3BSD/usr/man/man2/vfork.2

.UC
.TH VFORK 2
.SH NAME
vfork \- spawn new process in a virtual memory efficient way
.SH SYNOPSIS
\fBvfork()\fR
.SH DESCRIPTION
.I Vfork
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
.I fork
(2) would have been to create a new system context for an
.I exec.
.I Vfork
differs from fork in that the parent's memory and thread of control run in
the child's system context till a call to
.I exec
(2) or an exit
(either by a call to
.I exit
(2) or abnormally.)
The parent's process slot is suspended while it runs in the child's.
.I Vfork
returns 0 in the child's context and (later) the pid of the child in
the parents context.
.PP
.I Vfork
can normally be used just like
.I fork.
It does not work, however, to return while running in the childs context
from the procedure which called
.I vfork
since the eventual return from
.I vfork
would then return to a no longer existant stack frame.
Be careful, also, to call
.I _exit
rather than
.I exit
if you can't
.I exec,
since
.I exit
will flush and close standard I/O channels, and thereby mess up the
parent processes standard I/O data structures.  (Even with
.I fork
it is wrong to call
.I exit
since buffered data would then be flushed twice.)
.SH SEE ALSO
fork(2), exec(2), wait(2),
.SH DIAGNOSTICS
Same as for fork.
.SH BUGS
Would be unnecessary if
.I fork
were implemented by a mechanism similar to copy-on-write.
The current system does not support this mechanism, however, necessitating
.I vfork.