xref: /dragonfly/lib/libc/sys/dup.2 (revision b40e316c) 
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32.\"     @(#)dup.2	8.1 (Berkeley) 6/4/93
33.\" $FreeBSD: src/lib/libc/sys/dup.2,v 1.9.2.6 2001/12/14 18:34:00 ru Exp $
34.\" $DragonFly: src/lib/libc/sys/dup.2,v 1.2 2003/06/17 04:26:47 dillon Exp $
35.\"
36.Dd June 4, 1993
37.Dt DUP 2
38.Os
39.Sh NAME
40.Nm dup ,
41.Nm dup2
42.Nd duplicate an existing file descriptor
43.Sh LIBRARY
44.Lb libc
45.Sh SYNOPSIS
46.In unistd.h
47.Ft int
48.Fn dup "int oldd"
49.Ft int
50.Fn dup2 "int oldd" "int newd"
51.Sh DESCRIPTION
52.Fn Dup
53duplicates an existing object descriptor and returns its value to
54the calling process
55.Fa ( newd
56=
57.Fn dup oldd ) .
58The argument
59.Fa oldd
60is a small non-negative integer index in
61the per-process descriptor table.  The value must be less
62than the size of the table, which is returned by
63.Xr getdtablesize 2 .
64The new descriptor returned by the call
65is the lowest numbered descriptor
66currently not in use by the process.
67.Pp
68The object referenced by the descriptor does not distinguish
69between
70.Fa oldd
71and
72.Fa newd
73in any way.
74Thus if
75.Fa newd
76and
77.Fa oldd
78are duplicate references to an open
79file,
80.Xr read 2 ,
81.Xr write 2
82and
83.Xr lseek 2
84calls all move a single pointer into the file,
85and append mode, non-blocking I/O and asynchronous I/O options
86are shared between the references.
87If a separate pointer into the file is desired, a different
88object reference to the file must be obtained by issuing an
89additional
90.Xr open 2
91call.
92The close-on-exec flag on the new file descriptor is unset.
93.Pp
94In
95.Fn dup2 ,
96the value of the new descriptor
97.Fa newd
98is specified.  If this descriptor is already in use and
99.Fa oldd
100\*(Ne
101.Fa newd ,
102the descriptor is first deallocated as if a
103.Xr close 2
104call had been used.
105If
106.Fa oldd
107is not a valid descriptor, then
108.Fa newd
109is not closed.
110If
111.Fa oldd
112==
113.Fa newd
114and
115.Fa oldd
116is a valid descriptor, then
117.Fn dup2
118is successful, and does nothing.
119.Sh RETURN VALUES
120The value -1 is returned if an error occurs in either call.
121The external variable
122.Va errno
123indicates the cause of the error.
124.Sh ERRORS
125.Fn Dup
126and
127.Fn dup2
128fail if:
129.Bl -tag -width Er
130.It Bq Er EBADF
131.Fa Oldd
132or
133.Fa newd
134is not a valid active descriptor
135.It Bq Er EMFILE
136Too many descriptors are active.
137.El
138.Sh SEE ALSO
139.Xr accept 2 ,
140.Xr close 2 ,
141.Xr fcntl 2 ,
142.Xr getdtablesize 2 ,
143.Xr open 2 ,
144.Xr pipe 2 ,
145.Xr socket 2 ,
146.Xr socketpair 2
147.Sh STANDARDS
148The
149.Fn dup
150and
151.Fn dup2
152function calls are expected to conform to
153.St -p1003.1-90 .
154.Sh HISTORY
155A
156.Fn dup
157and a
158.Fn dup2
159function call appeared in
160.At v7 .
161