NetBSD/lib/libc/sys/vfork.2

.\"	$NetBSD: vfork.2,v 1.12 1998/06/07 04:56:21 enami Exp $
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.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
.\"
.Dd January 3, 1998
.Dt VFORK 2
.Os BSD 4
.Sh NAME
.Nm vfork
.Nd spawn new process in a virtual memory efficient way
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
The
.Nm
system call creates a new process that that does not have a new
virtual address space, but rather shares address space with the
parent, thus avoiding potentially expensive copy-on-write operations
normally associated with creating a new process.
It is useful when the purpose of
.Xr fork 2
would have been to create a new system context for an
.Xr execve 2 .
The
.Nm
system call differs from
.Xr fork 2
in that the child borrows the parent's memory and thread of
control until a call to
.Xr execve 2
or an exit (either by a call to
.Xr _exit 2
or abnormally).
The parent process is suspended while the child is using its resources.
.Pp
The 
.Nm
system call returns 0 in the child's context and (later) the pid
of the child in the parent's context.
.Pp
The 
.Nm
system call can normally be used just like
.Xr fork 2 .
It does not work, however, to return while running in the childs context
from the procedure that called
.Fn vfork
since the eventual return from
.Fn vfork
would then return to a no longer existent stack frame.
Be careful, also, to call
.Xr _exit 2
rather than
.Xr exit 3
if you can't
.Xr execve 2 ,
since
.Xr exit 3
will flush and close standard I/O channels, and thereby mess up the
parent processes standard I/O data structures (even with
.Xr fork 2
it is wrong to call
.Xr exit 3
since buffered data would then be flushed twice).
.Sh RETURN VALUES and ERRORS
Same as for
.Xr fork 2 .
.Sh SEE ALSO
.Xr fork 2 ,
.Xr execve 2 ,
.Xr sigaction 2 ,
.Xr wait 2
.Sh BUGS
.\" XXX We reintroduced the original semantics.
.\" This system call will be eliminated when proper system sharing
.\" mechanisms are implemented. 
Users should not depend on the memory sharing semantics of
.Xr vfork 2
as other ways of speeding up the fork process may be developed in
the future.
.Pp
To avoid a possible deadlock situation, processes that are children
in the middle of a
.Fn vfork
are never sent
.Dv SIGTTOU
or
.Dv SIGTTIN
signals; rather, output or
.Xr ioctl 2
calls are allowed and input attempts result in an end-of-file indication.
.Sh HISTORY
The
.Fn vfork
function call appeared in
.Bx 3.0 .
In
.Bx 4.4 ,
the semantics were changed to only suspend the parent.
The original semantics were reintroduced in
.Nx 1.4 .