.\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999
+.\" and Copyright 2006, 2012, 2017 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
+.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
.\"
.\" 1999-11-10: Merged text taken from the page contributed by
.\" Reed H. Petty (rhp@draper.net)
.\"
-.TH VFORK 2 2010-09-20 "Linux" "Linux Programmer's Manual"
+.TH VFORK 2 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
vfork \- create a child process and block parent
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
-.sp
+.PP
.B pid_t vfork(void);
-.sp
+.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
-.sp
+.PP
.BR vfork ():
.ad l
.RS 4
.TP 4
Since glibc 2.12:
.nf
-_BSD_SOURCE ||
- (_XOPEN_SOURCE\ >=\ 500 ||
- _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
- !(_POSIX_C_SOURCE\ >=\ 200809L || _XOPEN_SOURCE\ >=\ 700)
+(_XOPEN_SOURCE\ >=\ 500) && ! (_POSIX_C_SOURCE\ >=\ 200809L)
+ || /* Since glibc 2.19: */ _DEFAULT_SOURCE
+ || /* Glibc versions <= 2.19: */ _BSD_SOURCE
.TP 4
.fi
Before glibc 2.12:
-_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
-_XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
+_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
+.\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
.PD
.RE
.ad b
.SH DESCRIPTION
-.SS "Standard Description"
+.SS Standard description
(From POSIX.1)
The
.BR vfork ()
or one of the
.BR exec (3)
family of functions.
-.SS "Linux Description"
+.SS Linux description
.BR vfork (),
just like
.BR fork (2),
.BR vfork ()
differs from
.BR fork (2)
-in that the parent is suspended until the child terminates
+in that the calling thread is suspended until the child terminates
(either normally,
by calling
.BR _exit (2),
Until that point, the child shares all memory with its parent,
including the stack.
The child must not return from the current function or call
-.BR exit (3),
-but may call
+.BR exit (3)
+(which would have the effect of calling exit handlers
+established by the parent process and flushing the parent's
+.BR stdio (3)
+buffers), but may call
.BR _exit (2).
.PP
-Signal handlers are inherited, but not shared.
-Signals to the parent
+As with
+.BR fork (2),
+the child process created by
+.BR vfork ()
+inherits copies of various of the caller's process attributes
+(e.g., file descriptors, signal dispositions, and current working directory);
+the
+.BR vfork ()
+call differs only in the treatment of the virtual address space,
+as described above.
+.PP
+Signals sent to the parent
arrive after the child releases the parent's memory
(i.e., after the child terminates
or calls
.BR execve (2)).
-.SS "Historic Description"
+.SS Historic description
Under Linux,
.BR fork (2)
is implemented using copy-on-write pages, so the only penalty incurred by
However, in the bad old days a
.BR fork (2)
would require making a complete copy of the caller's data space,
-often needlessly, since usually immediately afterwards an
+often needlessly, since usually immediately afterward an
.BR exec (3)
is done.
Thus, for greater efficiency, BSD introduced the
was tricky: for example, not modifying data
in the parent process depended on knowing which variables were
held in a register.
-.SH "CONFORMING TO"
-4.3BSD, POSIX.1-2001.
+.SH CONFORMING TO
+4.3BSD; POSIX.1-2001 (but marked OBSOLETE).
POSIX.1-2008 removes the specification of
.BR vfork ().
+.PP
The requirements put on
.BR vfork ()
by the standards are weaker than those put on
and cannot rely on any specific behavior with respect to shared memory.
.\" In AIXv3.1 vfork is equivalent to fork.
.SH NOTES
-.SS Linux Notes
+.PP
+Some consider the semantics of
+.BR vfork ()
+to be an architectural blemish, and the 4.2BSD man page stated:
+"This system call will be eliminated when proper system sharing mechanisms
+are implemented.
+Users should not depend on the memory sharing semantics of
+.BR vfork ()
+as it will, in that case, be made synonymous to
+.BR fork (2).\c
+"
+However, even though modern memory management hardware
+has decreased the performance difference between
+.BR fork (2)
+and
+.BR vfork (),
+there are various reasons why Linux and other systems have retained
+.BR vfork ():
+.IP * 3
+Some performance-critical applications require the small performance
+advantage conferred by
+.BR vfork ().
