1 .\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999
3 .\" Permission is granted to make and distribute verbatim copies of this
4 .\" manual provided the copyright notice and this permission notice are
5 .\" preserved on all copies.
7 .\" Permission is granted to copy and distribute modified versions of this
8 .\" manual under the conditions for verbatim copying, provided that the
9 .\" entire resulting derived work is distributed under the terms of a
10 .\" permission notice identical to this one.
12 .\" Since the Linux kernel and libraries are constantly changing, this
13 .\" manual page may be incorrect or out-of-date. The author(s) assume no
14 .\" responsibility for errors or omissions, or for damages resulting from
15 .\" the use of the information contained herein. The author(s) may not
16 .\" have taken the same level of care in the production of this manual,
17 .\" which is licensed free of charge, as they might when working
20 .\" Formatted or processed versions of this manual, if unaccompanied by
21 .\" the source, must acknowledge the copyright and authors of this work.
23 .\" 1999-11-10: Merged text taken from the page contributed by
24 .\" Reed H. Petty (rhp@draper.net)
26 .TH VFORK 2 2012-08-05 "Linux" "Linux Programmer's Manual"
28 vfork \- create a child process and block parent
30 .B #include <sys/types.h>
32 .B #include <unistd.h>
37 Feature Test Macro Requirements for glibc (see
38 .BR feature_test_macros (7)):
49 (_XOPEN_SOURCE\ >=\ 500 ||
50 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED) &&
51 !(_POSIX_C_SOURCE\ >=\ 200809L || _XOPEN_SOURCE\ >=\ 700)
55 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500 ||
56 _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
61 .SS "Standard description"
65 function has the same effect as
67 except that the behavior is undefined if the process created by
69 either modifies any data other than a variable of type
71 used to store the return value from
73 or returns from the function in which
75 was called, or calls any other function before successfully calling
80 .SS "Linux description"
84 creates a child process of the calling process.
85 For details and return value and errors, see
91 It is used to create new processes without copying the page tables of
93 It may be useful in performance-sensitive applications
94 where a child is created which then immediately issues an
100 in that the calling thread is suspended until the child terminates
104 or abnormally, after delivery of a fatal signal),
105 or it makes a call to
107 Until that point, the child shares all memory with its parent,
109 The child must not return from the current function or call
116 the child process created by
118 inherits copies of various of the caller's process attributes
119 (e.g., file descriptors, signal dispositions, and current working directory);
122 call differs only in the treatment of the virtual address space,
125 Signals sent to the parent
126 arrive after the child releases the parent's memory
127 (i.e., after the child terminates
130 .SS "Historic description"
133 is implemented using copy-on-write pages, so the only penalty incurred by
135 is the time and memory required to duplicate the parent's page tables,
136 and to create a unique task structure for the child.
137 However, in the bad old days a
139 would require making a complete copy of the caller's data space,
140 often needlessly, since usually immediately afterward an
143 Thus, for greater efficiency, BSD introduced the
145 system call, which did not fully copy the address space of
146 the parent process, but borrowed the parent's memory and thread
147 of control until a call to
150 The parent process was suspended while the
151 child was using its resources.
154 was tricky: for example, not modifying data
155 in the parent process depended on knowing which variables were
158 4.3BSD; POSIX.1-2001 (but marked OBSOLETE).
159 POSIX.1-2008 removes the specification of
162 The requirements put on
164 by the standards are weaker than those put on
166 so an implementation where the two are synonymous is compliant.
167 In particular, the programmer cannot rely on the parent
168 remaining blocked until the child either terminates or calls
170 and cannot rely on any specific behavior with respect to shared memory.
171 .\" In AIXv3.1 vfork is equivalent to fork.
174 Some consider the semantics of
176 to be an architectural blemish, and the 4.2BSD man page stated:
177 "This system call will be eliminated when proper system sharing mechanisms
179 Users should not depend on the memory sharing semantics of
181 as it will, in that case, be made synonymous to
184 However, even though modern memory management hardware
185 has decreased the performance difference between
189 there are various reasons why Linux and other systems have retained
192 Some performance-critical applications require the small performance
193 advantage conferred by
197 can be implemented on systems that lack a memory-management unit (MMU), but
199 can't be implemented on such systems.
200 (POSIX.1-2008 removed
202 from the standard; the POSIX rationale for the
204 function notes that that function,
205 which provides functionality equivalent to
206 .BR fork (2)+ exec (3),
207 is designed to be implementable on systems that lack an MMU.)
208 .\" http://stackoverflow.com/questions/4259629/what-is-the-difference-between-fork-and-vfork
209 .\" http://developers.sun.com/solaris/articles/subprocess/subprocess.html
210 .\" http://mailman.uclinux.org/pipermail/uclinux-dev/2009-April/000684.html
212 Fork handlers established using
213 .BR pthread_atfork (3)
214 are not called when a multithreaded program employing
215 the NPTL threading library calls
217 Fork handlers are called in this case in a program using the
218 LinuxThreads threading library.
221 for a description of Linux threading libraries.)
225 is equivalent to calling
231 CLONE_VM | CLONE_VFORK | SIGCHLD
235 system call appeared in 3.0BSD.
236 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
237 .\" present, but definitely on its way out'.
238 In 4.4BSD it was made synonymous to
240 but NetBSD introduced it again,
242 .UR http://www.netbsd.org\:/Documentation\:/kernel\:/vfork.html
244 In Linux, it has been equivalent to
246 until 2.2.0-pre6 or so.
247 Since 2.2.0-pre9 (on i386, somewhat later on
248 other architectures) it is an independent system call.
249 Support was added in glibc 2.0.112.
252 Details of the signal handling are obscure and differ between systems.
253 The BSD man page states:
254 "To avoid a possible deadlock situation, processes that are children
261 signals; rather, output or
263 are allowed and input attempts result in an end-of-file indication."
265 .\" As far as I can tell, the following is not true in 2.6.19:
266 .\" Currently (Linux 2.3.25),
270 .\" and requires a kernel patch.