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)
  25 .\"
  26 .TH VFORK 2 1999-11-01 "Linux 2.2.0" "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 .SH "STANDARD DESCRIPTION"
  36 (From SUSv2 / POSIX draft.)
  37 The
  38 .BR vfork ()
  39 function has the same effect as
  40 .BR fork (),
  41 except that the behaviour is undefined if the process created by
  42 .BR vfork ()
  43 either modifies any data other than a variable of type
  44 .I pid_t
  45 used to store the return value from
  46 .BR vfork (),
  47 or returns from the function in which
  48 .BR vfork ()
  49 was called, or calls any other function before successfully calling
  50 .BR _exit ()
  51 or one of the
  52 .BR exec ()
  53 family of functions.
  54 .SH "LINUX DESCRIPTION"
  55 .BR vfork (),
  56 just like
  57 .BR fork (2),
  58 creates a child process of the calling process.
  59 For details and return value and errors, see
  60 .BR fork (2).
  61 .PP
  62 .BR vfork ()
  63 is a special case of
  64 .BR clone (2).
  65 It is used to create new processes without copying the page tables of
  66 the parent process.  It may be useful in performance sensitive applications
  67 where a child will be created which then immediately issues an
  68 .BR execve () .
  69 .PP
  70 .BR vfork ()
  71 differs from
  72 .BR fork ()
  73 in that the parent is suspended until the child makes a call to
  74 .BR execve (2)
  75 or
  76 .BR _exit (2).
  77 The child shares all memory with its parent, including the stack, until
  78 .BR execve ()
  79 is issued by the child.  The child must not return from the
  80 current function or call
  81 .BR exit (),
  82 but may call
  83 .BR _exit ().
  84 .PP
  85 Signal handlers are inherited, but not shared.  Signals to the parent
  86 arrive after the child releases the parent's memory.
  87 .SH "HISTORIC DESCRIPTION"
  88 Under Linux,
  89 .BR fork ()
  90 is implemented using copy-on-write pages, so the only penalty incurred by
  91 .BR fork ()
  92 is the time and memory required to duplicate the parent's page tables,
  93 and to create a unique task structure for the child.
  94 However, in the bad old days a
  95 .BR fork ()
  96 would require making a complete copy of the caller's data space,
  97 often needlessly, since usually immediately afterwards an
  98 .BR exec ()
  99 is done. Thus, for greater efficiency, BSD introduced the
 100 .BR vfork ()
 101 system call, that did not fully copy the address space of
 102 the parent process, but borrowed the parent's memory and thread
 103 of control until a call to
 104 .BR execve ()
 105 or an exit occurred. The parent process was suspended while the
 106 child was using its resources.
 107 The use of
 108 .BR vfork ()
 109 was tricky: for example, not modifying data
 110 in the parent process depended on knowing which variables are
 111 held in a register.
 112 .SH BUGS
 113 It is rather unfortunate that Linux revived this spectre from the past.
 114 The BSD manpage states:
 115 "This system call will be eliminated when proper system sharing mechanisms
 116 are implemented. Users should not depend on the memory sharing semantics of
 117 .BR vfork ()
 118 as it will, in that case, be made synonymous to
 119 .BR fork ().\c
 120 "
 121
 122 Formally speaking, the standard description given above does not allow
 123 one to use
 124 .BR vfork ()
 125 since a following
 126 .BR exec ()
 127 might fail, and then what happens is undefined.
 128
 129 Details of the signal handling are obscure and differ between systems.
 130 The BSD manpage states:
 131 "To avoid a possible deadlock situation, processes that are children
 132 in the middle of a
 133 .BR vfork ()
 134 are never sent SIGTTOU or SIGTTIN signals; rather, output or
 135 .IR ioctl s
 136 are allowed and input attempts result in an end-of-file indication."
 137
 138 Currently (Linux 2.3.25),
 139 .BR strace (1)
 140 cannot follow
 141 .BR vfork ()
 142 and requires a kernel patch.
 143 .SH HISTORY
 144 The
 145 .BR vfork ()
 146 system call appeared in 3.0BSD.
 147 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
 148 .\" present, but definitely on its way out'.
 149 In 4.4BSD it was made synonymous to
 150 .BR fork ()
 151 but NetBSD introduced it again,
 152 cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
 153 In Linux, it has been equivalent to
 154 .BR fork ()
 155 until 2.2.0-pre6 or so. Since 2.2.0-pre9 (on i386, somewhat later on
 156 other architectures) it is an independent system call. Support was
 157 added in glibc 2.0.112.
 158 .SH "CONFORMING TO"
 159 4.3BSD, POSIX.1-2001.
 160
 161 The requirements put on
 162 .BR vfork ()
 163 by the standards are weaker than those put on
 164 .BR fork (),
 165 so an implementation where the two are synonymous
 166 is compliant. In particular, the programmer cannot
 167 rely on the parent remaining blocked until a call of
 168 .BR execve ()
 169 or
 170 .BR _exit ()
 171 and cannot rely on any specific behaviour w.r.t. shared memory.
 172 .\" In AIXv3.1 vfork is equivalent to fork.
 173 .SH "SEE ALSO"
 174 .BR clone (2),
 175 .BR execve (2),
 176 .BR fork (2),
 177 .BR unshare (2),
 178 .BR wait (2)