Thread: Creating / receiving timers

Hi,

Please could someone explain this simple timing code (to be used under Linux). I get most of it, but some of the struct members like sa_flags confuses me and I can't find something that explains it clearly.

Code:

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


volatile int stop=0;


void sigalrm_handler( int sig )
{
    stop = 1;
}


int main(int argc, char **argv)
{
    struct sigaction sact;
    int num_sent = 0;


    sigemptyset (&sact.sa_mask);
    sact.sa_flags = 0;
    sact.sa_handler = sigalrm_handler;
    sigaction(SIGALRM, &sact, NULL);


    alarm(1);  /* Request SIGALRM in 60 seconds */
    while (!stop) {
        printf("Sent");
        num_sent++;
    }


    printf("sent %d packets\n", num_sent);
    return(0);
}

I hope to use timing code in some of my projects. Is it possible to create multiple sigalarms at once?

Thanks in advance,
Joshun

You should always read the documentation for all the functions you use. On Linux, this is usually available in sections 2 or 3 of the man pages (i.e. type "man 2 sigaction" or "man 3 sigaction" from the command line). For example, the sigaction man pages (sigaction(2): examine/change signal action - Linux man page and sigaction(3): examine/change signal action - Linux man page) discuss the flags

Originally Posted by man 2 sigaction

sa_flags specifies a set of flags which modify the behavior of the signal. It is formed
by the bitwise OR of zero or more of the following:


SA_NOCLDSTOP
If signum is SIGCHLD, do not receive notification when child processes stop
(i.e., when they receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or resume
(i.e., they receive SIGCONT) (see wait(2)). This flag is only meaningful when
establishing a handler for SIGCHLD.


SA_NOCLDWAIT (Since Linux 2.6)
If signum is SIGCHLD, do not transform children into zombies when they termi‐
nate. See also waitpid(2). This flag is only meaningful when establishing a
handler for SIGCHLD, or when setting that signal's disposition to SIG_DFL.


If the SA_NOCLDWAIT flag is set when establishing a handler for SIGCHLD,
POSIX.1 leaves it unspecified whether a SIGCHLD signal is generated when a
child process terminates. On Linux, a SIGCHLD signal is generated in this
case; on some other implementations, it is not.


SA_NODEFER
Do not prevent the signal from being received from within its own signal han‐
dler. This flag is only meaningful when establishing a signal handler.
SA_NOMASK is an obsolete, non-standard synonym for this flag.


SA_ONSTACK
Call the signal handler on an alternate signal stack provided by sigalt‐
stack(2). If an alternate stack is not available, the default stack will be
used. This flag is only meaningful when establishing a signal handler.


SA_RESETHAND
Restore the signal action to the default state once the signal handler has been
called. This flag is only meaningful when establishing a signal handler.
SA_ONESHOT is an obsolete, non-standard synonym for this flag.


SA_RESTART
Provide behavior compatible with BSD signal semantics by making certain system
calls restartable across signals. This flag is only meaningful when establish‐
ing a signal handler. See signal(7) for a discussion of system call restart‐
ing.


SA_SIGINFO (since Linux 2.2)
The signal handler takes 3 arguments, not one. In this case, sa_sigaction
should be set instead of sa_handler. This flag is only meaningful when estab‐
lishing a signal handler.

A few other notes:
1. alarm(1) triggers a SIGALRM in 1 second, not 1 minute (60 sec).
2. alarm() and the SIGALRM don't play nicely with some of the other timing functions, like sleep, usleep, etc. Read the man page (sections 2 and 3) for details: alarm(2) - Linux man page and alarm(3): schedule alarm signal - Linux man page.