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signalssignal-handlinggoogle-breakpad

When does a signal get automatically rethrown?

The following comment in breakpad suggests that resetting the signal handler forces the signal to be rethrown. However, I wasn't able to find any documentation online that describes this behaviour.

Additionally, the comment here says once the signal handler returns, the signal will be rethrown. Is this also as a result of the signal handler being restored or reset to the default?

Solution

  • suggests that resetting the signal handler forces the signal to be rethrown

    says once the signal handler returns, the signal will be rethrown

    Neither is true. However, I don't believe that's what the comments imply. The likely case is that it leaves the signal unprocessed in some cases. So the original problem that triggered the signal triggers the same signal again. Consider this example:

    #include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>

void handler(int sig)
{
    write(1, "Received FPE\n", sizeof "Received FPE\n" - 1);
}

int  main(void)
{
    signal(SIGFPE, handler);
    printf("%f", 1/0);
}

    (This should cause an infinite loop).

    SIGFPE isn't really "handled" here. So the once the control returns back from the signal handler, SIGPFE is repeatedly triggered. I believe this is scenario that's referred to in those comments.

    Similarly, when signals are blocked they'll be queued and sent to the process once they're unblocked. They are masking it when installing their signal handler. This is likely what the second comment refers to. Masking can also be done via sigprocmask or pthread_sigmask.