Thread: When to re-enable signal handlers

I'm reading the GNU libc manual, the part about signals.

In this page it shows some code. I report it here:

Code:

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* This flag controls termination of the main loop. */
     volatile sig_atomic_t keep_going = 1;
     
     /* The signal handler just clears the flag and re-enables itself. */
     void
     catch_alarm (int sig)
     {
       keep_going = 0;
       signal (sig, catch_alarm);
     }
     
     void
     do_stuff (void)
     {
       puts ("Doing stuff while waiting for alarm....");
     }
     
     int
     main (void)
     {
       /* Establish a handler for SIGALRM signals. */
       signal (SIGALRM, catch_alarm);
     
       /* Set an alarm to go off in a little while. */
       alarm (2);
     
       /* Check the flag once in a while to see when to quit. */
       while (keep_going)
         do_stuff ();
     
       return EXIT_SUCCESS;
     }

I was curious about the signal handler function. Why does it have to re-enable itself? Is it always necessary? If not, when is it? I'd like to know when I have to re-enable signal handlers.
I'm in a situation where I can't use sigaction() function (exam rule).

PS: a user in IRC told me "It's necessary on some platforms (implementation-defined when) with signal(). But sigaction() should avoid that issue." Is this true?

Thanks for your support, grumpy.

I was reading 'man signal' to learn more about this behaviour, and it seems to change according to libc version and macros defined.

In the original Unix systems, when a handler that was established using signal() was invoked by the delivery of a
signal, the disposition of the signal would be reset to SIG_DFL, and the system did not block delivery of further
instances of the signal. System V also provides these semantics for signal(). This was bad because the signal
might be delivered again before the handler had a chance to reestablish itself. Furthermore, rapid deliveries of
the same signal could result in recursive invocations of the handler.

BSD improved on this situation by changing the semantics of signal handling (but, unfortunately, silently changed
the semantics when establishing a handler with signal()). On BSD, when a signal handler is invoked, the signal
disposition is not reset, and further instances of the signal are blocked from being delivered while the handler is
executing.

The situation on Linux is as follows:

* The kernel's signal() system call provides System V semantics.

* By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system call. Instead,
it calls sigaction(2) using flags that supply BSD semantics. This default behavior is provided as long as the
_BSD_SOURCE feature test macro is defined. By default, _BSD_SOURCE is defined; it is also implicitly defined if
one defines _GNU_SOURCE, and can of course be explicitly defined.

On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined, then signal() provides System V
semantics. (The default implicit definition of _BSD_SOURCE is not provided if one invokes gcc(1) in one of its
standard modes (-std=xxx or -ansi) or defines various other feature test macros such as _POSIX_SOURCE,
_XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)

* The signal() function in Linux libc4 and libc5 provide System V semantics. If one on a libc5 system includes
<bsd/signal.h> instead of <signal.h>, then signal() provides BSD semantics.

In my Ubuntu 10.04 system, I got libc6 installed, but it says version 2.11.1-0ubuntu7.10.
When the 'man' page talks about "libc 2 or later", what does it mean? Does it mean "libcX 2 or later" or "libc version 2.anything or later"? The man also says that _BSD_SOURCE macro is defined by default... even if I don't include any header? Probably I can check it with a #ifdef, right?
My concern is that I will arrive at the exam place, different computers, and I will not know how that computer behave. How can I check it fast? Check for _BSD_SOURCE and libc version? (still not sure if libcX or libc v2.blabla)

Thanks

Ok

Anyway, I was curious and I checked about that macro with this source code:

Code:

#include <stdio.h>

int main(void){
#ifdef _BSD_SOURCE
	printf("_BSD_SOURCE is defined\n");
#endif
	return 0;
}

I compiled it with gcc filename.c -Wall and it printed the text. So, yes, even without including any particular header file, on my system that macro is defined by default, so it means I got BSD-like behaviours. Which means:

On BSD, when a signal handler is invoked, the
signal disposition is not reset, and further instances of the signal are
blocked from being delivered while the handler is executing.

and

By default, in glibc 2 and later, the signal() wrapper function does not
invoke the kernel system call. Instead, it calls sigaction(2) using
flags that supply BSD semantics. This default behavior is provided as
long as the _BSD_SOURCE feature test macro is defined.

Sorry me for the repetition of informations, I did it to make it clear if anyone would find this topic later by search.

Thanks