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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 interchangeably.  They refer to the same open file descrip-
       tion (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  descrip-
          tor  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  condi-
              tion 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 information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-10-09                            DUP(2)

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