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vfork(2): Mention some risks of calling vfork() from application code.

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MFC after:	1 week
This commit is contained in:
Jilles Tjoelker 2016-05-22 13:32:45 +00:00
parent 306e53bce9
commit 050c87f9a1
1 changed files with 28 additions and 12 deletions
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40
lib/libc/sys/vfork.2
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@ -28,7 +28,7 @@
.\" @(#)vfork.2 8.1 (Berkeley) 6/4/93
.\" $FreeBSD$
.\"
.Dd April 6, 2015
.Dd May 22, 2016
.Dt VFORK 2
.Os
.Sh NAME
@ -41,11 +41,20 @@
.Ft pid_t
.Fn vfork void
.Sh DESCRIPTION
.Bf -symbolic
Since this function is hard to use correctly from application software,
it is recommended to use
.Xr posix_spawn 3
or
.Xr fork 2
instead.
.Ef
.Pp
The
.Fn vfork
system call
can be used to create new processes without fully copying the address
space of the old process, which is horrendously inefficient in a paged
space of the old process, which is inefficient in a paged
environment.
It is useful when the purpose of
.Xr fork 2
@ -56,13 +65,15 @@ The
system call
differs from
.Xr fork 2
in that the child borrows the parent's memory and thread of
control until a call to
in that the child borrows the parent process's address space and the
calling thread's stack
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.
The calling thread is suspended while the child is using its resources.
Other threads continue to run.
.Pp
The
.Fn vfork
@ -70,17 +81,21 @@ system call
returns 0 in the child's context and (later) the pid of the child in
the parent's context.
.Pp
The
.Fn vfork
system call
can normally be used just like
.Xr fork 2 .
It does not work, however, to return while running in the child's context
Many problems can occur when replacing
.Xr fork 2
with
.Fn vfork .
For example, it does not work to return while running in the child's 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.
Also, changing process state which is partially implemented in user space
such as signal handlers with
.Xr libthr 3
will corrupt the parent's state.
.Pp
Be careful, also, to call
.Xr _exit 2
rather than
@ -106,7 +121,8 @@ Same as for
.Xr rfork 2 ,
.Xr sigaction 2 ,
.Xr wait 2 ,
.Xr exit 3
.Xr exit 3 ,
.Xr posix_spawn 3
.Sh HISTORY
The
.Fn vfork