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" "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 DESCRIPTION
  36 .SS "Standard Description"
  37 (From SUSv2 / POSIX draft.)
  38 The
  39 .BR vfork ()
  40 function has the same effect as
  41 .BR fork (2),
  42 except that the behaviour is undefined if the process created by
  43 .BR vfork ()
  44 either modifies any data other than a variable of type
  45 .I pid_t
  46 used to store the return value from
  47 .BR vfork (),
  48 or returns from the function in which
  49 .BR vfork ()
  50 was called, or calls any other function before successfully calling
  51 .BR _exit (2)
  52 or one of the
  53 .BR exec (3)
  54 family of functions.
  55 .SS "Linux Description"
  56 .BR vfork (),
  57 just like
  58 .BR fork (2),
  59 creates a child process of the calling process.
  60 For details and return value and errors, see
  61 .BR fork (2).
  62 .PP
  63 .BR vfork ()
  64 is a special case of
  65 .BR clone (2).
  66 It is used to create new processes without copying the page tables of
  67 the parent process.
  68 It may be useful in performance sensitive applications
  69 where a child will be created which then immediately issues an
  70 .BR execve (2) .
  71 .PP
  72 .BR vfork ()
  73 differs from
  74 .BR fork (2)
  75 in that the parent is suspended until the child makes a call to
  76 .BR execve (2)
  77 or
  78 .BR _exit (2).
  79 The child shares all memory with its parent, including the stack, until
  80 .BR execve (2)
  81 is issued by the child.
  82 The child must not return from the current function or call
  83 .BR exit (3),
  84 but may call
  85 .BR _exit (2).
  86 .PP
  87 Signal handlers are inherited, but not shared.
  88 Signals to the parent
  89 arrive after the child releases the parent's memory.
  90 .SS "Historic Description"
  91 Under Linux,
  92 .BR fork (2)
  93 is implemented using copy-on-write pages, so the only penalty incurred by
  94 .BR fork (2)
  95 is the time and memory required to duplicate the parent's page tables,
  96 and to create a unique task structure for the child.
  97 However, in the bad old days a
  98 .BR fork (2)
  99 would require making a complete copy of the caller's data space,
 100 often needlessly, since usually immediately afterwards an
 101 .BR exec (3)
 102 is done.
 103 Thus, for greater efficiency, BSD introduced the
 104 .BR vfork ()
 105 system call, that did not fully copy the address space of
 106 the parent process, but borrowed the parent's memory and thread
 107 of control until a call to
 108 .BR execve (2)
 109 or an exit occurred.
 110 The parent process was suspended while the
 111 child was using its resources.
 112 The use of
 113 .BR vfork ()
 114 was tricky: for example, not modifying data
 115 in the parent process depended on knowing which variables are
 116 held in a register.
 117 .SH "CONFORMING TO"
 118 4.3BSD, POSIX.1-2001.
 119 .\" FIXME Mar 07: in the draft of the next POSIX revision, the spec for
 120 .\" vfork() has been removed.  See if this remains so in final version.
 121 The requirements put on
 122 .BR vfork ()
 123 by the standards are weaker than those put on
 124 .BR fork (2),
 125 so an implementation where the two are synonymous is compliant.
 126 In particular, the programmer cannot
 127 rely on the parent remaining blocked until a call of
 128 .BR execve (2)
 129 or
 130 .BR _exit (2)
 131 and cannot rely on any specific behaviour w.r.t. shared memory.
 132 .\" In AIXv3.1 vfork is equivalent to fork.
 133 .SH NOTES
 134 .SS Linux Notes
 135 Fork handlers established using
 136 .BR pthread_atfork (3)
 137 are not called when a multithreaded program employing
 138 the NPTL threading library calls
 139 .BR vfork ().
 140 Fork handlers are called in this case in a program using the
 141 LinuxThreads threading library.
 142 (See
 143 .BR pthreads (7)
 144 for a description of Linux threading libraries.)
 145 .SS History
 146 The
 147 .BR vfork ()
 148 system call appeared in 3.0BSD.
 149 .\" In the release notes for 4.2BSD Sam Leffler wrote: `vfork: Is still
 150 .\" present, but definitely on its way out'.
 151 In 4.4BSD it was made synonymous to
 152 .BR fork (2)
 153 but NetBSD introduced it again,
 154 cf. http://www.netbsd.org/Documentation/kernel/vfork.html .
 155 In Linux, it has been equivalent to
 156 .BR fork (2)
 157 until 2.2.0-pre6 or so.
 158 Since 2.2.0-pre9 (on i386, somewhat later on
 159 other architectures) it is an independent system call.
 160 Support was added in glibc 2.0.112.
 161 .SH BUGS
 162 It is rather unfortunate that Linux revived this spectre from the past.
 163 The BSD man page states:
 164 "This system call will be eliminated when proper system sharing mechanisms
 165 are implemented.
 166 Users should not depend on the memory sharing semantics of
 167 .BR vfork ()
 168 as it will, in that case, be made synonymous to
 169 .BR fork (2).\c
 170 "
 171
 172 Details of the signal handling are obscure and differ between systems.
 173 The BSD man page states:
 174 "To avoid a possible deadlock situation, processes that are children
 175 in the middle of a
 176 .BR vfork ()
 177 are never sent SIGTTOU or SIGTTIN signals; rather, output or
 178 .IR ioctl s
 179 are allowed and input attempts result in an end-of-file indication."
 180 .\"
 181 .\" As far as I can tell, the following is not true in 2.6.19:
 182 .\" Currently (Linux 2.3.25),
 183 .\" .BR strace (1)
 184 .\" cannot follow
 185 .\" .BR vfork ()
 186 .\" and requires a kernel patch.
 187 .SH "SEE ALSO"
 188 .BR clone (2),
 189 .BR execve (2),
 190 .BR fork (2),
 191 .BR unshare (2),
 192 .BR wait (2)