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 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 equiv-
       alent 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)

Linux								    2017-09-15								    DUP(2)