The tss_create() function creates a thread-specific storage pointer identified by a key. Threads can allocate thread-specific storage and associate the storage with a key that uniquely identifies the storage by calling the tss_set() function. If not properly freed, this memory may be leaked. Ensure that thread-specific storage is freed.
Noncompliant Code Example
In this noncompliant code example, each thread dynamically allocates storage in the get_data() function, which is then associated with the global key by the call to tss_set() in the add_data() function. This memory is subsequently leaked when the threads terminate.
#include <threads.h>
#include <stdlib.h>
/* Global key to the thread-specific storage */
tss_t key;
enum { MAX_THREADS = 3 };
int *get_data(void) {
int *arr = (int *)malloc(2 * sizeof(int));
if (arr == NULL) {
return arr; /* Report error */
}
arr[0] = 10;
arr[1] = 42;
return arr;
}
int add_data(void) {
int *data = get_data();
if (data == NULL) {
return -1; /* Report error */
}
if (thrd_success != tss_set(key, (void *)data)) {
/* Handle error */
}
return 0;
}
void print_data(void) {
/* Get this thread's global data from key */
int *data = tss_get(key);
if (data != NULL) {
/* Print data */
}
}
int function(void *dummy) {
if (add_data() != 0) {
return -1; /* Report error */
}
print_data();
return 0;
}
int main(void) {
thrd_t thread_id[MAX_THREADS];
/* Create the key before creating the threads */
if (thrd_success != tss_create(&key, NULL)) {
/* Handle error */
}
/* Create threads that would store specific storage */
for (size_t i = 0; i < MAX_THREADS; i++) {
if (thrd_success != thrd_create(&thread_id[i], function, NULL)) {
/* Handle error */
}
}
for (size_t i = 0; i < MAX_THREADS; i++) {
if (thrd_success != thrd_join(thread_id[i], NULL)) {
/* Handle error */
}
}
tss_delete(key);
return 0;
}
Compliant Solution
In this compliant solution, each thread explicitly frees the thread-specific storage returned by the tss_get() function before terminating:
#include <threads.h>
#include <stdlib.h>
/* Global key to the thread-specific storage */
tss_t key;
int function(void *dummy) {
if (add_data() != 0) {
return -1; /* Report error */
}
print_data();
free(tss_get(key));
return 0;
}
/* ... Other functions are unchanged */
Compliant Solution
This compliant solution invokes a destructor function registered during the call to tss_create() to automatically free any thread-specific storage:
#include <threads.h>
#include <stdlib.h>
/* Global key to the thread-specific storage */
tss_t key;
enum { MAX_THREADS = 3 };
/* ... Other functions are unchanged */
void destructor(void *data) {
free(data);
}
int main(void) {
thrd_t thread_id[MAX_THREADS];
/* Create the key before creating the threads */
if (thrd_success != tss_create(&key, destructor)) {
/* Handle error */
}
/* Create threads that would store specific storage */
for (size_t i = 0; i < MAX_THREADS; i++) {
if (thrd_success != thrd_create(&thread_id[i], function, NULL)) {
/* Handle error */
}
}
for (size_t i = 0; i < MAX_THREADS; i++) {
if (thrd_success != thrd_join(thread_id[i], NULL)) {
/* Handle error */
}
}
tss_delete(key);
return 0;
}
Risk Assessment
Failing to free thread-specific objects results in memory leaks and could result in a denial-of-service attack.
Rule | Severity | Likelihood | Detectable | Repairable | Priority | Level |
|---|---|---|---|---|---|---|
CON30-C | Medium | Unlikely | No | No | P2 | L3 |
Automated Detection
| Tool | Version | Checker | Description |
|---|---|---|---|
| Astrée | 24.04 | Supported, but no explicit checker | |
| CodeSonar | 9.1p0 | ALLOC.LEAK | Leak |
| Coverity | 2017.07 | ALLOC_FREE_MISMATCH | Partially implemented, correct implementation is more involved |
| Cppcheck Premium | 24.11.0 | premium-cert-con30-c | |
| Helix QAC | 2025.2 | C1780, C1781, C1782, C1783, C1784 | |
| Parasoft C/C++test | 2024.2 | CERT_C-CON30-a | Ensure resources are freed |
R2025b | CERT C: Rule CON30-C | Checks for thread-specific memory leak (rule fully covered) |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
9 Comments
Robert Seacord
From Geoff Clare:
The pthread_setspecific() call in the destructor() function in the CS is not needed. The system automatically sets the value to NULL on thread exit if there is a destructor function associated with the key; there is no need to do it explicitly.
The statement "free(NULL) which most compilers might ignore" is incorrect. The standard requires free() to accept a null pointer (and take no action).
Also the malloc.h includes should be stdlib.h.
Robert Seacord
One more thing. For the first compliant solution, should I add the following line:
intfunction(void*dummy) {if(add_data() != 0) {return-1;/* Report error */}print_data();free(tss_get(key));(void) tss_set(key,NULL)return0;}Geoff Clare suggests that pthreads does this automatically from the destructor, although I don't see any such guarantees in the C Standard.
Aaron Ballman
The standard is vague on tss destruction, but I would guess that it is harmless, but not helpful, to do that. Once you call
tss_delete(), calls totss_get()for that key would return 0 in a sane implementation.Robert Seacord
OK, maybe I'll just omit it.
Will Snavely
At first glance, I thought this rule was about the tss_delete function (e.g. ensure you call tss_delete), but that isn't the case. Rather, the gist of it seems to be, if you associate dynamically allocated memory with a thread-specific-storage pointer, make sure you free it. I feel like this scenario is an instantiation of MEM31-C. Free dynamically allocated memory when no longer needed, and may not require a seperate rule.
Yozo TODA
sorry for only commenting... (I don't have edit privilege now)
the URL of Defect Report #416 should be http://www.open-std.org/JTC1/SC22/WG14/www/docs/summary.htm#dr_416
Aaron Ballman
I've corrected the link, thank you for pointing it out!
Joseph C. Sible
Is there a reason that the second compliant example needs to write its own destructor function? Why can't it just do
tss_create(&key, free)?David Svoboda
For that particular code example, yes, you could simply pass
freetotss_create()rather than creating a distinct destructor function. The destructor function is useful if it needs to do anything more than a simple free, though.