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             /* See feature_test_macros(7) */
       #include <fcntl.h>              /* Obtain O_* constant definitions */
       #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 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 modi-
       fied 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
       On success, these system calls return the new descriptor.  On error, -1 is  returned,  and
       errno is set appropriately.

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 ver-
       sion 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 3.53 of the Linux man-pages project.  A  description  of  the
       project,     and    information    about    reporting    bugs,    can    be    found    at
       http://www.kernel.org/doc/man-pages/.



Linux                                       2012-02-14                                     DUP(2)