Clever Geek Handbook

Some questions about Linux signals

  • Is ( SIGRTMIN + 1

    ) safe for interprocess communication? Will it change in different processes?

  • Is there any difference in using sigqueue(2)

    or kill(2)

    for sending standard signals and signals in real time?
    If I use sigqueue(2)

    to send a series SIGUSR1

    (standard signals) and process them slowly, are there multiple instances in the queue SIGUSR1

    ?
    How about using kill(2)

    to send SIGRTMIN

    (real-time signals)? Will they be queued up?

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2 answers


sigqueue () can be used to send only realtime signals and kill () can be used to send only standard signals. I have not tried to send the wrong signal using the API. But I would expect it to fail with some relevant error. Linux does not queue standard signals. Real-time signals are queued. The maximum number of real-time signals that can be queued is defined as RLIMIT_SIGPENDING

You can use any real-time signal as long as the receiver has a handler set up to send a specific sender.

EDIT



I was wrong earlier in my answer. It looks like kill () can send signals in real time. But from the comment in __send_signal () it looks like using kill to send signals in real time may not have the desired effect in some cases, that is: signals may not be queued.

   /*
     * Real-time signals must be queued if sent by sigqueue, or
     * some other real-time mechanism.  It is implementation
     * defined whether kill() does so.  We attempt to do so, on
     * the principle of least surprise, but since kill is not
     * allowed to fail with EAGAIN when low on memory we just
     * make sure at least one signal gets delivered and don't
     * pass on the info struct.
     */
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This post is old news, but it ranks high on Google and is unfortunately misleading, so here are some clarifications:



  • You can send any signal using any of these functions.
  • sigqueue allows more "context" to be passed (eg info, @see siginfo et. al] as opposed to kill / raise
  • According to real-time signals - it doesn't matter if they are sent or queued - they will behave the same [for example, in queue cf. grouped as signals without RT]
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