fork()ing hell!! Post: 13998

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Top Forums Programming fork()ing hell!! Post 13998 by Perderabo on Saturday 26th of January 2002 11:25:44 AM

01-26-2002

Administrator Emeritus

You guys are making this much harder than it needs to be. Let's start by forking one process and storing its pid:

Code:

#include &lt;stdio.h>
#include &lt;unistd.h>

void main()
{
        int pid, parentpid, childpid;

        parentpid=getpid();
        printf("I am the parent process and my pid is %d\n", getpid());

        if (pid=fork()) {
                childpid=pid;
        } else {
                printf("I am a child process and my pid is %d\n", getpid());
                exit(0);
        }

        printf("I am still the parent process and my pid is %d\n", getpid());
        exit(0);
}

Here the child process just displays its pid then exits. You will probably want to do more with your child processes, but after your children take care of business they must exit so they participate in any further forking. Also in my example the parent process exits fairly quickly. This means that init will inherit the child process and will reap it when it dies. If I wanted to keep the parent around, I would need to insure that it issues wait() calls for each child who dies. If I didn't do this, the children would become zombies. I usually just let the parent die.

Once we have some code that does what we want, if we want to do it n times, we use a loop:

Code:

#include &lt;stdio.h>
#include &lt;unistd.h>

void main()
{
        int n, pid, parentpid, childpids[10];

        parentpid=getpid();
        printf("I am the parent process and my pid is %d\n", getpid());

        for(n=0; n<5; n++) {
                if (pid=fork()) {
                        childpids[n]=pid;
                } else {
                        printf("I am a child process and my pid is %d\n", 
                                        getpid());
                        exit(0);
                }
        }

        printf("I am still the parent process and my pid is %d\n", getpid());
        exit(0);
}

As requested by the OP, the children's pids are recorded in an array. But I still just let the parent die.

Perderabo

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LEARN ABOUT FREEBSD

vfork

VFORK(2)						      BSD System Calls Manual							  VFORK(2)

NAME

     vfork -- create a new process without copying the address space

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <unistd.h>

     pid_t
     vfork(void);

DESCRIPTION

     The vfork() system call can be used to create new processes without fully copying the address space of the old process, which is horrendously
     inefficient in a paged environment.  It is useful when the purpose of fork(2) would have been to create a new system context for an
     execve(2).  The vfork() system call differs from fork(2) in that the child borrows the parent's memory and thread of control until a call to
     execve(2) or an exit (either by a call to _exit(2) or abnormally).  The parent process is suspended while the child is using its resources.

     The vfork() system call returns 0 in the child's context and (later) the pid of the child in the parent's context.

     The vfork() system call can normally be used just like fork(2).  It does not work, however, to return while running in the child's context
     from the procedure that called vfork() since the eventual return from vfork() would then return to a no longer existent stack frame.  Be
     careful, also, to call _exit(2) rather than exit(3) if you cannot execve(2), since exit(3) will flush and close standard I/O channels, and
     thereby mess up the parent processes standard I/O data structures.  (Even with fork(2) it is wrong to call exit(3) since buffered data would
     then be flushed twice.)

RETURN VALUES

     Same as for fork(2).

SEE ALSO

     _exit(2), execve(2), fork(2), rfork(2), sigaction(2), wait(2), exit(3)

HISTORY

     The vfork() system call appeared in 2.9BSD.

BUGS

     To avoid a possible deadlock situation, processes that are children in the middle of a vfork() are never sent SIGTTOU or SIGTTIN signals;
     rather, output or ioctl(2) calls are allowed and input attempts result in an end-of-file indication.

BSD
								 November 13, 2009							       BSD