c - Why signal handling is malfunctioning? -

i have signal handling snippet somehow malfunctioning on mac , virtual linux box @ koding.com on office linux pc working..can please tell me why..

#include <stdio.h> #include <signal.h> #include <unistd.h>  void my_isr(int n){    printf("hello world");    signal(sigint, sig_dfl);  }   int main(){    signal(sigint, my_isr);    printf("pid = %d\n", getpid());    while(1);     return 0;  } 

when pressing ctrl+c not printing hello world on first time re-modifying sigint signal action & hence exiting program when press ctrl+c second time. can explain me why?

you not allowed call every function in signal handler.

read signal(7). only async signal safe functions can called (directly or indirectly) from signal handler, , printf not such function. if want reliably "print" inside signal handler (which don't recommend), can use low-level write(2) syscall (it async signal safe).

so you've got undefined behavior. this explains why bad.

the recommended way set volatile sigatomic_t flag in signal handler, , test outside of (e.g. in while loop...). , forgot call fflush(3). might more lucky ending printf format string \n since stdout line-buffered!

of course, changing printf inside signal handler still ub, \n, appear work.

here conforming version of program....

#include <signal.h> #include <unistd.h> #include <stdio.h>  volatile sig_atomic_t got_signal;  void my_sigint_handler (int signum) {   if (signum == sigint) // true!     got_signal = 1; #define interrupt_message "interrupted!\n"   write(stdout_fileno, interrupt_message, strlen(interrupt_message)); };  int main(int argc, char**argv) {   struct sigaction act_int;   memset (&act_int, 0, sizeof(act_int));   act_int.sa_handler = my_sigint_handler;   if (sigaction(sigint, &act_int, null)) {      perror("sigaction"); exit(exit_failure);   };   printf ("start %s pid %d\n", argv[0], (int)getpid());   while (!got_signal) {   };   printf ("ended %s after signal\n", argv[0]);   return 0; } 

a useful (and permissible) trick write(2) single byte -inside signal handler- on pipe(7) self (you set pipe using pipe(2) @ program initialization), , in event loop poll(2) read end of pipe.