CERT C: Rec. POS04-C

Avoid using PTHREAD_MUTEX_NORMAL type mutex locks

Since R2026a

Description

Avoid using PTHREAD_MUTEX_NORMALL type mutex locks¹

Polyspace Implementation

The rule checker checks for Use of PTHREAD_MUTEX_NORMAL type mutex locks.

Examples

Use of PTHREAD_MUTEX_NORMAL type mutex locks

Issue

This issue occurs when the second argument of a call to the function pthread_mutexattr_settype() is PTHREAD_MUTEX_NORMAL instead of PTHREAD_MUTEX_RECURSIVE or PTHREAD_MUTEX_ERRORCHECK.

Risk

Mutex locks of type PTHREAD_MUTEX_NORMAL do not detect deadlocks. As a result, use of this type of mutex can lead to unexpected deadlocks and undefined behaviors.

Say your code declares a PTHREAD_MUTEX_NORMAL mutex called normal_mutex. These outcomes can result from using such a mutex:

• If a thread attempts to relock normal_mutex without unlocking it first, execution deadlocks without returning an error to the caller.

• If a thread attempts to unlock normal_mutex after it is locked by a different thread, the behavior of the code is undefined.

• If a thread attempts to unlock normal_mutex after it is unlocked, the behavior of the code is undefined.

Fix

Use PTHREAD_MUTEX_RECURSIVE or PTHREAD_MUTEX_ERRORCHECK instead of PTHREAD_MUTEX_NORMAL.

Example

This example uses a PTHREAD_MUTEX_NORMAL type mutex counter_mutex to protect the shared resource counter. Polyspace^® reports a violation.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 5
#define NUM_INCREMENTS 100000

// Shared resource
int counter = 0;

// Mutex for protecting the shared resource
pthread_mutex_t counter_mutex;

// Function executed by each thread
void* incrementCounter(void* arg) {
    for (int i = 0; i < NUM_INCREMENTS; i++) {
        // Lock the mutex before accessing the shared resource
        pthread_mutex_lock(&counter_mutex);
        
        // Increment the shared counter
        counter++;
        
        // Unlock the mutex after accessing the shared resource
        pthread_mutex_unlock(&counter_mutex);
    }
    return NULL;
}

int main() {
    pthread_t threads[NUM_THREADS];
    pthread_mutexattr_t attr;

    // Initialize the mutex attribute
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_NORMAL);  //Noncompliant

    // Initialize the mutex with the specified attributes
    pthread_mutex_init(&counter_mutex, &attr);

    // Create threads
    for (int i = 0; i < NUM_THREADS; i++) {
        if (pthread_create(&threads[i], NULL, incrementCounter, NULL) != 0) {
            perror("Failed to create thread");
            return EXIT_FAILURE;
        }
    }

    // Wait for all threads to finish
    for (int i = 0; i < NUM_THREADS; i++) {
        if (pthread_join(threads[i], NULL) != 0) {
            perror("Failed to join thread");
            return EXIT_FAILURE;
        }
    }

    // Print the final value of the counter
    printf("Final counter value: %d\n", counter);

    // Destroy the mutex
    pthread_mutex_destroy(&counter_mutex);

    return EXIT_SUCCESS;
}

Correction

To fix this issue, define counter_mutex as a PTHREAD_MUTEX_ERRORCHECK type mutex.

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define NUM_THREADS 5
#define NUM_INCREMENTS 100000

// Shared resource
int counter = 0;

// Mutex for protecting the shared resource
pthread_mutex_t counter_mutex;

// Function executed by each thread
void* incrementCounter(void* arg) {
    for (int i = 0; i < NUM_INCREMENTS; i++) {
        // Lock the mutex before accessing the shared resource
        pthread_mutex_lock(&counter_mutex);
        
        // Increment the shared counter
        counter++;
        
        // Unlock the mutex after accessing the shared resource
        pthread_mutex_unlock(&counter_mutex);
    }
    return NULL;
}

int main() {
    pthread_t threads[NUM_THREADS];
    pthread_mutexattr_t attr;

    // Initialize the mutex attribute
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_ERRORCHECK);  //Compliant

    // Initialize the mutex with the specified attributes
    pthread_mutex_init(&counter_mutex, &attr);

    // Create threads
    for (int i = 0; i < NUM_THREADS; i++) {
        if (pthread_create(&threads[i], NULL, incrementCounter, NULL) != 0) {
            perror("Failed to create thread");
            return EXIT_FAILURE;
        }
    }

    // Wait for all threads to finish
    for (int i = 0; i < NUM_THREADS; i++) {
        if (pthread_join(threads[i], NULL) != 0) {
            perror("Failed to join thread");
            return EXIT_FAILURE;
        }
    }

    // Print the final value of the counter
    printf("Final counter value: %d\n", counter);

    // Destroy the mutex
    pthread_mutex_destroy(&counter_mutex);

    return EXIT_SUCCESS;
}

Check Information

Group: Rec. 50. POSIX (POS)

PQL Name: std.cert.POS04_C

Version History

Introduced in R2026a