Mutexes are used to protect shared data structures being concurrently accessed. If a mutex is destroyed while a thread is blocked waiting for that mutex, critical sections and shared data are no longer protected.
The C Standard, 7.26.4.1, paragraph 2 [ISO/IEC 9899:2011], states
The
mtx_destroyfunction releases any resources used by the mutex pointed to bymtx. No threads can be blocked waiting for the mutex pointed to bymtx.
This statement implies that destroying a mutex while a thread is waiting on it is undefined behavior.
Noncompliant Code Example
This noncompliant code example creates several threads that each invoke the do_work() function, passing a unique number as an ID. The do_work() function initializes the lock mutex if the argument is 0 and destroys the mutex if the argument is max_threads - 1. In all other cases, the do_work() function provides normal processing. Each thread, except the final cleanup thread, increments the atomic completed variable when it is finished.
Unfortunately, this code contains several race conditions, allowing the mutex to be destroyed before it is unlocked. Additionally, there is no guarantee that lock will be initialized before it is passed to mtx_lock(). Each of these behaviors is undefined.
#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
mtx_t lock;
/* Atomic so multiple threads can modify safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 };
int do_work(void *arg) {
int *i = (int *)arg;
if (*i == 0) { /* Creation thread */
if (thrd_success != mtx_init(&lock, mtx_plain)) {
/* Handle error */
}
atomic_store(&completed, 1);
} else if (*i < max_threads - 1) { /* Worker thread */
if (thrd_success != mtx_lock(&lock)) {
/* Handle error */
}
/* Access data protected by the lock */
atomic_fetch_add(&completed, 1);
if (thrd_success != mtx_unlock(&lock)) {
/* Handle error */
}
} else { /* Destruction thread */
mtx_destroy(&lock);
}
return 0;
}
int main(void) {
thrd_t threads[max_threads];
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_create(&threads[i], do_work, &i)) {
/* Handle error */
}
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_join(threads[i], 0)) {
/* Handle error */
}
}
return 0;
}
Compliant Solution
This compliant solution eliminates the race conditions by initializing the mutex in main() before creating the threads and by destroying the mutex in main() after joining the threads:
#include <stdatomic.h>
#include <stddef.h>
#include <threads.h>
mtx_t lock;
/* Atomic so multiple threads can increment safely */
atomic_int completed = ATOMIC_VAR_INIT(0);
enum { max_threads = 5 };
int do_work(void *dummy) {
if (thrd_success != mtx_lock(&lock)) {
/* Handle error */
}
/* Access data protected by the lock */
atomic_fetch_add(&completed, 1);
if (thrd_success != mtx_unlock(&lock)) {
/* Handle error */
}
return 0;
}
int main(void) {
thrd_t threads[max_threads];
if (thrd_success != mtx_init(&lock, mtx_plain)) {
/* Handle error */
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_create(&threads[i], do_work, NULL)) {
/* Handle error */
}
}
for (size_t i = 0; i < max_threads; i++) {
if (thrd_success != thrd_join(threads[i], 0)) {
/* Handle error */
}
}
mtx_destroy(&lock);
return 0;
}
Risk Assessment
Destroying a mutex while it is locked may result in invalid control flow and data corruption.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON31-C | Medium | Probable | High | P4 | L3 |
Automated Detection
Tool | Version | Checker | Description |
|---|---|---|---|
| Astrée | 24.04 | Supported, but no explicit checker | |
| CodeSonar | 8.1p0 | CONCURRENCY.LOCALARG | Local Variable Passed to Thread |
| Helix QAC | 2024.1 | DF4961, DF4962 | |
| Parasoft C/C++test | 2023.1 | CERT_C-CON31-a | Do not destroy another thread's mutex |
R2023b | CERT C: Rule CON31-C | Checks for destruction of locked mutex (rule fully covered) |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
Key here (explains table format and definitions)
Taxonomy | Taxonomy item | Relationship |
|---|---|---|
| CWE 2.11 | CWE-667, Improper Locking | 2017-07-10: CERT: Rule subset of CWE |
CERT-CWE Mapping Notes
Key here for mapping notes
CWE-667 and CON31-C/POS48-C
Intersection( CON31-C, POS48-C) = Ø
CWE-667 = Union, CON31-C, POS48-C, list) where list =
- Locking & Unlocking issues besides unlocking another thread’s C mutex or pthread mutex.
Bibliography
| [ISO/IEC 9899:2011] | 7.26.4.1, "The mtx_destroy Function" |
9 Comments
Alex Volkovitsky
What happens if the worker thread dies while holding the mutex? This would cause the clean up thread to block forever... is there a fix for this? Should we mention it?
Geoff Clare
This page seems to be in an inconsistent state. It says "This solution requires the cleanup function to acquire the lock before destroying it" but then goes on to give a quote from POSIX that effectively forbids doing this ("Attempting to destroy a locked mutex results in undefined behavior") and the code given as the solution does not lock the mutex in the cleanup function. Instead the code assumes that pthread_mutex_lock() will return an error if the mutex has been destroyed, but this is no good either, because according to POSIX the behaviour is undefined ("A destroyed mutex object can be reinitialized using pthread_mutex_init(); the results of otherwise referencing the object after it has been destroyed are undefined.")
The proper solution is to design the application in such a way that the cleanup cannot be done while worker threads might still have the mutex locked. For example, don't do the cleanup until all the worker threads have been joined.
Alex Volkovitsky
This is correct.. the quote from POSIX kind of hoses this entire rule, as there is really no way to destroy another thread's mutex that would be problematic since
pthread_mutex_destroy()acquires all necessary locks...Looking at this now, the compliant and non-compliant solutions are actually the same
We should probably delete this rule since it is in contradiction with what POSIX says
another quote from the man page:
Jonathan Leffler
This seems to be confused about processes and threads. Linux doesn't help by making threads close to processes.
David Keaton
I believe the issues are addressed now.
Geoff Clare
There were still some things that implied an application could expect pthread_mutex_lock() to fail if the mutex has been destroyed. The error is optional in POSIX, and on systems that don't detect it you get undefined behaviour. There was also an incorrect statement about pthread_mutex_destroy() acquiring locks. I have applied fixes.
The quote from POSIX before the compliant code now seems to be orphaned, but I wasn't sure what should be done with it.
David Keaton
I agree. The quote didn't seem to serve a purpose any more, so I removed it.
Aaron Ballman
I removed the unenforceable flag since this is enforceable via dynamic analysis.
Robert Seacord
We appear to be referring to a static analysis checker in Fortify that can detect violations of this rule. That suggests that this rule could also be checked through static analysis, or that this is a mistake.