POS44-C. Do not use signals to terminate threads

Do not send an uncaught signal to kill a thread because the signal kills the entire process, not just the individual thread. This rule is a specific instance of SIG02-C. Avoid using signals to implement normal functionality.

In POSIX systems, using the signal() function in a multithreaded program falls under exception CON37C-C-EX0 of rule CON37-C. Do not call signal() in a multithreaded program.

Noncompliant Code Example

This code uses the pthread_kill() function to send a SIGTERM signal to the created thread. The thread receives the signal, and the entire process is terminated.

void func(void *foo) {
  /* Execution of thread */
}

int main(void) {
  int result;
  pthread_t thread;

  if ((result = pthread_create(&thread, NULL, func, 0)) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_kill(thread, SIGTERM)) != 0) {
    /* Handle Error */
  }

  /* This point is not reached because the process terminates in pthread_kill() */

  return 0;
}

Compliant Solution

This compliant code uses instead the pthread_cancel() function to terminate the thread. The thread continues to run until it reaches a cancellation point. See The Open Group Base Specifications Issue 6, IEEE Std 1003.1, 2004 Edition [Open Group 2004] for lists of functions that are required and allowed to be cancellation points. If the cancellation type is set to asynchronous, the thread is terminated immediately. However, POSIX requires only the pthread_cancel(), pthread_setcancelstate(), and pthread_setcanceltype() functions to be async-cancel safe. An application that calls other POSIX functions with asynchronous cancellation enabled is nonconforming. Consequently, we recommend disallowing asynchronous cancellation, as explained by POS47-C. Do not use threads that can be canceled asynchronously.

void func(void *foo) {
  /* Execution of thread */
}

int main(void) {
  int result;
  pthread_t thread;

  if ((result = pthread_create(&thread, NULL, func, 0)) != 0) {
    /* Handle Error */
  }
  if ((result = pthread_cancel(thread)) != 0) {
    /* Handle Error */
  }

  /* Continue executing */

  return 0;
}

Risk Assessment

Sending the signal to a process causes it to be abnormally terminated.

Automated Detection

Related Vulnerabilities

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

Bibliography