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
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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13 .\" manual page may be incorrect or out-of-date.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)):
42 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
44 .SS "Standard Description"
48 function has the same effect as
50 except that the behavior is undefined if the process created by
52 either modifies any data other than a variable of type
54 used to store the return value from
56 or returns from the function in which
58 was called, or calls any other function before successfully calling
63 .SS "Linux Description"
67 creates a child process of the calling process.
68 For details and return value and errors, see
74 It is used to create new processes without copying the page tables of
76 It may be useful in performance-sensitive applications
77 where a child will be created which then immediately issues an
83 in that the parent is suspended until the child terminates
87 or abnormally, after delivery of a fatal signal),
90 Until that point, the child shares all memory with its parent,
92 The child must not return from the current function or call
97 Signal handlers are inherited, but not shared.
99 arrive after the child releases the parent's memory
100 (i.e., after the child terminates
103 .SS "Historic Description"
106 is implemented using copy-on-write pages, so the only penalty incurred by
108 is the time and memory required to duplicate the parent's page tables,
109 and to create a unique task structure for the child.
110 However, in the bad old days a
112 would require making a complete copy of the caller's data space,
113 often needlessly, since usually immediately afterwards an
116 Thus, for greater efficiency, BSD introduced the
118 system call, which did not fully copy the address space of
119 the parent process, but borrowed the parent's memory and thread
120 of control until a call to
123 The parent process was suspended while the
124 child was using its resources.
127 was tricky: for example, not modifying data
128 in the parent process depended on knowing which variables are
131 4.3BSD, POSIX.1-2001.
132 POSIX.1-2008 removes the specification of
134 The requirements put on
136 by the standards are weaker than those put on
138 so an implementation where the two are synonymous is compliant.
139 In particular, the programmer cannot rely on the parent
140 remaining blocked until the child either terminates or calls
142 and cannot rely on any specific behavior with respect to shared memory.
143 .\" In AIXv3.1 vfork is equivalent to fork.
146 Fork handlers established using
147 .BR pthread_atfork (3)
148 are not called when a multithreaded program employing
149 the NPTL threading library calls
151 Fork handlers are called in this case in a program using the
152 LinuxThreads threading library.
155 for a description of Linux threading libraries.)
159 system call appeared in 3.0BSD.
160 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
161 .\" present, but definitely on its way out'.
162 In 4.4BSD it was made synonymous to
164 but NetBSD introduced it again,
165 cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
166 In Linux, it has been equivalent to
168 until 2.2.0-pre6 or so.
169 Since 2.2.0-pre9 (on i386, somewhat later on
170 other architectures) it is an independent system call.
171 Support was added in glibc 2.0.112.
173 It is rather unfortunate that Linux revived this specter from the past.
174 The BSD man page states:
175 "This system call will be eliminated when proper system sharing mechanisms
177 Users should not depend on the memory sharing semantics of
179 as it will, in that case, be made synonymous to
183 Details of the signal handling are obscure and differ between systems.
184 The BSD man page states:
185 "To avoid a possible deadlock situation, processes that are children
192 signals; rather, output or
194 are allowed and input attempts result in an end-of-file indication."
196 .\" As far as I can tell, the following is not true in 2.6.19:
197 .\" Currently (Linux 2.3.25),
201 .\" and requires a kernel patch.