xref: /netbsd/lib/libc/sys/vfork.2 (revision bf9ec67e)
1.\"	$NetBSD: vfork.2,v 1.19 2002/05/16 21:23:31 wiz Exp $
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34.\"     @(#)vfork.2	8.1 (Berkeley) 6/4/93
35.\"
36.Dd January 3, 1998
37.Dt VFORK 2
38.Os
39.Sh NAME
40.Nm vfork
41.Nd spawn new process in a virtual memory efficient way
42.Sh LIBRARY
43.Lb libc
44.Sh SYNOPSIS
45.Fd #include \*[Lt]unistd.h\*[Gt]
46.Ft pid_t
47.Fn vfork void
48.Sh DESCRIPTION
49The
50.Nm
51system call creates a new process that does not have a new
52virtual address space, but rather shares address space with the
53parent, thus avoiding potentially expensive copy-on-write operations
54normally associated with creating a new process.
55It is useful when the purpose of
56.Xr fork 2
57would have been to create a new system context for an
58.Xr execve 2 .
59The
60.Nm
61system call differs from
62.Xr fork 2
63in that the child borrows the parent's memory and thread of
64control until a call to
65.Xr execve 2
66or an exit (either by a call to
67.Xr _exit 2
68or abnormally).
69The parent process is suspended while the child is using its resources.
70.Pp
71The
72.Nm
73system call returns 0 in the child's context and (later) the pid
74of the child in the parent's context.
75.Pp
76The
77.Nm
78system call can normally be used just like
79.Xr fork 2 .
80It does not work, however, to return while running in the childs context
81from the procedure that called
82.Fn vfork
83since the eventual return from
84.Fn vfork
85would then return to a no longer existent stack frame.
86Be careful, also, to call
87.Xr _exit 2
88rather than
89.Xr exit 3
90if you can't
91.Xr execve 2 ,
92since
93.Xr exit 3
94will flush and close standard I/O channels, and thereby mess up the
95standard I/O data structures
96in the parent process.  (Even with
97.Xr fork 2
98it is wrong to call
99.Xr exit 3
100since buffered data would then be flushed twice.)
101.Sh RETURN VALUES
102Same as for
103.Xr fork 2 .
104.Sh ERRORS
105Same as for
106.Xr fork 2 .
107.Sh SEE ALSO
108.Xr execve 2 ,
109.Xr fork 2 ,
110.Xr sigaction 2 ,
111.Xr wait 2
112.Sh HISTORY
113The
114.Fn vfork
115function call appeared in
116.Bx 3.0 .
117In
118.Bx 4.4 ,
119the semantics were changed to only suspend the parent.
120The original semantics were reintroduced in
121.Nx 1.4 .
122.Sh BUGS
123.\" XXX We reintroduced the original semantics.
124.\" This system call will be eliminated when proper system sharing
125.\" mechanisms are implemented.
126Users should not depend on the memory sharing semantics of
127.Xr vfork 2
128as other ways of speeding up the fork process may be developed in
129the future.
130.Pp
131To avoid a possible deadlock situation, processes that are children
132in the middle of a
133.Fn vfork
134are never sent
135.Dv SIGTTOU
136or
137.Dv SIGTTIN
138signals; rather, output or
139.Xr ioctl 2
140calls are allowed and input attempts result in an end-of-file indication.
141