'\" te
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.TH DUP2 3C "June 21, 2024"
.SH NAME
dup2, dup3 \- duplicate an open file descriptor
.SH SYNOPSIS
.nf
#include <unistd.h>

\fBint\fR \fBdup2\fR(\fBint\fR \fIfildes\fR, \fBint\fR \fIfildes2\fR);
.fi

.LP
.nf
\fBint\fR \fBdup3\fR(\fBint\fR \fIfildes\fR, \fBint\fR \fIfildes2\fR, \fBint\fR \fIflags\fR);
.fi

.SH DESCRIPTION
The \fBdup2()\fR function causes the file descriptor \fIfildes2\fR to refer to
the same file as \fIfildes\fR. The \fIfildes\fR argument is a file descriptor
referring to an open file, and \fIfildes2\fR is a non-negative integer less
than the current value for the maximum number of open file descriptors  allowed
the calling process.  See \fBgetrlimit\fR(2). If \fIfildes2\fR already refers
to an open file, not \fIfildes\fR, it is closed first. If \fIfildes2\fR refers
to \fIfildes\fR, or if \fIfildes\fR is not a valid open file descriptor,
\fIfildes2\fR will not be closed first.
.sp
.LP
The \fBdup2()\fR function is equivalent to  \fBfcntl\fR(\fIfildes\fR,
\fBF_DUP2FD\fR, \fIfildes2\fR).  The returned file descriptor will have neither
the close-on-exec (\fBFD_CLOEXEC\fR) or close-on-fork (\fBFD_CLOFORK\fR) flags
set on it, regardless of what is set on \fIfildes\fR.
.sp
.LP
The \fBdup3()\fR function works similarly to the \fBdup2()\fR function with
two exceptions.  If \fIfildes\fR and \fIfildes2\fR point to the same file
descriptor, -1 is returned and errno set to \fBEINVAL\fR.  The \fIflags\fR
argument allows for control of the returned file descriptor's flags.  Valid
values are the bitwise-inclusive-OR of:

.sp
.ne 2
.na
\fB\fBO_CLOEXEC\fR\fR
.ad
.RS 12n
The returned file descriptor will have the close-on-exec flag, \fBFD_CLOEXEC\fR
set on it.  The file descriptor will be automatically closed when the process
calls the \fBexec\fR(2) family of functions.
.RE

.sp
.ne 2
.na
\fB\fBO_CLOFORK\fR\fR
.ad
.RS 12n
The returned file descriptor will have the close-on-fork flag, \fBFD_CLOFORK\fR
set on it.  The file descriptor will be automatically closed in any child
processes created with the \fBfork\fR(2) family of functions.  It will remain
open in the parent.
.RE

.SH RETURN VALUES
Upon successful completion a non-negative integer representing the file
descriptor is returned. Otherwise, \fB\(mi1\fR is returned and \fBerrno\fR is
set to indicate the error.
.SH ERRORS
The \fBdup2()\fR and \fBdup3()\fR functions will fail if:
.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 10n
The \fIfildes\fR argument is not a valid open file descriptor.
.RE

.sp
.ne 2
.na
\fB\fBEBADF\fR\fR
.ad
.RS 10n
The \fIfildes2\fR argument is negative or is not less than the current resource
limit returned by \fBgetrlimit(RLIMIT_NOFILE, .\|.\|.)\fR.
.RE

.sp
.ne 2
.na
\fB\fBEINTR\fR\fR
.ad
.RS 10n
A signal was caught during the \fBdup2()\fR call.
.RE

.sp
.ne 2
.na
\fB\fBEMFILE\fR\fR
.ad
.RS 10n
The process has too many open files.  See  \fBfcntl\fR(2).
.RE

.sp
.LP
Additionally, the \fBdup3()\fR function will fail if:
.sp
.ne 2
.na
\fB\fBEINVAL\fR\fR
.ad
.RS 10n
\fIflags\fR has a value other than 0 or the bitwise-inclusive-OR of
\fBO_CLOEXEC\fR and \fBO_CLOFORK\fR. \fIfildes\fR and \fIfildes2\fR point to
the same file descriptor.
.RE

.SH ATTRIBUTES
See \fBattributes\fR(7) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	Async-Signal-Safe
.TE

.SH SEE ALSO
.BR close (2),
.BR creat (2),
.BR exec (2),
.BR fcntl (2),
.BR fork (2),
.BR getrlimit (2),
.BR open (2),
.BR pipe (2),
.BR attributes (7),
.BR standards (7)