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



NAME
       dup, dup2, dup3 - duplicate a file descriptor

SYNOPSIS
       #include <unistd.h>

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

       #define _GNU_SOURCE
       #include <unistd.h>

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

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

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

       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  inter-
       changeably.   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 off-
       set 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
       On success, these system calls return the new descriptor.  On error, -1 is returned, and errno is set appropri-
       ately.

ERRORS
       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 signal(7).

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

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

       dup3() is Linux-specific.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-10-09                            DUP(2)