.\" Copyright, the authors of the Linux man-pages project
.\"
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.TH F_DUPFD 2const 2025-07-19 "Linux man-pages 6.15"
.SH NAME
F_DUPFD,
F_DUPFD_CLOEXEC
\-
duplicate a file descriptor
.SH LIBRARY
Standard C library
.RI ( libc ,\~ \-lc )
.SH SYNOPSIS
.nf
.B #include <fcntl.h>
.P
.BI "int fcntl(int " fd ", F_DUPFD, int " arg );
.BI "int fcntl(int " fd ", F_DUPFD_CLOEXEC, int " arg );
.fi
.SH DESCRIPTION
.TP
.B F_DUPFD
Duplicate the file descriptor
.I fd
using the lowest-numbered available file descriptor greater than or equal to
.IR arg .
This is different from
.BR dup2 (2),
which uses exactly the file descriptor specified.
.IP
On success, the new file descriptor is returned.
.IP
See
.BR dup (2)
for further details.
.TP
.B F_DUPFD_CLOEXEC
As for
.BR F_DUPFD ,
but additionally set the
close-on-exec flag for the duplicate file descriptor.
Specifying this flag permits a program to avoid an additional
.BR F_SETFD (2const)
operation to set the
.B FD_CLOEXEC
flag.
For an explanation of why this flag is useful,
see the description of
.B O_CLOEXEC
in
.BR open (2).
.SH RETURN VALUE
The new file descriptor.
.P
On error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
See
.BR fcntl (2).
.TP
.B EINVAL
.I arg
is negative or is greater than the maximum allowable value
(see the discussion of
.B RLIMIT_NOFILE
in
.BR getrlimit (2)).
.TP
.B EMFILE
The per-process limit on the number of open file descriptors
has been reached.
.SH VERSIONS
POSIX.1-2024 specifies
.BR F_DUPFD_CLOFORK ,
but Linux doesn't support it.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
.TP
.B F_DUPFD
SVr4, 4.3BSD, POSIX.1-2001.
.TP
.B F_DUPFD_CLOEXEC
Linux 2.6.24.
POSIX.1-2008.
(To get this definition, define
.B _POSIX_C_SOURCE
with the value 200809L or greater, or
.B _XOPEN_SOURCE
with the value 700 or greater.)
.SH CAVEATS
The errors returned by
.BR dup2 (2)
are different from those returned by
.BR F_DUPFD .
.SH SEE ALSO
.BR dup2 (2),
.BR fcntl (2)