and definitions)

Taxonomy

Taxonomy item

Relationship

CERT CLCK09-J. Do not perform operations that can block while holding a lockPrior to 2018-01-12: CERT: Unspecified Relationship
CWE 2.11CWE-5572017-07-10: CERT: Rule subset of CWE

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