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



NAME
       dup, dup2 - duplicate a file descriptor

SYNOPSIS
       #include <&lt;unistd.h>&gt;

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

DESCRIPTION
       dup() and dup2() create a copy of the file descriptor oldfd.

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

       dup2() makes newfd be the copy of oldfd, closing newfd first if  neces-
       sary, 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 dup() or dup2(), 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 sta-
       tus 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.

RETURN VALUE
       dup() and dup2() return the new descriptor, or -1 if an error  occurred
       (in which case, 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() during a race condi-
              tion with open(2) and dup().

       EINTR  The dup2() call was interrupted by a signal; see signal(7).

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

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

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() without clos-
       ing newfd first.

SEE ALSO
       close(2), fcntl(2), open(2)

COLOPHON
       This page is part of release 3.05 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                             2008-01-12                            DUP(2)