I am generating signal and facing the strange behaviour

Question

I have started getting my hands of signals in Linux but here is some strange behavior happening in my code. I have just started and I searched for it too but I have not found anything, sorry if the question is too lame, here is the code-

void Handler(int sig ){
    printf("Inside Handler\n");
}

int main(int argc, char * argv[], char * envp[]){
    if( signal(SIGINT, Handler) ==SIG_ERR )
        exit(EXIT_FAILURE);

    for(size_t i = 0; ; i++){
        printf("%d\n", i);
        sleep(2);
    }
}

I know printf, signal calls are not the good idea to use but I have not studied sigaction till now. Now according to my book and others tutorial if I press ctrl+c then it has to call Handler every time but here is the strange thing: when I am pressing ctrl+c once, it's calling Handler but the next time it's terminating the program. Why is this strange thing happening?

See [How to avoid using `printf()` in a signal handler](http://stackoverflow.com/questions/16891019/how-to-avoid-using-printf-in-a-signal-handler/) for some discussion/commentary on the wisdom of your signal handler. — Jonathan Leffler, Apr 07 '17 at 05:30
Note that what you see depends on the platform you're using. You're using Linux which behaves as you describe (the signal handler is reset), but BSD and macOS behave differently (the signal handler is not reset) — and the standards (POSIX and C) sanction both sets of behaviour. — Jonathan Leffler, Apr 07 '17 at 05:37

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

The manual page for signal syscall says:

If the signal signum is delivered to the process, then one of the following happens:

If the disposition is set to SIG_IGN, then the signal is ignored.

If the disposition is set to SIG_DFL, then the default action associated with the signal (see signal(7)) occurs.

If the disposition is set to a function, then first either the disposition is reset to SIG_DFL, or the signal is blocked (see Portability below), and then handler is called with argument signum. If invocation of the handler caused the signal to be blocked, then the signal is unblocked upon return from the handler.

Since the default behavior of SIGINT is to terminate once the signal is reset to default behavior, the subsequent behavior is different.

score 0 · Answer 2 · answered Apr 07 '17 at 05:13

The behavior of signal when passed a handler other than SIG_DFL or SIG_IGN is undefined. Use sigaction for this purpose.

As the docs say:

The only portable use of signal() is to set a signal's disposition to SIG_DFL or SIG_IGN. The semantics when using signal() to establish a signal handler vary across systems (and POSIX.1 explicitly permits this variation); do not use it for this purpose.

score 0 · Accepted Answer · answered Apr 07 '17 at 06:21

0

Here is the code which I have written with your help which is working perfectly fine and I believe this will work nicely in each and every platform

#define _GNU_SOURCE

void Handler(int sig ){
printf("Inside Handler\n");
}

int main(int argc, char *argv[], char *envp[] ){
sighandler_t Action =Handler;

if( Action == SIG_DFL ){
Action = Handler;
}

if( signal(SIGINT, Action ) == SIG_ERR )
  exit(EXIT_FAILURE );

for(size_t k = 0 ; ; k++ ){
 printf("%d\n", k );
 sleep(2);
}

}

answered Apr 07 '17 at 06:21

Devesh Pratap Singh

46
1
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Which platform did you test it on? – Jonathan Leffler Apr 07 '17 at 06:42
Linux , ubuntu. – Devesh Pratap Singh Apr 07 '17 at 07:39
OK. And what do you consider is covered by "each and every platform"? – Jonathan Leffler Apr 07 '17 at 07:41
Because there are only two issues which are mentioned in man page, they are i) if we are using `-std=cxx` or `-ansi` then `SIG_DFL` is gonna set automatically and ii) if handler function returns `SIG_DFL` is gonna set automatically, `_BSD_SOURCE` has improved this behavior but still again its depend on versions. So, according to me only problem is `SIG_DFL` which we are handling in within `if`. The only problem I see in this is `sig_t` data type as `_BSD_SOURCE` provides this one. That something we can solve by using preprocessors. Thanks!! – Devesh Pratap Singh Apr 07 '17 at 14:23

I am generating signal and facing the strange behaviour

3 Answers3