dup(2) BSD System Calls Manual dup(2)
NAME
dup, dup2 -- duplicate an existing file descriptor
SYNOPSIS
#include <unistd.h>
int
dup(int fildes);
int
dup2(int fildes, int fildes2);
DESCRIPTION
Dup() 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 sepa-
rate 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 fildes2 is specified. If this
descriptor is already in use, the descriptor is first deallocated as if a
close(2) call had been done first.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and the global integer variable errno is set to indi-
cate the error.
ERRORS
The dup() and dup2() system calls will fail if:
[EBADF] fildes or fildes2 is not an active, valid file
descriptor.
[EINTR] Execution is interrupted by a signal.
[EMFILE] Too many file descriptors are active.
SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2),
socket(2), socketpair(2)
STANDARDS
Dup() and dup2() are expected to conform to IEEE Std 1003.1-1988
(``POSIX.1'').
4th Berkeley Distribution June 4, 1993 4th Berkeley Distribution
Mac OS X 10.7 - Generated Sun Nov 6 10:37:59 CST 2011