[C]Fork and waitpid Post: 302580781

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Top Forums Programming [C]Fork and waitpid Post 302580781 by Corona688 on Friday 9th of December 2011 02:40:32 PM

12-09-2011

Registered User

You're not quitting in your child code, so they finish executing the child code then continue blithely on into the parent code. Since you haven't posted your complete code I can't tell what they'd be doing, but it's possible that some of your children may be fork()-ing and ending up printing like the parent should. That would explain some of the strange PID values.

Instead of using a 9-long if/else chain for 9 pid's, use a loop that can handle any number of PID's with the same code:

You should put \n at the end of the line, not the beginning, because printf only actually prints AFTER it receives a \n. printf("\nthis is a string"); will leave stuff in the buffer that might be printed twice (since fork() copies the buffer along with the rest of the process).

I'm not sure what your status1, status2, ...variables are since you didn't post that code, but since you're not using & anywhere, you're probably doing it wrong. You should give waitpid() a pointer to an integer -- not an integer, and not a bare pointer.

Code:

int main(void)
{
        int n;
        pid_t pids[10];
        for(n=0; n<10; n++)
        {
                pid[n]=fork();
                if(pid[n] == 0) // child code
                {
                        printf("My PID is %d\n", getpid());
                        exit(0); // DON'T FORGET THIS or child will keep executing!
                }
        }

        for(n=0; n<10; n++)
        {
                int s;
                waitpid(pid[n], &s, 0); // It needs a pointer TO an integer -- an address pointing to a valid 'int'.
                printf("Child %d has completed\n", pid[n]);
        }

        return(0);
}

Last edited by Corona688; 12-09-2011 at 04:44 PM..

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pthread_atfork

PTHREAD_ATFORK(3)					     Library Functions Manual						 PTHREAD_ATFORK(3)

NAME

       pthread_atfork - register handlers to be called at fork(2) time

SYNOPSIS

       #include <pthread.h>

       int pthread_atfork(void (*prepare)(void), void (*parent)(void), void (*child)(void));

DESCRIPTION

       pthread_atfork  registers handler functions to be called just before and just after a new process is created with fork(2). The prepare han-
       dler will be called from the parent process, just before the new process is created. The parent handler will  be  called  from  the  parent
       process, just before fork(2) returns. The child handler will be called from the child process, just before fork(2) returns.

       One or several of the three handlers prepare, parent and child can be given as NULL, meaning that no handler needs to be called at the cor-
       responding point.

       pthread_atfork can be called several times to install several sets of handlers. At fork(2) time, the prepare handlers are  called  in  LIFO
       order  (last  added  with  pthread_atfork,  first  called before fork), while the parent and child handlers are called in FIFO order (first
       added, first called).

       To understand the purpose of pthread_atfork, recall that fork(2) duplicates the whole memory space,  including  mutexes	in  their  current
       locking	state, but only the calling thread: other threads are not running in the child process.  The mutexes are not usable after the fork
       and must be initialized with pthread_mutex_init in the child process.  This is a limitation of the  current  implementation  and  might	or
       might not be present in future versions.

RETURN VALUE

       pthread_atfork returns 0 on success and a non-zero error code on error.

ERRORS

       ENOMEM insufficient memory available to register the handlers.

AUTHOR

       Xavier Leroy <Xavier.Leroy@inria.fr>

SEE ALSO

       fork(2), pthread_mutex_lock(3), pthread_mutex_unlock(3).

								   LinuxThreads 						 PTHREAD_ATFORK(3)

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