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Full Discussion: Sinal-processing alarm()-function
Top Forums Programming Sinal-processing alarm()-function Post 302283445 by otheus on Tuesday 3rd of February 2009 11:00:11 AM
Old 02-03-2009
Hopefully, 1 month delay is not too long for an answer:

The main thing is this: just before a signal handler is called, subsequent signals on that same signal are blocked. The return function from a signal handler is "special" in that it unblocks that mask. I'm not sure where this is documented, but you can demonstrated it by adding a couple of printf("%x\n",siggetmask()); calls, in the signal handler, and right after the alarm() call. But because your signal handler does a longjmp instead of a return, the signal mask never gets reset.

Or, to do it "by hand", you can do this in your signal handler (or on return from it):
Code:
    sigset_t sigset;
    sigfillset(&sigset);
    sigprocmask(SIG_UNBLOCK,&sigset,NULL);

All you need to do is set savemask=1 when you declare it. Then things will work the way you intended. This tells the setlongjmp() call to restore the original signal mask when it does the jump -- in effect, emulating the normal return-from-signal code.

There is also a problem mixing SIGALRM and wait(). From the wait(3) man page:
Code:
BUGS
       sleep()  may  be implemented using SIGALRM; mixing calls to alarm() and
       sleep() is a bad idea.

       Using longjmp() from a signal handler  or  modifying  the  handling  of
       SIGALRM while sleeping will cause undefined results.

Last edited by otheus; 02-03-2009 at 12:03 PM.. Reason: added sigprocmask()
 

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LEARN ABOUT MINIX

sigprocm

SIGPROCMASK(2)							System Calls Manual						    SIGPROCMASK(2)

NAME

       sigprocmask - manipulate the signal mask

SYNOPSIS

       #include <signal.h>

       int sigprocmask(int how, const sigset_t *set, sigset_t *oset)

DESCRIPTION

       Sigprocmask()  examines	or  manipulates  the  signal  mask.  This mask is the set of signals that are currently blocked.  The how argument
       determines the action that must be performed.  In all cases the signal set referenced by oset, if not NULL, will be used to receive the old
       signal mask.  The set argument, if not NULL, will be used to set or modify the current signal mask.

       How can be one of:

       SIG_BLOCK      Add the signals referenced by set to the mask.

       SIG_UNBLOCK    Remove the signals referenced by set from the mask.

       SIG_SETMASK    Set the signal mask to the set referenced by set.

       The value of how is ignored if set is NULL.

SEE ALSO

       sigaction(2), sigpending(2), sigsuspend(2), sigset(3).

DIAGNOSTICS

       Returns 0 on success and -1 on error.  The error code is EFAULT for a bad set or oset address, or EINVAL for a bad how argument.

AUTHOR

       Kees J. Bot (kjb@cs.vu.nl)

																    SIGPROCMASK(2)
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