bsd/man/man2/vfork.2

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  34 .\"     @(#)vfork.2     8.1 (Berkeley) 6/4/93
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  36 .Dd June 4, 1993
  37 .Dt VFORK 2
  38 .Os BSD 4
  39 .Sh NAME
  40 .Nm vfork
  41 .Nd spawn new process in a virtual memory efficient way
  42 .Sh SYNOPSIS
  43 .Fd #include <unistd.h>
  44 .Ft pid_t
  45 .Fn vfork void
  46 .Sh DESCRIPTION
  47 .Fn Vfork
  48 can be used to create new processes without fully copying the address
  49 space of the old process, which is horrendously inefficient in a paged
  50 environment.  It is useful when the purpose of
  51 .Xr fork 2
  52 would have been to create a new system context for an
  53 .Xr execve .
  54 .Fn Vfork
  55 differs from
  56 .Xr fork
  57 in that the child borrows the parent's memory and thread of
  58 control until a call to
  59 .Xr execve 2
  60 or an exit (either by a call to
  61 .Xr exit 2
  62 or abnormally.)
  63 The parent process is suspended while the child is using its resources.
  64 .Pp
  65 .Fn Vfork
  66 returns 0 in the child's context and (later) the pid of the child in
  67 the parent's context.
  68 .Pp
  69 .Fn Vfork
  70 can normally be used just like
  71 .Xr fork .
  72 It does not work, however, to return while running in the childs context
  73 from the procedure that called
  74 .Fn vfork
  75 since the eventual return from
  76 .Fn vfork
  77 would then return to a no longer existent stack frame.
  78 Be careful, also, to call
  79 .Xr _exit
  80 rather than
  81 .Xr exit
  82 if you can't
  83 .Xr execve ,
  84 since
  85 .Xr exit
  86 will flush and close standard I/O channels, and thereby mess up the
  87 parent processes standard I/O data structures.
  88 (Even with
  89 .Xr fork
  90 it is wrong to call
  91 .Xr exit
  92 since buffered data would then be flushed twice.)
  93 .Sh SEE ALSO
  94 .Xr fork 2 ,
  95 .Xr execve 2 ,
  96 .Xr sigaction 2 ,
  97 .Xr wait 2
  98 .Sh DIAGNOSTICS
  99 Same as for
 100 .Xr fork .
 101 .Sh BUGS
 102 This system call will be eliminated when proper system sharing
 103 mechanisms are implemented.
 104 Users should not depend on the memory
 105 sharing semantics of
 106 .Xr vfork
 107 as it will, in that case, be made synonymous to
 108 .Xr fork .
 109 .Pp
 110 To avoid a possible deadlock situation,
 111 processes that are children in the middle
 112 of a
 113 .Fn vfork
 114 are never sent
 115 .Dv SIGTTOU
 116 or
 117 .Dv SIGTTIN
 118 signals; rather,
 119 output or
 120 .Xr ioctl 2
 121 calls
 122 are allowed
 123 and input attempts result in an end-of-file indication.
 124 .Sh HISTORY
 125 The
 126 .Fn vfork
 127 function call appeared in
 128 .Bx 3.0 .