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1 .\" $NetBSD: vfork.2,v 1.6 1995/02/27 12:39:30 cgd Exp $
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34 .\" @(#)vfork.2 8.1 (Berkeley) 6/4/93
35 .\"
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 .