MISRA C:2012 Rule 22.18

Non-recursive mutexes shall not be recursively locked

Since R2025a

Description

Rule Definition

Non-recursive mutexes shall not be recursively locked.¹

Rationale

Recursive mutexes are usually locked by recursively called functions. Recursive mutexes can be locked multiple times without causing a deadlock as long as it is unlocked the same number of times.

Locking a nonrecursive mutex multiple times without unlocking it first is undefined behavior. Similarly, unlocking a nonrecursive mutex multiple times without locking it first is undefined behavior.

Polyspace Implementation

Polyspace^® report a violation of this rule when:

• A thread locks a nonrecursive mutex.

• The thread locks the mutex again without unlocking it first.

Recursive mutexes are initialized by using the constant mtx_recursive in the function mtx_init(). Polyspace recognizes recursive_mutex as a recursive mutex:

mtx_t recursive_mutex;

void foo(){
    mtx_init( &recursive_mutex, mtx_recursive); // Recursive mutex
    
}

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

Nonrecursive Mutex Double Locked

In this example, the mutex object plain_mutex is nonrecursive. Locking this mutex multiple times without unlocking it first is undefined behavior. Polyspace reports a violation. Locking the recursive mutex recusrsive_mutex multiple times is not a violation.

#include<threads.h>

mtx_t recursive_mutex;
mtx_t plain_mutex;

void foo(){
    mtx_lock(&recursive_mutex);
    mtx_lock(&recursive_mutex);
    mtx_lock(&recursive_mutex);
    mtx_unlock(&recursive_mutex);
    mtx_unlock(&recursive_mutex);
    mtx_unlock(&recursive_mutex);  //Compliant
    mtx_lock(&plain_mutex);
    mtx_lock(&plain_mutex);  //Noncompliant
    
}

thrd_t id1;

int main(){
    mtx_init( &recursive_mutex, mtx_recursive); // Recursive mutex
    mtx_init( &plain_mutex, mtx_plain); // Plain mutex
    thrd_create(&id1, foo, 0);
    //...
}

Check Information

Group: Resources

Category: Required

AGC Category: Required

Version History

Introduced in R2025a