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 .Fo vfork
  46 .Fa void
  47 .Fc
  48 .Sh DESCRIPTION
  49 .Fn vfork
  50 can be used to create new processes without fully copying the address
  51 space of the old process, which is horrendously inefficient in a paged
  52 environment.  It is useful when the purpose of
  53 .Xr fork 2
  54 would have been to create a new system context for an
  55 .Xr execve .
  56 .Fn vfork
  57 differs from
  58 .Xr fork
  59 in that the child borrows the parent's memory and thread of
  60 control until a call to
  61 .Xr execve 2
  62 or an exit (either by a call to
  63 .Xr exit 2
  64 or abnormally.)
  65 The parent process is suspended while the child is using its resources.
  66 .Pp
  67 .Fn vfork
  68 returns 0 in the child's context and (later) the pid of the child in
  69 the parent's context.
  70 .Pp
  71 .Fn vfork
  72 can normally be used just like
  73 .Xr fork .
  74 It does not work, however, to return while running in the childs context
  75 from the procedure that called
  76 .Fn vfork
  77 since the eventual return from
  78 .Fn vfork
  79 would then return to a no longer existent stack frame.
  80 Be careful, also, to call
  81 .Xr _exit
  82 rather than
  83 .Xr exit
  84 if you can't
  85 .Xr execve ,
  86 since
  87 .Xr exit
  88 will flush and close standard I/O channels, and thereby mess up the
  89 parent processes standard I/O data structures.
  90 (Even with
  91 .Xr fork
  92 it is wrong to call
  93 .Xr exit
  94 since buffered data would then be flushed twice.)
  95 .Sh SEE ALSO
  96 .Xr execve 2 ,
  97 .Xr fork 2 ,
  98 .Xr sigaction 2 ,
  99 .Xr wait 2
 100 .Sh ERRORS
 101 The
 102 .Fn vfork
 103 system call will fail for any of the reasons described
 104 in the
 105 .Xr fork
 106 man page.
 107 In addition, it will fail if:
 108 .Bl -tag -width Er
 109 .\" ===========
 110 .It Bq Er EINVAL
 111 A system call other than
 112 .Xr _exit()
 113 or
 114 .Xr execve()
 115 (or libc functions that make no system calls other than those)
 116 is called following calling a
 117 .Fn vfork
 118 call.
 119 .El
 120 .Sh BUGS
 121 This system call will be eliminated when proper system sharing
 122 mechanisms are implemented.
 123 Users should not depend on the memory
 124 sharing semantics of
 125 .Xr vfork
 126 as it will, in that case, be made synonymous to
 127 .Xr fork .
 128 .Pp
 129 To avoid a possible deadlock situation,
 130 processes that are children in the middle
 131 of a
 132 .Fn vfork
 133 are never sent
 134 .Dv SIGTTOU
 135 or
 136 .Dv SIGTTIN
 137 signals; rather,
 138 output or
 139 .Xr ioctl 2
 140 calls
 141 are allowed
 142 and input attempts result in an end-of-file indication.
 143 .Sh HISTORY
 144 The
 145 .Fn vfork
 146 function call appeared in
 147 .Bx 3.0 .