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.TH DUP 2
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.SH NAME
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dup \- duplicate an open file descriptor
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.SH SYNOPSIS
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.B #include <u.h>
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.br
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.B #include <libc.h>
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.PP
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.B
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int dup(int oldfd, int newfd)
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.SH DESCRIPTION
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Given a file descriptor,
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.IR oldfd ,
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referring to an open file,
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.I dup
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returns a new file descriptor referring to the same file.
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.PP
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If
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.I newfd
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is \-1 the system chooses the lowest available file descriptor.
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Otherwise,
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.I dup
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will use
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.I newfd
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for the new file descriptor
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(closing any old file associated with
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.IR newfd ).
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File descriptors are allocated dynamically,
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so to prevent unwarranted growth of the file descriptor table,
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.I dup
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requires that
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.I newfd
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be no greater than 20 more than the highest file descriptor ever used by
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the program.
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.SH SOURCE
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.B /sys/src/libc/9syscall
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.SH SEE ALSO
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.IR intro (2),
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.IR dup (3)
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.SH DIAGNOSTICS
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Sets
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.IR errstr .