vfork.2

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 2 代码 · 共 80 行

.\" SCCSID: @(#)vfork.2	8.1	9/11/90
.TH vfork 2
.SH Name
vfork \- spawn new process in a virtual memory-efficient way
.SH Syntax
.B pid = vfork()
.br
.B int pid;
.SH Description
.NXR "vfork system call"
.NXR "process" "creating efficiently"
.NXA "fork system call" "vfork system call"
.NXA "vfork system call" "fork system call"
The
.PN vfork
can be used to create new processes without fully copying the address
space of the old process, which is inefficient in a paged
environment.  It is useful when the purpose of 
.PN fork 
would have been to create a new system context for an
.PN execve .
The
.PN vfork
system call differs from
.PN fork
in that the child borrows the parent's memory and thread of
control until a call to 
.PN execve 
or an exit (either by a call to
.MS exit 2
or abnormally.)
The parent process is suspended while the child is using its resources.
.PP
The
.PN vfork
system call returns a value of zero (0) in the child's context and,
later, the pid of the child in
the parent's context.
.PP
The
.PN vfork
system call
can normally be used just like
.PN fork .
It does not work, however, to return while running in the childs context
from the procedure which called
.PN vfork ,
because the eventual return from
.PN vfork
would then return to a nonexistent stack frame.
Be careful, also, to call
.I _exit
rather than
.I exit
if you cannot call
.PN execve ,
because
.I exit
will flush and close standard I/O channels and thereby cause problems in the
parent process's standard I/O data structures.
Even with
.PN fork
it is wrong to call
.IR exit ,
because buffered data would then be flushed twice.
.SH Restrictions
To avoid a possible deadlock situation,
processes which are children in the middle
of a
.PN vfork
are never sent SIGTTOU or SIGTTIN signals.  Rather,
output or
.IR ioctl s
are allowed, and input attempts result in an end-of-file indication.
.SH Diagnostics
Same as for
.PN fork .
.SH See Also
execve(2), fork(2), sigvec(2), wait(2)