V10/man/man2/dup.2

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.TH DUP 2
.CT 2 file_io
.SH NAME
dup, dup2 \(mi duplicate an open file descriptor
.SH SYNOPSIS
.nf
.B int dup(fildes)
.B int fildes;
.PP
.B int dup2(fildes, fildes2)
.B int fildes, fildes2;
.fi
.SH DESCRIPTION
Given
a file descriptor
.I dup
allocates another file descriptor
synonymous
with
the original.
The new file descriptor is returned.
.PP
In
.IR dup2 ,
.I fildes
is a file descriptor referring to an open file, and
.I fildes2
is an integer in the range of legal file descriptors.
.I Dup2
causes
.I fildes2
to refer to the same file
and returns 
.I fildes2.
If
.I fildes2
already referred to another open file, it is closed first.
.SH "SEE ALSO"
.IR open (2), 
.IR pipe (2), 
.IR fd (4)
.SH DIAGNOSTICS
.BR EBADF ,
.BR EMFILE
.SH BUGS
.I Dup
of a file descriptor greater than 63
turns into a
.I dup2
with a random second argument.