dup(2)



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
       The  dup() system call creates a copy of the file descriptor oldfd, us-
       ing 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 de-
       scriptors, 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 de-
       scriptor 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 per-
       formed 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  descrip-
          tion 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 er-
       ror, -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  sig-
              nal(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  getr-
              limit(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  fc-
       ntl(...,  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 sin-
       gle-threaded and does not allocate  file  descriptors  in  signal  han-
       dlers--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 4.16 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/.

Linux                             2017-09-15                            DUP(2)

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