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CERT C: Rule. POS53-C

Do not use more than one mutex for concurrent waiting operations on a condition variable

Since R2020a

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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

Do not use more than one mutex for concurrent waiting operations on a condition variable.1

Polyspace Implementation

The rule checker checks for Multiple mutexes used with same conditional variable.

Examples

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Issue

This issue occurs when multiple threads use more than one mutex to concurrently wait on the same condition variable. A thread waits on a condition variable by calling the functions pthread_cond_timedwait or pthread_cond_wait. These functions take a condition variable and a locked mutex as arguments, and the condition variable is bound to that mutex when the thread waits on the condition variable.

The checkers flags the use of pthread_cond_timedwait or pthread_cond_wait in one of the threads. See the Event column in the Results Details pane to view the threads waiting on the same condition variable and using a different mutex.

Risk

When a thread waits on a condition variable using a mutex, the condition variable is bound to that mutex. Any other thread using a different mutex to wait on the same condition variable is undefined behavior according to the POSIX standard.

Fix

Use the same mutex argument for pthread_cond_timedwait or pthread_cond_wait when threads are concurrently waiting on the same condition variable, or use separate condition variables for each mutex.

Example - Concurrent Waiting on Condition Variable with Multiple Mutexes
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#define Thrd_return_t                    void *
#define __USE_XOPEN2K8



#define COUNT_LIMIT 5

static void fatal_error(void)
{
    exit(1);
}


pthread_mutex_t mutex1;
pthread_mutex_t mutex2;
pthread_mutex_t mutex3;
pthread_cond_t cv;

int count1 = 0, count2 = 0, count3 = 0;
#define DELAY 8

Thrd_return_t waiter1(void* arg)
{
    int ret;
    while (count1 < COUNT_LIMIT) {
        if ((ret = pthread_mutex_lock(&mutex1)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex1)) != 0) { //Noncompliant
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count1 = %d\n", ++count1);
        if ((ret = pthread_mutex_unlock(&mutex1)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t waiter2(void* arg)
{
    int ret;
    while (count2 < COUNT_LIMIT) {
        if ((ret = pthread_mutex_lock(&mutex2)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex2)) != 0) { //Noncompliant
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count2 = %d\n", ++count2);
        if ((ret = pthread_mutex_unlock(&mutex2)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t signaler(void* arg)
{
    int ret;
    while ((count1 < COUNT_LIMIT) || (count2 < COUNT_LIMIT)) {
        sleep(1);
        printf("signaling\n");
        if ((ret = pthread_cond_broadcast(&cv)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t waiter3(void* arg)
{
    int ret;
    while (count3 % COUNT_LIMIT != 0) {
        if ((ret = pthread_mutex_lock(&mutex3)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex3)) != 0) { //Noncompliant
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count3 = %d\n", ++count3);
        if ((ret = pthread_mutex_unlock(&mutex3)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

int main(void)
{
    int ret;
    pthread_t thread1, thread2, thread3;

    pthread_mutexattr_t attr;

    if ((ret = pthread_mutexattr_init(&attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
        /* Handle error */
        fatal_error();
    }

    if ((ret = pthread_mutex_init(&mutex1, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutex_init(&mutex2, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutex_init(&mutex3, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_cond_init(&cv, NULL)) != 0) {
        /* handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread1, NULL, &waiter1, NULL))) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread2, NULL, &waiter2, NULL))) {
        /* handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread3, NULL, &signaler, NULL))) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread1, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread2, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread3, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }

    while (1) { ; }

    return 0;
}

In this example, a different mutex is used to protect each count variable. Since all three waiter functions wait on the same condition variable cv with different mutexes, the call to pthread_cond_wait will succeed for one of the threads and the call will be undefined for the other two.

The checker raises a defect for function waiter3 even though the function is not invoked directly or indirectly by a thread, entry-point, or interrupt. The analysis considers function waiter3 called by the main program through its function address or an unidentified thread whose creation is the missing source code.

Correction — Use the Same Mutex for All Threads Waiting on Same Condition Variable

One possible correction is to pass the same mutex argument to all the call to pthread_cond_wait that are used to wait on the same condition variable.

 #include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <pthread.h>
#define Thrd_return_t                    void *
#define __USE_XOPEN2K8



#define COUNT_LIMIT 5

static void fatal_error(void)
{
    exit(1);
}


pthread_mutex_t mutex;

pthread_cond_t cv;

int count1 = 0, count2 = 0, count3 = 0;
#define DELAY 8

Thrd_return_t waiter1(void* arg)
{
    int ret;
    while (count1 < COUNT_LIMIT) {
        if ((ret = pthread_mutex_lock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count1 = %d\n", ++count1);
        if ((ret = pthread_mutex_unlock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t waiter2(void* arg)
{
    int ret;
    while (count2 < COUNT_LIMIT) {
        if ((ret = pthread_mutex_lock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count2 = %d\n", ++count2);
        if ((ret = pthread_mutex_unlock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t signaler(void* arg)
{
    int ret;
    while ((count1 < COUNT_LIMIT) || (count2 < COUNT_LIMIT)) {
        sleep(1);
        printf("signaling\n");
        if ((ret = pthread_cond_broadcast(&cv)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}

Thrd_return_t waiter3(void* arg)
{
    int ret;
    while (count3 % COUNT_LIMIT != 0) {
        if ((ret = pthread_mutex_lock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        if ((ret =
                 pthread_cond_wait(&cv, &mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
        sleep(random() % DELAY);
        printf("count3 = %d\n", ++count3);
        if ((ret = pthread_mutex_unlock(&mutex)) != 0) {
            /* Handle error */
            fatal_error();
        }
    }
    return (Thrd_return_t)0;
}
/* 
void user_task(void)
{
    (void)waiter3(NULL);
} */

int main(void)
{
    int ret;
    pthread_t thread1, thread2, thread3;

    pthread_mutexattr_t attr;

    if ((ret = pthread_mutexattr_init(&attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK)) != 0) {
        /* Handle error */
        fatal_error();
    }

    if ((ret = pthread_mutex_init(&mutex, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutex_init(&mutex, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_mutex_init(&mutex, &attr)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_cond_init(&cv, NULL)) != 0) {
        /* handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread1, NULL, &waiter1, NULL))) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread2, NULL, &waiter2, NULL))) {
        /* handle error */
        fatal_error();
    }
    if ((ret = pthread_create(&thread3, NULL, &signaler, NULL))) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread1, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread2, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }
    if ((ret = pthread_join(thread3, NULL)) != 0) {
        /* Handle error */
        fatal_error();
    }

    while (1) { ; }

    return 0;
}

Check Information

Group: Rule 50. POSIX (POS)

Version History

Introduced in R2020a

See Also

Topics

External Websites

1 This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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