Rewrite for clarity and add an example.
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dholland
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@ -1,4 +1,4 @@
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.\" $NetBSD: dup.2,v 1.28 2012/01/25 00:28:35 christos Exp $
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.\" $NetBSD: dup.2,v 1.29 2013/12/24 22:31:11 dholland Exp $
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.\"
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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@ -29,7 +29,7 @@
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.\"
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.\" @(#)dup.2 8.1 (Berkeley) 6/4/93
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.\"
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.Dd January 23, 2012
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.Dd December 24, 2013
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.Dt DUP 2
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.Os
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.Sh NAME
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@ -42,129 +42,190 @@
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.Sh SYNOPSIS
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.In unistd.h
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.Ft int
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.Fn dup "int oldd"
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.Fn dup "int oldfd"
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.Ft int
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.Fn dup2 "int oldd" "int newd"
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.Fn dup2 "int oldfd" "int newfd"
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.Ft int
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.Fn dup3 "int oldd" "int newd" "int flags"
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.Fn dup3 "int oldfd" "int newfd" "int flags"
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.Sh DESCRIPTION
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The
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.Fn dup
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duplicates an existing object descriptor and returns its value to
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the calling process
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.Fa ( newd
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=
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.Fn dup oldd ) .
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The argument
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.Fa oldd
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is a small non-negative integer index in
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the per-process descriptor table.
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The value must be less than the size of the table, which is returned by
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.Xr getdtablesize 3 .
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The new descriptor returned by the call
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is the lowest numbered descriptor currently not in use by the process.
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.Pp
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The object referenced by the descriptor does not distinguish
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between
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.Fa oldd
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and
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.Fa newd
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in any way.
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Thus if
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.Fa newd
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and
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.Fa oldd
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are duplicate references to an open
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file,
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family of calls duplicates an existing file descriptor
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.Fa oldfd .
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A new file descriptor is produced; it is a new reference to the same
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underlying system object.
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The object in question does not distinguish between the descriptors
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referencing it in any way.
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Thus for files,
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.Xr read 2 ,
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.Xr write 2
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and
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.Xr lseek 2
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calls all move a single pointer into the file,
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and append mode, non-blocking I/O and asynchronous I/O options
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are shared between the references.
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If a separate pointer into the file is desired, a different
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object reference to the file must be obtained by issuing an
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additional
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calls all move a single shared seek position.
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Similarly, all object modes, settings, properties, and behavior other
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than the close-on-exec flag are shared between references.
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This includes the setting of append mode, non-blocking I/O actions,
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asynchronous I/O operations in progress, socket options, and so forth.
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The close-on-exec flag, however, is a property of the descriptor
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rather than the object and can be set independently for each
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reference.
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.Pp
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To get an independent handle with its own seek position and settings,
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an additional
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.Xr open 2
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call.
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The close-on-exec flag on the new file descriptor is unset.
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call must be issued.
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.Pq This is not generally possible for pipes and sockets.
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.Pp
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In
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.Fn dup2 ,
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the value of the new descriptor
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.Fa newd
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is specified.
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If this descriptor is already
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in use, the descriptor is first deallocated as if a
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.Xr close 2
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call had been done first.
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If
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.Fa newd
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The
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.Nm dup
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call chooses the new descriptor: it is the lowest-numbered descriptor
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not currently in use.
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The
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.Nm dup2
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and
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.Fa oldd
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are the same, the call has no effect.
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.Nm dup3
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calls allow the caller to choose the new descriptor by passing
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.Fa newfd ,
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which must be within the range of valid descriptors.
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If
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.Fa newfd
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is the same as
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.Fa oldfd ,
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the call has no effect.
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Otherwise, if
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.Fa newfd
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is already in use, it is closed as if
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.Xr close 2
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had been called.
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.Pp
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File descriptors are small non-negative integers that index into the
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per-process file table.
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Values 0, 1, and 2 have the special property that they are treated as
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standard input, standard output, and standard error respectively.
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(The constants
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.Dv STDIN_FILENO ,
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.Dv STDOUT_FILENO ,
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and
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.Dv STDERR_FILENO
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are provided as symbolic forms for these values.)
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The maximum value for a file descriptor is one less than the file
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table size.
