|DUP(2)||System Calls Manual||DUP(2)|
duplicate an existing file descriptor
duplicates an existing object descriptor and returns its value to the
calling process (fildes2 =
dup(fildes)). The argument
fildes 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(2). 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 fildes and fildes2 in any way. Thus if fildes2 and fildes 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.
value of the new descriptor fildes2 is specified. If
fildes and fildes2 are equal,
dup2() just returns
fildes2; no other changes are made to the existing
descriptor. Otherwise, if descriptor fildes2 is
already in use, it is first deallocated as if a
close(2) call had been done first.
Upon successful completion, the new file descriptor is returned. Otherwise, a value of -1 is returned and the global integer variable errno is set to indicate the error.
dup2() system calls will fail if:
dup2() system call will fail if:
are expected to conform to IEEE Std 1003.1-1988
|December 1, 2010||BSD 4|