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Full Discussion: fork and stdin
Top Forums UNIX for Dummies Questions & Answers fork and stdin Post 302262603 by milouz on Friday 28th of November 2008 05:26:04 AM
Old 11-28-2008
fork and stdin
When a process fork(), the child share the same file descriptors as his father. Thus, they share the same stdin. Quick and dirty exemple below (sorry for the ugly gets() call) :

Code:
#include <stdio.h>
#include <unistd.h>

int main()
{
        char buf[512];

        if (fork()) { /*parent */
                while(1) {
                        gets(buf);
                        printf("[%d] parent: %s\n", getpid(), buf);
                }
        } else { /* child */
                while(1) {
                        gets(buf);
                        printf("[%d] children: %s\n", getpid(), buf);
                }
        }
}

Question 1 :
If I'm right, after the fork(), the parent and the child belong to the same group process which is a foreground group and which share the same console. As there is a concurrent read on stdin, I was thinking that the process that read the lines in input is pick up randomly. However, on my Linux system, the child always read first. Is it normal ?

Question 2:
If I launch the command like that :
Code:
$ ./a.out > /tmp/foobar

I was thinking that after the redirection, the parent and the child would both write on /tmp/foobar. But the /tmp/foobar file remains totaly empty !
Why ?

Thanks for your help,

Arnauld
 

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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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