dup(2) - NetBSD Manual Pages

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DUP(2)                    NetBSD System Calls Manual                    DUP(2)


NAME

     dup, dup2, dup3 -- duplicate an existing file descriptor


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

     #include <unistd.h>

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);

     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, the descriptor is first deallocated as if a
     close(2) call had been done first.  If newd and oldd are the same, the
     call has no effect.

     dup3() behaves exactly like dup2() only it allows extra flags to be set
     on the returned file descriptor.  The following flags are valid:

           O_CLOEXEC   Set the ``close-on-exec'' property.

           O_NONBLOCK  Sets non-blocking I/O.

           O_NOSIGPIPE
                       Return EPIPE instead of raising SIGPIPE.


RETURN VALUES

     The value -1 is returned if an error occurs in either call.  The external
     variable errno indicates the cause of the error.


ERRORS

     All three functions may fail if:

     [EBADF]            oldd is not a valid active descriptor or newd is not
                        in the range of valid file descriptors.

     The dup() function may also fail if:

     [EMFILE]           Too many descriptors are active.

     The dup3() function will also fail if:

     [EINVAL]           flags is other than O_NONBLOCK or O_CLOEXEC.


SEE ALSO

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


STANDARDS

     The dup() and dup2() functions conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').


HISTORY

     The dup3() function is inspired from Linux and appeared in NetBSD 6.0.

NetBSD 6.0                     January 23, 2012                     NetBSD 6.0