kill signal example
25,806
Your code has a major race condition. You do not ensure that the child has finished calling signal
before the parent sends the signals. You either need to use some kind of synchronization primitive to make the parent wait for the child to install the handlers, or you need to install the signal handlers before forking so the child inherits them.
Here's the easiest way I know to synchronize processes like this:
- Before forking, call
pipe(p)
to create a pipe. fork()
.- In the parent,
close(p[1]);
(the writing end) andread(p[0], &dummy, 1);
- In the child,
close(p[0]);
andclose(p[1]);
after installing the signal handlers. - When the parent returns from
read
, you can be sure the child has setup its signal handlers. You can alsoclose(p[0]);
in the parent at this point.
Edit 2: Perhaps a better and easier approach:
- Before forking, call
sigprocmask
to block all signals and save the old signal mask. - In the parent, call
sigprocmask
again right after forking to restore the original signal mask. - In the child, call
sigprocmask
right after installing the signal handlers to restore the original signal mask.
Author by
Christian Wagner
Updated on December 19, 2020Comments
-
Christian Wagner over 3 years
I'm trying this example that I took from: http://www.cs.cf.ac.uk/Dave/C/node24.html:
#include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <signal.h> void sighup(); /* routines child will call upon sigtrap */ void sigint(); void sigquit(); main() { int pid; /* get child process */ if ((pid = fork()) < 0) { perror("fork"); exit(1); } if (pid == 0) { /* child */ printf("\nI am the new child!\n\n"); signal(SIGHUP,sighup); /* set function calls */ signal(SIGINT,sigint); signal(SIGQUIT, sigquit); printf("\nChild going to loop...\n\n"); for(;;); /* loop for ever */ } else /* parent */ { /* pid hold id of child */ printf("\nPARENT: sending SIGHUP\n\n"); kill(pid,SIGHUP); sleep(3); /* pause for 3 secs */ printf("\nPARENT: sending SIGINT\n\n"); kill(pid,SIGINT); sleep(3); /* pause for 3 secs */ printf("\nPARENT: sending SIGQUIT\n\n"); kill(pid,SIGQUIT); sleep(3); } } void sighup() { signal(SIGHUP,sighup); /* reset signal */ printf("CHILD: I have received a SIGHUP\n"); } void sigint() { signal(SIGINT,sigint); /* reset signal */ printf("CHILD: I have received a SIGINT\n"); } void sigquit() { printf("My DADDY has Killed me!!!\n"); exit(0); }
But I do not see any output from the child process.
Is it the expected behaviour? If so; why?
Thank you very much!
-
danglingpointer almost 7 yearsOP, mentioned no output from the child process(). getpid() will only give the id of the process. will that solve the problem?