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DUP(2)                      BSD System Calls Manual                     DUP(2)

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

SYNOPSIS
     #include <&lt;unistd.h>&gt;

     int
     dup(int oldd);

     int
     dup2(int oldd, int newd);

     #include <&lt;fcntl.h>&gt;
     #include <&lt;unistd.h>&gt;

     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 nega-
                        tive 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 sys-
                        tem.

     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 stan-
     dard.

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.

BSD                             April 28, 2017                             BSD