Date: Sun, 29 May 2016 12:16:58 +0000 (UTC) From: Jilles Tjoelker <jilles@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-stable@freebsd.org, svn-src-stable-10@freebsd.org Subject: svn commit: r300954 - stable/10/lib/libc/sys Message-ID: <201605291216.u4TCGwBB034344@repo.freebsd.org>
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Author: jilles Date: Sun May 29 12:16:58 2016 New Revision: 300954 URL: https://svnweb.freebsd.org/changeset/base/300954 Log: MFC r300420: vfork(2): Mention some risks of calling vfork() from application code. Modified: stable/10/lib/libc/sys/vfork.2 Modified: stable/10/lib/libc/sys/vfork.2 ============================================================================== --- stable/10/lib/libc/sys/vfork.2 Sun May 29 11:54:09 2016 (r300953) +++ stable/10/lib/libc/sys/vfork.2 Sun May 29 12:16:58 2016 (r300954) @@ -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
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