The C Standard, 7.26.5.6 [ISO/IEC 9899:2011], states that a thread shall not be joined once it was previously joined or detached. Similarly, subclause 7.26.5.3 states that a thread shall not be detached once it was previously joined or detached. Violating either of these subclauses results in undefined behavior.

Noncompliant Code Example

This noncompliant code example detaches a thread that is later joined.

#include <stddef.h>
#include <threads.h>
 
int thread_func(void *arg) {
  /* Do work */
  thrd_detach(thrd_current());
  return 0;
}

int main(void) {
  thrd_t t;

  if (thrd_success != thrd_create(&t, thread_func, NULL)) {
    /* Handle error */
    return 0;
  }

  if (thrd_success != thrd_join(t, 0)) {
    /* Handle error */
    return 0;
  }
  return 0;
}

Compliant Solution

 This compliant solution does not detach the thread. Its resources are released upon successfully joining with the main thread:

#include <stddef.h>
#include <threads.h>
  
int thread_func(void *arg) {
  /* Do work */
  return 0;
}

int main(void) {
  thrd_t t;

  if (thrd_success != thrd_create(&t, thread_func, NULL)) {
    /* Handle error */
    return 0;
  }

  if (thrd_success != thrd_join(t, 0)) {
    /* Handle error */
    return 0;
  }
  return 0;
} 

Risk Assessment

Joining or detaching a previously joined or detached thread is undefined behavior.

Automated Detection

Tool	Version	Checker	Description
Astrée			Supported, but no explicit checker
Helix QAC		C1776
Parasoft C/C++test		CERT_C-CON39-a	Do not join or detach a thread that was previously joined or detached
Polyspace Bug Finder		CERT C: Rule CON39-C	Checks for join or detach of a joined or detached thread (rule fully covered)

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography