dup



DUP(2)                        System Calls Manual                       DUP(2)


NAME

     dup, dup2, dup3 - duplicate an existing file descriptor


SYNOPSIS

     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);

     #include <fcntl.h>
     #include <unistd.h>

     int
     dup3(int oldd, int newd, int flags);


DESCRIPTION

     dup() duplicates an existing object descriptor and returns its value to
     the calling process (newd = dup(oldd)).  The argument oldd is a small
     non-negative integer index in the per-process descriptor table.  The
     value must be less than the size of the table, which is returned by
     getdtablesize(3).  The new descriptor returned by the call is the lowest
     numbered descriptor currently not in use by the process.

     The object referenced by the descriptor does not distinguish between oldd
     and newd in any way.  Thus if newd and oldd are duplicate references to
     an open file, read(2), write(2) and lseek(2) calls all move a single
     pointer into the file, and append mode, non-blocking I/O and asynchronous
     I/O options are shared between the references.  If a separate pointer
     into the file is desired, a different object reference to the file must
     be obtained by issuing an additional open(2) call.  The close-on-exec
     flag on the new file descriptor is unset.

     In dup2(), the value of the new descriptor newd is specified.  If this
     descriptor is already in use, it is first deallocated as if a close(2)
     call had been done first.  When newd equals oldd, dup2() just returns
     without affecting the close-on-exec flag.

     In dup3(), both the value of the new descriptor and the close-on-exec
     flag on the new file descriptor are specified: newd specifies the value
     and the O_CLOEXEC bit in flags specifies the close-on-exec flag.  Unlike
     dup2(), if oldd and newd are equal then dup3() fails.  Otherwise, if
     flags is zero then dup3() is identical to a call to dup2().


RETURN VALUES

     Upon successful completion, the value of the new descriptor is returned.
     The value -1 is returned if an error occurs in either call.  The external
     variable errno indicates the cause of the error.


ERRORS

     dup() will fail if:

     [EBADF]            oldd is not a valid active descriptor.

     [EMFILE]           Too many descriptors are active.

     dup2() and dup3() will fail if:

     [EBADF]            oldd is not a valid active descriptor or newd is
                        negative or greater than or equal to the process's
                        RLIMIT_NOFILE limit.

     [EINTR]            An interrupt was received.

     [EIO]              An I/O error occurred while writing to the file
                        system.

     In addition, dup3() will return the following error:

     [EINVAL]           oldd is equal to newd or flags is invalid.


SEE ALSO

     accept(2), close(2), fcntl(2), getrlimit(2), open(2), pipe(2), socket(2),
     socketpair(2), getdtablesize(3)


STANDARDS

     dup() and dup2() conform to IEEE Std 1003.1-2008 (``POSIX.1'').  The
     dup3() function is expected to conform to a future revision of that
     standard.


HISTORY

     The dup() system call first appeared in Version 3 AT&T UNIX, dup2() in
     Version 7 AT&T UNIX, and dup3() in OpenBSD 5.7.

OpenBSD 5.7                    December 10, 2014                   OpenBSD 5.7

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