POS44-C. Do not use signals to terminate threads

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

Noncompliant Code Example

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

int main(void){
  pthread_t thread;

  pthread_create(&thread, NULL, func, 0);
  pthread_kill(thread, SIGKILL);

  /* May continue executing briefly until the signal kills the process */

  return 0;
}

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

Compliant Solution

This code instead uses the pthread_cancel() to terminate the thread. The thread continues to run until it reaches a cancellation point. See the second referenced article for a list of functions that are cancellation points. If the cancellation type is set to asynchronous, the thread is terminated immediately. However, POSIX only requires 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 non-conforming.

int main(void){
  pthread_t thread;

  pthread_create(&thread, NULL, func, (void*)0);
  pthread_cancel(thread);

  /* Continues */

  return 0;
}

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

Risk Assessment

Using signals as described has the simple consequence of terminating the process, which is clearly undesired. However there is no other direct risk.

References

[[OpenBSD]] signal() Man Page
http://www.mkssoftware.com/docs/man3/pthread_cancel.3.asp
http://www.opengroup.org/onlinepubs/007908799/xsh/threads.html