DUP(2)                    (2020-11-01)                     DUP(2)

          dup, dup2, dup3 - duplicate a file descriptor

          #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

          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

          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 par-
          allel thread allocates a file descriptor.

          Note the following points:

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          *  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() 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 rea-
             sons why this may be useful.

          *  If oldfd equals newfd, then dup3() fails with the error

          On success, these system calls return the new file descrip-
          tor.  On error, -1 is returned, and errno is set appropri-

               oldfd isn't an open file descriptor.

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

               (Linux only) This may be returned by dup2() or dup3()
               during a race condition with open(2) and dup().

               The dup2() or dup3() call was interrupted by a signal;
               see signal(7).

               (dup3()) flags contain an invalid value.

               (dup3()) oldfd was equal to newfd.

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

          dup3() was added to Linux in version 2.6.27; glibc support

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          is available starting with version 2.9.

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

          dup3() is Linux-specific.

          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

          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 aqnewfdaq that can subsequently
                 be used to check for close() errors; an EBADF error
                 means that aqnewfdaq was not open. */

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

              /* Atomically duplicate aqoldfdaq on aqnewfdaq */

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

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

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

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

          This page is part of release 5.10 of the Linux man-pages
          project.  A description of the project, information about
          reporting bugs, and the latest version of this page, can be

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          found at https://www.kernel.org/doc/man-pages/.

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