+.IP *
+.BR vfork ()
+can be implemented on systems that lack a memory-management unit (MMU), but
+.BR fork (2)
+can't be implemented on such systems.
+(POSIX.1-2008 removed
+.BR vfork ()
+from the standard; the POSIX rationale for the
+.BR posix_spawn (3)
+function notes that that function,
+which provides functionality equivalent to
+.BR fork (2)+ exec (3),
+is designed to be implementable on systems that lack an MMU.)
+.\" http://stackoverflow.com/questions/4259629/what-is-the-difference-between-fork-and-vfork
+.\" http://developers.sun.com/solaris/articles/subprocess/subprocess.html
+.\" http://mailman.uclinux.org/pipermail/uclinux-dev/2009-April/000684.html
+.\"
+.IP *
+On systems where memory is constrained,
+.BR vfork ()
+avoids the need to temporarily commit memory (see the description of
+.IR /proc/sys/vm/overcommit_memory
+in
+.BR proc (5))
+in order to execute a new program.
+(This can be especially beneficial where a large parent process wishes
+to execute a small helper program in a child process.)
+By contrast, using
+.BR fork (2)
+in this scenario requires either committing an amount of memory equal
+to the size of the parent process (if strict overcommitting is in force)
+or overcommitting memory with the risk that a process is terminated
+by the out-of-memory (OOM) killer.
+.\"
+.SS Caveats
+The child process should take care not to modify the memory in unintended ways,
+since such changes will be seen by the parent process once
+the child terminates or executes another program.
+In this regard, signal handlers can be especially problematic:
+if a signal handler that is invoked in the child of
+.BR vfork ()
+changes memory, those changes may result in an inconsistent process state
+from the perspective of the parent process
+(e.g., memory changes would be visible in the parent,
+but changes to the state of open file descriptors would not be visible).
+.PP
+When
+.BR vfork ()
+is called in a multithreaded process,
+only the calling thread is suspended until the child terminates
+or executes a new program.
+This means that the child is sharing an address space with other running code.
+This can be dangerous if another thread in the parent process
+changes credentials (using
+.BR setuid (2)
+or similar),
+since there are now two processes with different privilege levels
+running in the same address space.
+As an example of the dangers,
+suppose that a multithreaded program running as root creates a child using
+.BR vfork ().
+After the
+.BR vfork (),
+a thread in the parent process drops the process to an unprivileged user
+in order to run some untrusted code
+(e.g., perhaps via plug-in opened with
+.BR dlopen (3)).
+In this case, attacks are possible where the parent process uses
+.BR mmap (2)
+to map in code that will be executed by the privileged child process.
+.\"
+.SS Linux notes
Fork handlers established using
.BR pthread_atfork (3)
are not called when a multithreaded program employing
(See
.BR pthreads (7)
for a description of Linux threading libraries.)
+.PP
+A call to
+.BR vfork ()
+is equivalent to calling
+.BR clone (2)
+with
+.I flags
+specified as:
+.PP
+ CLONE_VM | CLONE_VFORK | SIGCHLD
.SS History
The
.BR vfork ()
.\" present, but definitely on its way out'.
In 4.4BSD it was made synonymous to
.BR fork (2)
-but NetBSD introduced it again,
-cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
+but NetBSD introduced it again;
+see
+.UR http://www.netbsd.org\:/Documentation\:/kernel\:/vfork.html
+.UE .
In Linux, it has been equivalent to
.BR fork (2)
until 2.2.0-pre6 or so.
other architectures) it is an independent system call.
Support was added in glibc 2.0.112.
.SH BUGS
-It is rather unfortunate that Linux revived this specter from the past.
-The BSD man page states:
-"This system call will be eliminated when proper system sharing mechanisms
-are implemented.
-Users should not depend on the memory sharing semantics of
-.BR vfork ()
-as it will, in that case, be made synonymous to
-.BR fork (2).\c
-"
-
+.PP
Details of the signal handling are obscure and differ between systems.
The BSD man page states:
"To avoid a possible deadlock situation, processes that are children
.\" cannot follow
.\" .BR vfork ()
.\" and requires a kernel patch.
-.SH "SEE ALSO"
+.SH SEE ALSO
.BR clone (2),
.BR execve (2),
+.BR _exit (2),
.BR fork (2),
.BR unshare (2),
.BR wait (2)
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