Boost Semaphores under linux and EINTR return code

Question

In boost (I use 1.54.0) I see implementation for posix semaphore wait:

inline void semaphore_wait(sem_t *handle)
{
   int ret = sem_wait(handle);
   if(ret != 0){
      throw interprocess_exception(system_error_code());
   }
}

Manual on posix semaphore says:

ERRORS

  EINTR The call was interrupted by a signal handler; see signal(7). 

Am I right that boost semaphore throw exception if I send kill to the waiting thread? If so how do you handle this situation?

Answer 1

In my opinion, this is probably a bug in Boost.Interprocess. Please, report it to developers, at the very least they will be able to provide a rationale if this is intentional.

Commenting on signal management suggestion in the comments above. It is true that a typical multi-threaded application should mask out signals that are not intended to be processed by threads, leaving only one thread to handle signals. However, this is not a mandatory rule.

First, auxiliary threads can be spawned by libraries which do not internally handle signals, leaving that to the application. Signal handlers can potentially be called in these threads.

Second, some signals may be intentionally left unmasked to catch events related to that particular thread. For example, one can register a handler for SIGSEGV to detect segmentation errors. This handler will be invoked in the offending thread, and the application can theoretically deal with the error. Similarly, SIGUSR1 or SIGUSR2 can be used to signal application-defined events to particular threads.

The bottom line is that even though a well designed application should extract signal handling to a separate thread, libraries should not assume that and be prepared that it doesn't. In any case, throwing in case of EINTR doesn't look like a correct behavior.

Answer 2

The implementation looks OK. SA_RESTART flag can be used so the call is restarted automatically. http://man7.org/linux/man-pages/man7/signal.7.html