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