SIGCHLD interrupts its own handler Post: 302405364

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Top Forums Programming SIGCHLD interrupts its own handler Post 302405364 by Loic Domaigne on Thursday 18th of March 2010 04:05:51 PM

03-18-2010

Registered User

Quote:

Originally Posted by jrichemont

Hi Corona688. You are right. I already use WNOHANG with waitpid in a loop but only until that child has been dealt with. This is still liable to be interrupted though.

However you gave me an idea: I am using threads, I don't really need signals anyway and they mess with my head. What I did was start a thread and get it to sit in a loop with a blocking call to waitpid. As children die it gets unblocked, deals with it and loops back to blocking again. All in a nice serial manner.

I can simply ignore SIGCHLD now, that thread will pick up any children needing attention via waitpid.

Now, try as I might, I cannot break the system and I have it running in production today.

Cheers;

Jeremy

Good job, Jeremy!

You just discovered the way we usually deal with asynchronous signal in multi-threaded application. We spend a thread that waits (and consumes) synchronously the signal.

The function to wait synchronously for a signal is called sigwait(). You need in addition to ensure that the signal shall be delivered to the right thread, that is the one blocked in sigwait(). Pthreads offers such possibility with pthread_sigmask().

In your particular application, you luckier. First you can use waitpid(). And second, you can ignore SIGCHLD "signal redirection", since the default signal action for SIGCHLD is IGN.

Cheers,
Lo�c.

Loic Domaigne

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pause

pause(2)							System Calls Manual							  pause(2)

NAME

       pause - suspend process until signal

SYNOPSIS

DESCRIPTION

       suspends  the  calling  process	until  it  receives  a	signal.  The signal must be one that is not currently set to be ignored or blocked
       (masked) by the calling process.

       If the signal causes termination of the calling process, does not return.

       If the signal is by the calling process and control is returned from the signal-catching function  (see	signal(5)),  the  calling  process
       resumes execution from the point of suspension; with a return value of -1 from and set to

APPLICATION USAGE

   Threads Considerations
       Signal dispositions (such as catch/default/ignore) are shared by all threads in the process and blocked signal masks are maintained by each
       thread.	Therefore, the signals being waited for should not be ignored by the process or blocked by the calling thread.

       will suspend only the calling thread until it receives a signal.

       If other threads in the process do not block the signal, the signal may be delivered to another thread in the process and the thread in may
       continue waiting.  For this reason, the use of is recommended instead of for multi-threaded applications.

       For more information regarding signals and threads, refer to signal(5).

SEE ALSO

       alarm(2), kill(2), sigwait(2), wait(2), signal(5).

STANDARDS CONFORMANCE

																	  pause(2)