trouble with loop counting Post: 302571189

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Top Forums Programming trouble with loop counting Post 302571189 by Corona688 on Sunday 6th of November 2011 04:27:59 PM

11-06-2011

Registered User

This code is intentionally obtuse. They modify the same counting variable many times per loop. They allow the child to continue looping after fork() -- meaning, the child itself will call fork() on the next loop! And the child will have its own, independent value of var!

The most effective way I see to tackle this program is brute force. Make a big chart of what changes when. Mentally work your way through the program and fill it out.

Their random indenting makes it impossible to see where anything begins or ends, too; tis should help.

Code:

#include<stdio.h>
#include<stdlib.h>
#include<sys/types.h>
#include<unistd.h>
#include<wait.h>

int main (int argc, char *argv[]){

    int var = 0;
    pid_t p;
    
    while(++var <5)
    {/*as long as it is less than this it will do fork*/
        if((p=fork()) < 0)
        {
            perror("fork error");
            exit(1);
        }
        else if(p ==0)
        { /*if it is the child*/
            var--; /*count down*/
        }
        else
        {
            if(waitpid(p,NULL,0) != p){/*if waipid is not equal p*/
                perror("waitpid error");
                exit(1);/*exit with status signal1*/
            }/*end if*/

            var++;
        } /*end if/else*/
        var++;
    }/*end while*/

    printf("pid=%d, ppid=%d var=%d, p=%d\n", getpid(), getppid(), var, p);
    exit(0);
}/*end main*/

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LEARN ABOUT REDHAT

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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