dup3(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

   The  dup()  system  call  creates  a copy of the file descriptor oldfd,
   using  the  lowest-numbered  unused  file  descriptor   for   the   new
   descriptor.

   After a successful return, 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  file
   descriptors, the offset is also changed for the other.

   The two file 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.

   dup2()
   The  dup2() system call performs the same task as dup(), but instead of
   using the lowest-numbered unused file  descriptor,  it  uses  the  file
   descriptor number specified in newfd.  If the file descriptor newfd was
   previously open, it is silently closed before being reused.

   The steps  of  closing  and  reusing  the  file  descriptor  newfd  are
   performed  atomically.   This is important, because trying to implement
   equivalent functionality using close(2) and dup() would be  subject  to
   race  conditions,  whereby newfd might be reused between the two steps.
   Such reuse could happen because the main program is  interrupted  by  a
   signal  handler that allocates a file descriptor, or because a parallel
   thread allocates a file descriptor.

   Note the following points:

   *  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.

   dup3()
   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 file descriptor.  On
   error, -1 is returned, and errno is set appropriately.

ERRORS

   EBADF  oldfd isn't an open file descriptor.

   EBADF  newfd is out of the allowed range for file descriptors (see  the
          discussion of RLIMIT_NOFILE in getrlimit(2)).

   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.

   EINVAL (dup3()) oldfd was equal to newfd.

   EMFILE The per-process limit on the number of open file descriptors has
          been  reached  (see   the   discussion   of   RLIMIT_NOFILE   in
          getrlimit(2)).

VERSIONS

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

CONFORMING TO

   dup(), dup2(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

   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.   If  this  is  of concern, then—unless the program is
   single-threaded and  does  not  allocate  file  descriptors  in  signal
   handlers—the  correct  approach  is  not  to close newfd before calling
   dup2(), because of the race condition described above.   Instead,  code
   something like the following could be used:

       /* Obtain a duplicate of 'newfd' that can subsequently
          be used to check for close() errors; an EBADF error
          means that 'newfd' was not open. */

       tmpfd = dup(newfd);
       if (tmpfd == -1 && errno != EBADF) {
           /* Handle unexpected dup() error */
       }

       /* Atomically duplicate 'oldfd' on 'newfd' */

       if (dup2(oldfd, newfd) == -1) {
           /* Handle dup2() error */
       }

       /* Now check for close() errors on the file originally
          referred to by 'newfd' */

       if (tmpfd != -1) {
           if (close(tmpfd) == -1) {
               /* Handle errors from close */
           }
       }

SEE ALSO

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

COLOPHON

   This  page  is  part of release 4.09 of the Linux man-pages project.  A
   description of the project, information about reporting bugs,  and  the
   latest     version     of     this    page,    can    be    found    at
   https://www.kernel.org/doc/man-pages/.

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