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The C Standard, subclause 7.2628.5.6 paragraph 2 [ISO/IEC 9899:20112024],  specifically states that a thread shall not be joined once it was previously joined or detached.

The termination of the other thread synchronizes with the completion of the thrd_join function. The thread identified by thr shall not have been previously detached or joined with another thread.

Similarly, subclause 7.2628.5.3 states that a paragraph 2 [ISO/IEC 9899:2024], states that a thread shall not be detached once it was previously joined or detached. 

The thrd_detach function tells the operating system to dispose of any resources allocated to the thread identified by thr when that thread terminates. The thread identified by thr shall not have been previously detached or joined with another thread.

Violating either of these subclauses results in in undefined behavior 211.

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, 0NULL)) {
    /* 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, 0NULL)) {
    /* 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 causes is undefined behavior 211.

Automated Detection

Related Vulnerabilities

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

Bibliography

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