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MISRA C:2012 Rule 22.17

No thread shall unlock a mutex or call cnd_wait() or cnd_timedwait() for a mutex it has not locked before

Since R2024b

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Description

This checker is deactivated in a default Polyspace® as You Code analysis. See Checkers Deactivated in Polyspace as You Code Analysis (Polyspace Access).

Rule Definition

No thread shall unlock a mutex or call cnd_wait() or cnd_timedwait() for a mutex it has not locked before.

This rule comes from MISRA C™: 2012 Amendment 4.

Rationale

If a thread attempts to unlock a mutex that it did not lock previously, the resulting behavior is undefined. Consider this code:

mtx_t mutex;

void foo() {
	//...
	mtx_unlock(&mutex);
}
The thread foo() does not lock mutex but attempts to unlock it, resulting in undefined behavior.

If a thread attempts to call the functions cnd_wait() or cnd_timedwait() using a mutex without locking the mutex first, the resulting behavior is undefined. Consider this code:

mtx_t mutex;
cnd_t cnd1
void foo() {
	//...
	cnd_wait(&mutex, &cnd1);
}
The thread foo does not lock mutex but calls cnd_wait() using the mutex object, resulting in undefined behavior.

To avoid undefined behavior, a thread must lock a mutex before attempting either of these operations:

  • Unlocking the mutex

  • Calling cnd_wait() or cnd_timedwait() using the mutex

Polyspace Implementation

Polyspace reports a violation of this rule if either of these conditions is true:

  • A thread unlocks a mutex object before locking it.

  • A thread attempts to unlock a previously locked mutex twice without a locking operation between the unlocking operations.

Troubleshooting

If you expect a rule violation but do not see it, refer to Diagnose Why Coding Standard Violations Do Not Appear as Expected.

Examples

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In this example, the thread t2 containing the function worker2() attempts to unlock mutex but t2 does not lock the mutex object first, resulting in undefined behavior. Polyspace reports a violation.

#include <stdio.h>
#include <threads.h>
void doWork(void);
// Mutex declaration
mtx_t mutex;

// Worker function 1: Correctly locks and then unlocks the mutex
int worker1(void* arg) {
    printf("Worker 1 is trying to lock the mutex.\n");
    if (mtx_lock(&mutex) == thrd_success) {
        printf("Worker 1 has locked the mutex.\n");

        doWork();
        printf("Worker 1 is unlocking the mutex.\n");
        mtx_unlock(&mutex);
    } else {
        printf("Worker 1 failed to lock the mutex.\n");
    }
    return 0;
}

// Worker function 2: Tries to unlock the mutex without locking it first
int worker2(void* arg) {

    printf("Worker 2 is attempting to unlock the mutex without locking it first.\n");
    if (mtx_unlock(&mutex) == thrd_success) { /* Noncompliant */
        printf("Worker 2 successfully unlocked the mutex.\n");
        doWork();
    } else {
        printf("Worker 2 failed to unlock the mutex. This is expected behavior.\n");
    }
    return 0;
}


int main() {
	thrd_t t1, t2;

	// Initialize mutex
	mtx_init(&mutex, mtx_plain);

	// Create worker threads
	if(thrd_create(&t1, worker1, NULL) != thrd_success) {
		printf("Failed to create thread 1\n");
		return 1;
	}
	if(thrd_create(&t2, worker2, NULL) != thrd_success) {
		printf("Failed to create thread 2\n");
		return 1;
	}

	// Wait for threads to finish
	thrd_join(t1, NULL);
	thrd_join(t2, NULL);

	// Clean up
	mtx_destroy(&mutex);

	return 0;
}

For this example, specify worker1 and worker2 as entry-point functions. In the Polyspace User Interface, specify these options:

OptionValue
Configure multitasking manuallyOn
Tasks (-entry-points)

worker1

worker2

Alternatively, at the command line, use this command:

polyspace-bug-finder -entry-points worker1,worker2

In this example, the thread t1 containing the function worker1() attempts to unlock a mutex that is already unlocked, which results in undefined behavior. Polyspace reports a violation.

#include <stdio.h>
#include <threads.h>
void doWork(void);
// Mutex declaration
mtx_t mutex;

// Worker function 1: Locks the mutex and then attempts to unlock it twice
int worker1(void *arg) {
	printf("Worker 1 is trying to lock the mutex.\n");
	if(mtx_lock(&mutex) == thrd_success) {
		printf("Worker 1 has locked the mutex.\n");

		doWork();
		printf("Worker 1 is unlocking the mutex for the first time.\n");
		mtx_unlock(&mutex);

		printf("Worker 1 is attempting to unlock the mutex for the second time.\n");
		if(mtx_unlock(&mutex) == thrd_success) { /*Noncompliant*/
			printf("Worker 1 successfully unlocked the mutex a second time.\n");
		} else {
			printf("Worker 1 failed to unlock the mutex a second time.\n");
		}
	} else {
		printf("Worker 1 failed to lock the mutex.\n");
	}
	return 0;
}

// Worker function 2: Correctly locks and then unlocks the mutex
int worker2(void *arg) {
	printf("Worker 2 is trying to lock the mutex.\n");
	if(mtx_lock(&mutex) == thrd_success) {
		printf("Worker 2 has locked the mutex.\n");

		doWork();
		printf("Worker 2 is unlocking the mutex.\n");
		mtx_unlock(&mutex);
	} else {
		printf("Worker 2 failed to lock the mutex.\n");
	}
	return 0;
}


int main() {
	thrd_t t1, t2;

	// Initialize mutex
	mtx_init(&mutex, mtx_plain);

	// Create worker threads
	if(thrd_create(&t1, worker1, NULL) != thrd_success) {
		printf("Failed to create thread 1\n");
		return 1;
	}
	if(thrd_create(&t2, worker2, NULL) != thrd_success) {
		printf("Failed to create thread 2\n");
		return 1;
	}

	// Wait for threads to finish
	thrd_join(t1, NULL);
	thrd_join(t2, NULL);

	// Clean up
	mtx_destroy(&mutex);

	return 0;
}

For this example, specify worker1 and worker2 as entry-point functions. In the Polyspace User Interface, specify these options:

OptionValue
Configure multitasking manuallyOn
Tasks (-entry-points)

worker1

worker2

Alternatively, at the command line, use this command:

polyspace-bug-finder -entry-points worker1,worker2

Check Information

Group: Resources
Category: Required
AGC Category: Required

Version History

Introduced in R2024b

See Also

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