Problem with infinite loop in signal handler

This is a discussion on Problem with infinite loop in signal handler within the C Programming forums, part of the General Programming Boards category; Hi all, I have problem with handling signals in simple program. Program starts one thread (only task of this thread ...

  1. #1
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    Problem with infinite loop in signal handler

    Hi all,
    I have problem with handling signals in simple program. Program starts
    one thread (only task of this thread is printing "Hello World") and
    after while sends signal to this thread (signal handler is previously
    registered). In signal handler is infinite loop that should suspend
    thread forever. But it suspends whole application. I think this
    problem is related with printf function because when I replace it with
    with linux write sys function problem does not occures. And one more
    info: problem occures not always, but most of time; maybe somewhere
    there is a race condition.
    This is whole code of application:

    Code:
    #include <signal.h>
    #include <stdio.h>
    
    void suspend(int sig) {
       printf("suspending\n");
       fflush(stdout);
       while(1);
    }
    
    
    void* loop_func(void* arg) {
       int i = 0;
       while(1) {
           if(i % 10000 == 0) {
               printf("Hello World %d\n", i);
               fflush(stdout);
           }
           i++;
       }
    }
    
    void wait_some_time() {
       int i;
       for(i = 0; i < 10000000; i++) { }
    }
    
    int main(void) {
    
       struct sigaction sa;
       pthread_t thread1;
    
       sa.sa_handler = suspend;
       sa.sa_flags = SA_RESTART;
       sigemptyset(&sa.sa_mask);
       sigaction(SIGUSR1, &sa, NULL);
    
       pthread_create(&thread1, 0, loop_func, 0);
       wait_some_time();
    
       pthread_kill(thread1, SIGUSR1);
       printf("Waiting for terminate...\n");
       wait_some_time();
    
       return 0;
    }
    Could any body tell me why this signal handler hangs whole
    application? It should hangs only one thread. It seems to be be a bug
    in linux kernel or maybe in C library. How can I solve this problem?

  2. #2
    spurious conceit MK27's Avatar
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    Code:
    void wait_some_time() {
       int i;
       for(i = 0; i < 10000000; i++) { }
    }
    The absolute WORST way to do this. You max out the processor for nothing. Very bad thing to do. Use sleep() or if 1 second is too long, nanosleep().

    The reason the signal handler suspends the whole program is because the signal handler applies to the whole program, not just the thread. If you move the signal handler to the thread, it will work the way you want:
    Code:
    void* loop_func(void* arg) {
       struct sigaction sa; 
       int i = 0;
       sa.sa_handler = suspend;
       sa.sa_flags = SA_RESTART;
       sigemptyset(&sa.sa_mask);
       sigaction(SIGUSR1, &sa, NULL);
       while(1) {
           if(i % 10000 == 0) {
               printf("Hello World %d\n", i); 
               fflush(stdout);
           }
           i++;
       }   
    }
    [edit] Just noticed no <strike> tag...anyway I removed a comment quoted by bithub next (since s/he's right and I'm wrong) but that may seem confusing...
    Last edited by MK27; 07-22-2009 at 02:24 PM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

  3. #3
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    That should be void, not void*
    No, it's correct the way it is.

    Also, you cannot call printf() in a signal handler. Only asychronous-safe function calls are allowed in a signal handler.

  4. #4
    Captain Crash brewbuck's Avatar
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    You aren't allowed to call printf() from a signal handler, period.

    In fact, there is very little you are allowed to do from within a signal handler.
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  5. #5
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by bithub
    No, it's correct the way it is.
    hmm... but then where does loop_func return a pointer, or anything at all?
    C + C++ Compiler: MinGW port of GCC
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    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  6. #6
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    Quote Originally Posted by laserlight View Post
    hmm... but then where does loop_func return a pointer, or anything at all?
    His code doesn't which is a problem. That doesn't change that fact that thread procedure functions are defined as void* (*proc)(void*)

  7. #7
    spurious conceit MK27's Avatar
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    Quote Originally Posted by bithub View Post
    His code doesn't which is a problem. That doesn't change that fact that thread procedure functions are defined as void* (*proc)(void*)
    Yeah. I've barely written anything with threads but I just noticed I did use void*, so hopefully there was a good reason, since I didn't return anything either -- the threads have the same lifespan as the program. Will edit my post.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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