From: Jason Stubbs (j.stubbslinkthink.co.jp)
Date: Mon Nov 06 2006 - 03:20:14 CST

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On Monday 06 November 2006 17:54, Kasicass wrote:
> Hi all,
>
> I just run the following program in obsd 3.9, but it doesn't work as
> expected. As said that child process created by vfork should run in the
> address space of the parent, until it calls exec/exit.
>
> ---------
> #include <sys/types.h>
> #include <unistd.h>
> #include <stdio.h>
> #include <stdlib.h>
>
> int glob = 6; /* external variable in initialized data */
>
> int
> main(void)
> {
> int var; /* automatic variable on the stack */
> pid_t pid;
>
> var = 88;
> printf("before fork\n"); /* we don't flush stdout */
>
> if ( (pid = vfork()) < 0 )
> ;
> else if (pid == 0)
> { /* child */
> glob++; /* modify variables */
> var++;
> _exit(0); /* child terminates */
> }
>
> printf("pid = %d, glob = %d, var = %d\n", getpid(), glob, var);
> exit(0);
> }
> ---------
> In obsd, result is
>
> before fork
> pid = xxx, glob = 7, var = 89
>
> But in Linux 2.6/FreeBSD 5.4-RELEASE, it works fine. The result is
>
> before fork
> pid = xxx, glob = 6, var = 88
> ---------
>
> Could anyone give me a brief description about why it runs like that ?

The following excerpt is taken from the Linux man page for vfork.

       (From SUSv2 / POSIX draft.) The vfork() function has the same effect
       as fork(), except that the behaviour is undefined if the process cre-
       ated by vfork() either modifies any data other than a variable of type
       pid_t used to store the return value from vfork(), or returns from the
       function in which vfork() was called, or calls any other function
       before successfully calling _exit() or one of the exec() family of
       functions.

Of note here is "the behaviour is undefined if the process created by vfork()
... modifies any data other than a variable of type pid_t used to store the
return value from vfork()". I can't answer why OpenBSD differs (although I
could make some educated guesses) but either implementation is correct if
going by the above excerpt.

--
Jason Stubbs

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