man2/vfork.2

   1 .\" Copyright (c) 1999 Andries Brouwer (aeb@cwi.nl), 1 Nov 1999
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  23 .\" 1999-11-10: Merged text taken from the page contributed by
  24 .\" Reed H. Petty (rhp@draper.net)
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  26 .TH VFORK 2 2008-10-29 "Linux" "Linux Programmer's Manual"
  27 .SH NAME
  28 vfork \- create a child process and block parent
  29 .SH SYNOPSIS
  30 .B #include <sys/types.h>
  31 .br
  32 .B #include <unistd.h>
  33 .sp
  34 .B pid_t vfork(void);
  35 .sp
  36 .in -4n
  37 Feature Test Macro Requirements for glibc (see
  38 .BR feature_test_macros (7)):
  39 .in
  40 .sp
  41 .BR vfork ():
  42 _BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
  43 .SH DESCRIPTION
  44 .SS "Standard Description"
  45 (From SUSv2 / POSIX draft.)
  46 The
  47 .BR vfork ()
  48 function has the same effect as
  49 .BR fork (2),
  50 except that the behavior is undefined if the process created by
  51 .BR vfork ()
  52 either modifies any data other than a variable of type
  53 .I pid_t
  54 used to store the return value from
  55 .BR vfork (),
  56 or returns from the function in which
  57 .BR vfork ()
  58 was called, or calls any other function before successfully calling
  59 .BR _exit (2)
  60 or one of the
  61 .BR exec (3)
  62 family of functions.
  63 .SS "Linux Description"
  64 .BR vfork (),
  65 just like
  66 .BR fork (2),
  67 creates a child process of the calling process.
  68 For details and return value and errors, see
  69 .BR fork (2).
  70 .PP
  71 .BR vfork ()
  72 is a special case of
  73 .BR clone (2).
  74 It is used to create new processes without copying the page tables of
  75 the parent process.
  76 It may be useful in performance-sensitive applications
  77 where a child will be created which then immediately issues an
  78 .BR execve (2).
  79 .PP
  80 .BR vfork ()
  81 differs from
  82 .BR fork (2)
  83 in that the parent is suspended until the child makes a call to
  84 .BR execve (2)
  85 or
  86 .BR _exit (2).
  87 The child shares all memory with its parent, including the stack, until
  88 .BR execve (2)
  89 is issued by the child.
  90 The child must not return from the current function or call
  91 .BR exit (3),
  92 but may call
  93 .BR _exit (2).
  94 .PP
  95 Signal handlers are inherited, but not shared.
  96 Signals to the parent
  97 arrive after the child releases the parent's memory
  98 (i.e., after the child calls
  99 .BR _exit (2)
 100 or
 101 .BR execve (2)).
 102 .SS "Historic Description"
 103 Under Linux,
 104 .BR fork (2)
 105 is implemented using copy-on-write pages, so the only penalty incurred by
 106 .BR fork (2)
 107 is the time and memory required to duplicate the parent's page tables,
 108 and to create a unique task structure for the child.
 109 However, in the bad old days a
 110 .BR fork (2)
 111 would require making a complete copy of the caller's data space,
 112 often needlessly, since usually immediately afterwards an
 113 .BR exec (3)
 114 is done.
 115 Thus, for greater efficiency, BSD introduced the
 116 .BR vfork ()
 117 system call, that did not fully copy the address space of
 118 the parent process, but borrowed the parent's memory and thread
 119 of control until a call to
 120 .BR execve (2)
 121 or an exit occurred.
 122 The parent process was suspended while the
 123 child was using its resources.
 124 The use of
 125 .BR vfork ()
 126 was tricky: for example, not modifying data
 127 in the parent process depended on knowing which variables are
 128 held in a register.
 129 .SH "CONFORMING TO"
 130 4.3BSD, POSIX.1-2001.
 131 POSIX.1-2008 removes the specification of
 132 .BR vfork ().
 133 The requirements put on
 134 .BR vfork ()
 135 by the standards are weaker than those put on
 136 .BR fork (2),
 137 so an implementation where the two are synonymous is compliant.
 138 In particular, the programmer cannot
 139 rely on the parent remaining blocked until a call of
 140 .BR execve (2)
 141 or
 142 .BR _exit (2)
 143 and cannot rely on any specific behavior with respect to shared memory.
 144 .\" In AIXv3.1 vfork is equivalent to fork.
 145 .SH NOTES
 146 .SS Linux Notes
 147 Fork handlers established using
 148 .BR pthread_atfork (3)
 149 are not called when a multithreaded program employing
 150 the NPTL threading library calls
 151 .BR vfork ().
 152 Fork handlers are called in this case in a program using the
 153 LinuxThreads threading library.
 154 (See
 155 .BR pthreads (7)
 156 for a description of Linux threading libraries.)
 157 .SS History
 158 The
 159 .BR vfork ()
 160 system call appeared in 3.0BSD.
 161 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
 162 .\" present, but definitely on its way out'.
 163 In 4.4BSD it was made synonymous to
 164 .BR fork (2)
 165 but NetBSD introduced it again,
 166 cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
 167 In Linux, it has been equivalent to
 168 .BR fork (2)
 169 until 2.2.0-pre6 or so.
 170 Since 2.2.0-pre9 (on i386, somewhat later on
 171 other architectures) it is an independent system call.
 172 Support was added in glibc 2.0.112.
 173 .SH BUGS
 174 It is rather unfortunate that Linux revived this specter from the past.
 175 The BSD man page states:
 176 "This system call will be eliminated when proper system sharing mechanisms
 177 are implemented.
 178 Users should not depend on the memory sharing semantics of
 179 .BR vfork ()
 180 as it will, in that case, be made synonymous to
 181 .BR fork (2).\c
 182 "
 183
 184 Details of the signal handling are obscure and differ between systems.
 185 The BSD man page states:
 186 "To avoid a possible deadlock situation, processes that are children
 187 in the middle of a
 188 .BR vfork ()
 189 are never sent
 190 .B SIGTTOU
 191 or
 192 .B SIGTTIN
 193 signals; rather, output or
 194 .IR ioctl s
 195 are allowed and input attempts result in an end-of-file indication."
 196 .\"
 197 .\" As far as I can tell, the following is not true in 2.6.19:
 198 .\" Currently (Linux 2.3.25),
 199 .\" .BR strace (1)
 200 .\" cannot follow
 201 .\" .BR vfork ()
 202 .\" and requires a kernel patch.
 203 .SH "SEE ALSO"
 204 .BR clone (2),
 205 .BR execve (2),
 206 .BR fork (2),
 207 .BR unshare (2),
 208 .BR wait (2)