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

       These system calls create a copy of the file descriptor oldfd.

       dup() uses the lowest-numbered unused descriptor for the new

       dup2() makes newfd be the copy of oldfd, closing newfd first if
       necessary, but note the following:

       *  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.

       After a successful return from one of these system calls, 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 descriptors, the offset is also changed
       for the other.

       The two 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.

       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 descriptor.  On error,
       -1 is returned, and errno is set appropriately.

ERRORS         top

       EBADF  oldfd isn't an open file descriptor, or newfd is out of the
              allowed range for file descriptors.

       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.  Or, oldfd was equal
              to newfd.

       EMFILE The process already has the maximum number of file descriptors
              open and tried to open a new one.

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

       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.  A careful programmer will not use dup2() or
       dup3() without closing newfd first.

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can
       be found at

Linux                            2012-02-14                           DUP(2)