On Monday 15 November 2004 13:20, Russell Miller wrote:
> I'm having a hard time figuring out what the NODEFER flag in
> sigaction actually does.
[...]
> it looks like if SA_NODEFER is not set, then it blocks the signal.
> Does the signal get moved into the pending signal slot, or does it
> just get blocked? I don't understand.
>
> I also don't understand what happens if SA_NODEFER is set. I have
> some code (that reaps children) that can get triggered at almost the
> same time. With SA_NODEFER set, it appears that sometimes the
> handling routine doesn't get allowed to finish if it gets interrupted
> by another signal.
If SA_NODEFER is set, then the single handler can be called while
executing that single handler (from a previous signal).
> So, an explanation would be appreciated, along with an answer to this
> question: If I have a SIGCHLD handling routine that is likely to get
> called at the same time, and do NOT set SA_NODEFER, will the SIGCHLD
> signal get deferred until the handling routine gets called, and then
> processed? Or will the SIGCHLD signal just get lost? I can't afford
> to have hundreds of zombies sitting around because the "waitpid" call
> never happens.
The sigaction manpage has
sa_mask gives a mask of signals which should be blocked during
execution of the signal handler. In addition, the signal
which triggered the handler will be blocked, unless the
SA_NODEFER or SA_NOMASK flags are used.
SA_NOMASK or SA_NODEFER
Do not prevent the signal from being received from
within its own signal handler.
This shows what happens if you have signal handlers for both USR1 and
CHLD. The two columns are are the difference in USR1 having the
SA_NODEFER set or not.
SA_NODEFER set SA_NODEFER not set
--------------------------- ---------------------------------
SIGUSR1 sent SIGUSR1 sent
SIGUSR1 handler called SIGUSR1 handler called
SIGCHLD sent SIGCHLD sent
SIGCHLD handler called SIGCHLD handler called
SIGCHLD hander returns SIGCHLD handler returns
SIGUSR1 sent SIGUSR1 sent
SIGUSR1 handler called (again)
SIGUSR1 handler returns
SIGUSR1 handler returns (again) SIGUSR1 handler returns
SIGUSR1 handler called
SIGUSR1 handler returns
If you want to prevent the the CHLD handler from being called while
processing the USR1 signal, then you need to specify that signal in the
sa_mask field in the struct sigaction when registering the USR1
handler.
Defered signals (or masked ones) don't get lost, but they don't count
either. i.e. if you get N signals of a particular type, you're handler
will be called M times, where M <= N.
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