dup

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,
       using the lowest-numbered unused file descriptor for the  new  descrip-
       tor.

       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  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
       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 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
       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 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
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Linux                             2017-09-15                            DUP(2)
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