Nested signal handlers in C

Question

I want to work on signal handlers in the context of two independent processes namely writer and reader for notification. The writer sends a first signal SIGUSR1 to the reader which loops till it hears the second signal SIGUSR2 from the writer.

reader.c

static volatile sig_atomic_t done_waiting; 

int handler1(int signal){
    done_waiting = 0;
     while( !done_waiting ){
            (void)fprintf(stdout, " reader waiting for sigusr2: done_waiting = %d\n", done_waiting );
    }
(void)fprintf(stdout, " reader received sigusr2 \n);
}

int handler2 (int signal){
         done_waiting = 1;
}

main(){
    signal(SIGUSR1, handler1);
    signal(SIGUSR2, handler2);
    sleep(5); // sleep till we start worker
}

In writer.c, signals are sent to the reader as

main(){
    kill(pid_reader, SIGUSR1);
    sleep(5);
    kill (pid_reader, SIGUSR2);
}

When I execute reader first followed by worker, the program quits at the while loop. And the writer prints that "No matching processes belonging to you were found".

Is nesting signal handlers allowed and if yes, is it recommended? Also, is there any another alternative mechanism for writer to notify reader that it is ready?

See [How to avoid using `printf()` in signal handlers?](http://stackoverflow.com/questions/16891019/how-to-avoid-using-printf-in-a-signal-handler/) for basic information. You should do as little as possible in a signal handler. There are strict limits on which functions you may call in a signal handlier — and the `printf()` family of functions are not on the list of OK functions (neither, if it's any help, is `strlen()` — but there are good reasons for `printf()` et al and I'm not aware of a good reason for `strlen()`). — Jonathan Leffler, Sep 26 '17 at 22:24
You're not "nesting" signal handlers, you've installed handlers two for different signals. Also, you should look at the `sigsuspend(2)` and related calls, which allow waiting for a signal (rather than the busy-wait you've got here with the `while` loop). — bnaecker, Sep 26 '17 at 22:26

user1048576 · Accepted Answer · 2017-09-26T23:43:37.723

Is maybe nested signals actually what you meant, not nested signal handlers ? To clarify, what will happen if a SIGUSR2 is received while the handler for SIGUSR1 is executing, is that what you mean ? I assume so,

I tested your code, with some modifications, to get the pid for the reader process into the writer process I used the args to main.

The results I get is.

First reader is quiet
After receiving SIGUSR1 it starts continuously writing that it waits for SIGUSR2
When receiving SIGUSR2, it prints "reader received SIGUSR2"

This indicates that it is possible to have nested signals. However I would not say it is recommended as an intentional design. As mentioned in the comments, you should do as little as possible in the signal handlers, definitely not loop in a while-loop. And as also mentioned in the comments, be very careful what functions you call in signal-context, printf() is not OK, even though it may seem to work fine.

Tested on Linux, with the ancient kernel 3.16 and gcc 4.9

Nested signal handlers in C

1 Answers1