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The file table size can be interrogated with
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.Xr getdtablesize 3
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and can to some extent be adjusted with
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.Xr setrlimit 2 .
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.Pp
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The
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.Fn dup3
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behaves exactly like
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.Fn dup2
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only it allows extra
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call includs an additional
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.Fa flags
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to be set on the returned file descriptor.
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The following flags are valid:
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.Bl -tag -width O_NONBLOCK -offset indent
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argument supporting a subset of the
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.Xr open 2
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flags:
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.Bl -tag -width O_NOSIGPIPE -offset indent
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.It Dv O_CLOEXEC
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Set the
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.Dq close-on-exec
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property.
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Set the close-on-exec flag on
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.Fa newfd .
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.It Dv O_NONBLOCK
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Sets non-blocking I/O.
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.It Dv O_NOSIGPIPE
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Return
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.Er EPIPE
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instead of raising
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.Dv SIGPIPE .
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For pipes and sockets, do not raise
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.Dv SIGPIPE
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when a write is made to a broken pipe.
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Instead, the write will fail with
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.Er EPIPE .
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.El
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As described above, only the close-on-exec flag is
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per-file-descriptor, so passing any of the other
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.Fa flags
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will affect
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both
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.Fa oldfd
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and
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.Fa newfd .
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These settings are, however, applied atomically along with the rest of
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the
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.Fn dup3
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operation.
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.Pp
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In the case of
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.Fn dup
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and
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.Fn dup2
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the close-on-exec flag on the new file descriptor is always left
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unset and all the modes and settings of the underlying object are left
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unchanged.
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.Pp
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Functionality similar to
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.Fn dup
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with slightly different semantics is also available via
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.Xr fcntl 2 .
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.Sh RETURN VALUES
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The value \-1 is returned if an error occurs in either call.
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The external variable
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These calls return the new file descriptor value.
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In the case of
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.Fn dup2
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and
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.Fn dup3
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this is always the same as
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.Fa newfd .
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If an error occurs, the value \-1 is returned and
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.Va errno
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indicates the cause of the error.
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is set to indicate what happened.
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.Sh EXAMPLES
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A common use for these functions is to set up a pipe as the standard
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input or standard output of a subprocess.
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That is done approximately as follows (error handling omitted for
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clarity):
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.Bd -literal -offset indent
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#include \*[Lt]unistd.h\*[Gt]
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int fds[2];
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pid_t pid;
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pipe(fds);
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pid = fork();
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if (pid == 0) {
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/* child; use read end of pipe to stdin */
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dup2(fds[0], STDIN_FILENO);
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close(fds[0]);
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close(fds[1]);
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execv("/some/program", args);
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}
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/* parent process; return write end of pipe */
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close(fds[0]);
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return fds[1];
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.Ed
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.Sh ERRORS
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All three functions may fail if:
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These functions fail if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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.Fa oldd
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is not a valid active descriptor
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or
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.Fa newd
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.Fa oldfd
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is not a valid active descriptor, or for
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.Fn dup2
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and
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.Fn dup3 ,
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.Fa newfd
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is not in the range of valid file descriptors.
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.El
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.Pp
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The
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.Fn dup
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function may also fail if:
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.Bl -tag -width Er
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.It Bq Er EMFILE
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Too many descriptors are active.
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.El
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.Pp
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The
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.Fn dup3
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function will also fail if:
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.Bl -tag -width Er
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Only
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.Fn dup
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can generate this error.
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.It Bq Er EINVAL
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.Fa flags
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is other than
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.Dv O_NONBLOCK
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or
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.Dv O_CLOEXEC .
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contained an invalid value.
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Only
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.Fn dup3
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can generate this error.
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.El
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.Sh SEE ALSO
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.Xr accept 2 ,
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.Xr close 2 ,
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.Xr fcntl 2 ,
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.Xr getrlimit 2 ,
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.Xr open 2 ,
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.Xr pipe 2 ,
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.Xr setrlimit 2 ,
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.Xr socket 2 ,
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.Xr socketpair 2 ,
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.Xr getdtablesize 3
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@ -178,5 +239,5 @@ functions conform to
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.Sh HISTORY
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The
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.Fn dup3
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function is inspired from Linux and appeared in
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function originated in Linux and appeared in
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.Nx 6.0 .
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