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.TH DUP 3
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.SH NAME
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dup \- dups of open files
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.SH SYNOPSIS
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.nf
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.B bind #d /fd
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.B /fd/0
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.B /fd/0ctl
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.B /fd/1
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.B /fd/1ctl
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\&...
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.fi
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.SH DESCRIPTION
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The
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.I dup
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device serves a one-level directory containing files whose
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names are decimal numbers.
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Each such file also has an associated control file.
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A file of name
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.I n
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corresponds to open file descriptor
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.I n
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in the current process.
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.PP
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An
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.IR open (2)
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of file
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.I n
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results in a file descriptor identical to
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what would be returned from a system call
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.IB dup ( n ,
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.BR -1) .
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Note that the result is no longer a file in the
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.I dup
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device.
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.PP
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The
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.I stat
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operation returns information about the device file, not the open file it points to.
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A stat of
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.BI #d/ n
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will contain
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.I n
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for the name, 0 for the length, and 0400, 0200, or 0600
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for the mode, depending on whether the dup target is open
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for reading, writing, or both.
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.PP
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A file of name
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.IB n ctl
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may be read to discover the properties of the associated file descriptor, in format identical to that of the
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.B fd
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file in
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.IR proc (3).
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.SH SEE ALSO
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.IR dup (2)
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.SH SOURCE
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.B /sys/src/9/port/devdup.c