dup(2) — Linux manual page


DUP(2)                    Linux Programmer's Manual                   DUP(2)

NAME         top

       dup, dup2, dup3 - duplicate a file descriptor

SYNOPSIS         top

       #include <unistd.h>

       int dup(int oldfd);
       int dup2(int oldfd, int newfd);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Obtain O_* constant definitions */
       #include <unistd.h>

       int dup3(int oldfd, int newfd, int flags);

DESCRIPTION         top

       The dup() system call creates a copy of the file descriptor oldfd,
       using the lowest-numbered unused file descriptor for the new

       After a successful return, the old and new file descriptors may be
       used interchangeably.  They refer to the same open file description
       (see open(2)) and thus share file offset and file status flags; for
       example, if the file offset is modified by using 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 (FD_CLOEXEC; see
       fcntl(2)) for the duplicate descriptor is off.

       The dup2() system call performs the same task as dup(), but instead
       of using the lowest-numbered unused file descriptor, it uses the file
       descriptor number specified in newfd.  If the file descriptor newfd
       was previously open, it is silently closed before being reused.

       The steps of closing and reusing the file descriptor newfd are
       performed atomically.  This is important, because trying to implement
       equivalent functionality using close(2) and dup() would be subject to
       race conditions, whereby 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:

       *  If oldfd is not a valid file descriptor, then the call fails, and
          newfd is not closed.

       *  If oldfd is a valid file descriptor, and newfd has the same value
          as oldfd, then dup2() does nothing, and returns newfd.

       dup3() is the same as dup2(), except that:

       *  The caller can force the close-on-exec flag to be set for the new
          file descriptor by specifying O_CLOEXEC in flags.  See the
          description of the same flag in open(2) for reasons why this may
          be useful.

       *  If oldfd equals newfd, then dup3() fails with the error EINVAL.

RETURN VALUE         top

       On success, these system calls return the new file descriptor.  On
       error, -1 is returned, and errno is set appropriately.

ERRORS         top

       EBADF  oldfd isn't an open file descriptor.

       EBADF  newfd is out of the allowed range for file descriptors (see
              the discussion of RLIMIT_NOFILE in getrlimit(2)).

       EBUSY  (Linux only) This may be returned by dup2() or dup3() during a
              race condition with open(2) and dup().

       EINTR  The dup2() or dup3() call was interrupted by a signal; see

       EINVAL (dup3()) flags contain an invalid value.

       EINVAL (dup3()) oldfd was equal to newfd.

       EMFILE The per-process limit on the number of open file descriptors
              has been reached (see the discussion of RLIMIT_NOFILE in

VERSIONS         top

       dup3() was added to Linux in version 2.6.27; glibc support is
       available starting with version 2.9.

CONFORMING TO         top

       dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

       dup3() is Linux-specific.

NOTES         top

       The error returned by dup2() is different from that returned by
       fcntl(..., F_DUPFD, ...)  when newfd is out of range.  On some
       systems, dup2() also sometimes returns EINVAL like F_DUPFD.

       If newfd was open, any errors that would have been reported at
       close(2) time are lost.  If this is of concern, then—unless the
       program is single-threaded and does not allocate file descriptors in
       signal handlers—the correct approach is not to close newfd before
       calling dup2(), because of the race condition described above.
       Instead, code something like the following could be used:

           /* 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 */

SEE ALSO         top

       close(2), fcntl(2), open(2), pidfd_getfd(2)

COLOPHON         top

       This page is part of release 5.09 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                            2020-11-01                           DUP(2)

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