.\" This manpage is Copyright (C) 1992 Drew Eckhardt; .\" and Copyright (C) 1993 Michael Haardt, Ian Jackson. .\" and Copyright (C) 2005, 2008 Michael Kerrisk .\" and Copyright (C) 2014 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .\" Modified 1993-07-21, Rik Faith .\" Modified 1994-08-21, Michael Chastain : .\" Fixed typos. .\" Modified 1997-01-31, Eric S. Raymond .\" Modified 2002-09-28, aeb .\" 2009-01-12, mtk, reordered text in DESCRIPTION and added some .\" details for dup2(). .\" 2008-10-09, mtk: add description of dup3() .\" .TH DUP 2 2016-12-12 "Linux" "Linux Programmer's Manual" .SH NAME dup, dup2, dup3 \- duplicate a file descriptor .SH SYNOPSIS .nf .B #include .sp .BI "int dup(int " oldfd ); .BI "int dup2(int " oldfd ", int " newfd ); .sp .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" .BR "#include " " /* Obtain O_* constant definitions */ .B #include .sp .BI "int dup3(int " oldfd ", int " newfd ", int " flags ); .fi .SH DESCRIPTION The .BR dup () system call creates a copy of the file descriptor .IR oldfd , using the lowest-numbered unused file descriptor for the new descriptor. After a successful return, the old and new file descriptors may be used interchangeably. They refer to the same open file description (see .BR open (2)) and thus share file offset and file status flags; for example, if the file offset is modified by using .BR lseek (2) on one of the file descriptors, the offset is also changed for the other. The two file descriptors do not share file descriptor flags (the close-on-exec flag). The close-on-exec flag .RB ( FD_CLOEXEC ; see .BR fcntl (2)) for the duplicate descriptor is off. .\" .SS dup2() The .BR dup2 () system call performs the same task as .BR dup (), but instead of using the lowest-numbered unused file descriptor, it uses the file descriptor number specified in .IR newfd . If the file descriptor .IR newfd was previously open, it is silently closed before being reused. The steps of closing and reusing the file descriptor .IR newfd are performed .IR atomically . This is important, because trying to implement equivalent functionality using .BR close (2) and .BR dup () would be subject to race conditions, whereby .I newfd might be reused between the two steps. Such reuse could happen because the main program is interrupted by a signal handler that allocates a file descriptor, or because a parallel thread allocates a file descriptor. Note the following points: .IP * 3 If .I oldfd is not a valid file descriptor, then the call fails, and .I newfd is not closed. .IP * If .I oldfd is a valid file descriptor, and .I newfd has the same value as .IR oldfd , then .BR dup2 () does nothing, and returns .IR newfd . .\" .SS dup3() .BR dup3 () is the same as .BR dup2 (), except that: .IP * 3 The caller can force the close-on-exec flag to be set for the new file descriptor by specifying .BR O_CLOEXEC in .IR flags . See the description of the same flag in .BR open (2) for reasons why this may be useful. .IP * .\" Ulrich Drepper, LKML, 2008-10-09: .\" We deliberately decided on this change. Otherwise, what is the .\" result of dup3(fd, fd, O_CLOEXEC)? If .IR oldfd equals .IR newfd , then .BR dup3 () fails with the error .BR EINVAL . .SH RETURN VALUE On success, these system calls return the new file descriptor. On error, \-1 is returned, and .I errno is set appropriately. .SH ERRORS .TP .B EBADF .I oldfd isn't an open file descriptor. .TP .B EBADF .I newfd is out of the allowed range for file descriptors (see the discussion of .BR RLIMIT_NOFILE in .BR getrlimit (2)). .TP .B EBUSY (Linux only) This may be returned by .BR dup2 () or .BR dup3 () during a race condition with .BR open (2) and .BR dup (). .TP .B EINTR The .BR dup2 () or .BR dup3 () call was interrupted by a signal; see .BR signal (7). .TP .B EINVAL .RB ( dup3 ()) .I flags contain an invalid value. .TP .B EINVAL .RB ( dup3 ()) .I oldfd was equal to .IR newfd . .TP .B EMFILE The per-process limit on the number of open file descriptors has been reached (see the discussion of .BR RLIMIT_NOFILE in .BR getrlimit (2)). .SH VERSIONS .BR dup3 () was added to Linux in version 2.6.27; glibc support is available starting with version 2.9. .SH CONFORMING TO .BR dup (), .BR dup2 (): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD. .BR dup3 () is Linux-specific. .\" SVr4 documents additional .\" EINTR and ENOLINK error conditions. POSIX.1 adds EINTR. .\" The EBUSY return is Linux-specific. .SH NOTES The error returned by .BR dup2 () is different from that returned by .BR fcntl( "..., " F_DUPFD ", ..." ) when .I newfd is out of range. On some systems, .BR dup2 () also sometimes returns .B EINVAL like .BR F_DUPFD . If .I newfd was open, any errors that would have been reported at .BR close (2) time are lost. If this is of concern, then\(emunless the program is single-threaded and does not allocate file descriptors in signal handlers\(emthe correct approach is .I not to close .I newfd before calling .BR dup2 (), because of the race condition described above. Instead, code something like the following could be used: .nf /* Obtain a duplicate of 'newfd' that can subsequently be used to check for close() errors; an EBADF error means that 'newfd' was not open. */ tmpfd = dup(newfd); if (tmpfd == \-1 && errno != EBADF) { /* Handle unexpected dup() error */ } /* Atomically duplicate 'oldfd' on 'newfd' */ if (dup2(oldfd, newfd) == \-1) { /* Handle dup2() error */ } /* Now check for close() errors on the file originally referred to by 'newfd' */ if (tmpfd != \-1) { if (close(tmpfd) == \-1) { /* Handle errors from close */ } } .fi .SH SEE ALSO .BR close (2), .BR fcntl (2), .BR open (2) .SH COLOPHON This page is part of release 4.10 of the Linux .I man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at \%https://www.kernel.org/doc/man\-pages/.