Date: Thu, 26 Aug 1999 10:40:29 -0400 (EDT) From: Andrew Gallatin <gallatin@cs.duke.edu> To: freebsd-hackers@freebsd.org Subject: undocumented vfork behaviour? Message-ID: <14277.18939.326895.354112@grasshopper.cs.duke.edu>
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I'm working on getting linux/alpha compatability going in
FreeBSD/alpha & I stumbled over some odd fork behaviour that I was
hoping somebody could shed some light on.
Specifically, when a linux/alpha process forks, the child's return
value is the parent's pid rather than zero. I tracked this behaviour
down to the child's code path in the alpha's cpu_fork():
/*
* Set up return-value registers as fork() libc stub expects.
*/
p2tf->tf_regs[FRAME_V0] = p1->p_pid; /* parent's pid */
p2tf->tf_regs[FRAME_A3] = 0; /* no error */
p2tf->tf_regs[FRAME_A4] = 1; /* is child */
In the i386 & mips libc Ovfork stubs, I see comments which describe
this behaviour:
/*
* pid = vfork();
*
* %edx == 0 in parent process, %edx == 1 in child process.
* %eax == pid of child in parent, %eax == pid of parent in child.
*
*/
Is this comment purely historical? At least the i386 platform no
longer seems to do this:
If I run the following code on FreeBSD/i386, I see:
parent's pid = 6730
I am the child, result = 0
I must be the parent, result = 6731
Whereas FreeBSD/alpha does this:
parent's pid = 17721
I must be the parent, result = 17721
I must be the parent, result = 17722
Code:
#include <sys/syscall.h>
#include <unistd.h>
main()
{
int result;
printf("parent's pid = %d\n", getpid());
result = syscall(SYS_vfork);
if(result == 0)
printf("I am the child, result = %d\n", result);
else
printf("I must be the parent, result = %d\n", result);
exit();
}
Before I "fix it" to act like FreeBSD/i386 (I've already tried & it seems to
work OK for FreeBSD & OSF1 binaries, and fixes my linux/alpha binary's
problems), I'd like to make sure there aren't any hidden ramifications.
Thanks,
Drew
------------------------------------------------------------------------------
Andrew Gallatin, Sr Systems Programmer http://www.cs.duke.edu/~gallatin
Duke University Email: gallatin@cs.duke.edu
Department of Computer Science Phone: (919) 660-6590
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