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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 */
             }
         }