|DUP(2)||System Calls Manual||DUP(2)|
oldd, int newd,
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.
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
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
oldd and newd are equal then
dup3() fails. Otherwise, if
flags is zero then
identical to a call to
dup() will fail if:
will fail if:
dup3() will return the
dup2() conform to IEEE Std 1003.1-2008 (“POSIX.1”). The
dup3() function is expected to conform to a future revision of that standard.
dup() system call first appeared in Version 3 AT&T UNIX,
dup2() in Version 7 AT&T UNIX, and
dup3() in OpenBSD 5.7.
|June 25, 2018||OpenBSD-current|