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13 .\" manual page may be incorrect or out-of-date.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 2009-06-21 "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 parent 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
114 Signal handlers are inherited, but not shared.
115 Signals to the parent
116 arrive after the child releases the parent's memory
117 (i.e., after the child terminates
120 .SS "Historic Description"
123 is implemented using copy-on-write pages, so the only penalty incurred by
125 is the time and memory required to duplicate the parent's page tables,
126 and to create a unique task structure for the child.
127 However, in the bad old days a
129 would require making a complete copy of the caller's data space,
130 often needlessly, since usually immediately afterwards an
133 Thus, for greater efficiency, BSD introduced the
135 system call, which did not fully copy the address space of
136 the parent process, but borrowed the parent's memory and thread
137 of control until a call to
140 The parent process was suspended while the
141 child was using its resources.
144 was tricky: for example, not modifying data
145 in the parent process depended on knowing which variables were
148 4.3BSD, POSIX.1-2001.
149 POSIX.1-2008 removes the specification of
151 The requirements put on
153 by the standards are weaker than those put on
155 so an implementation where the two are synonymous is compliant.
156 In particular, the programmer cannot rely on the parent
157 remaining blocked until the child either terminates or calls
159 and cannot rely on any specific behavior with respect to shared memory.
160 .\" In AIXv3.1 vfork is equivalent to fork.
163 Fork handlers established using
164 .BR pthread_atfork (3)
165 are not called when a multithreaded program employing
166 the NPTL threading library calls
168 Fork handlers are called in this case in a program using the
169 LinuxThreads threading library.
172 for a description of Linux threading libraries.)
176 system call appeared in 3.0BSD.
177 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
178 .\" present, but definitely on its way out'.
179 In 4.4BSD it was made synonymous to
181 but NetBSD introduced it again,
182 cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
183 In Linux, it has been equivalent to
185 until 2.2.0-pre6 or so.
186 Since 2.2.0-pre9 (on i386, somewhat later on
187 other architectures) it is an independent system call.
188 Support was added in glibc 2.0.112.
190 It is rather unfortunate that Linux revived this specter from the past.
191 The BSD man page states:
192 "This system call will be eliminated when proper system sharing mechanisms
194 Users should not depend on the memory sharing semantics of
196 as it will, in that case, be made synonymous to
200 Details of the signal handling are obscure and differ between systems.
201 The BSD man page states:
202 "To avoid a possible deadlock situation, processes that are children
209 signals; rather, output or
211 are allowed and input attempts result in an end-of-file indication."
213 .\" As far as I can tell, the following is not true in 2.6.19:
214 .\" Currently (Linux 2.3.25),
218 .\" and requires a kernel patch.