DUP(2) System Calls DUP(2)
dup, dup2 - duplicate an existing file descriptor
int dup (int oldfd);
int dup2 (int oldfd, int newfd);
Dup duplicates an existing file descriptor and returns its value to the
calling process. The argument oldfd 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 cur-
rently not in use by the process.
The file referenced by the descriptor does not distinguish between
oldfd and newfd in any way. Thus if newfd and oldfd are duplicate ref-
erences 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 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 newfd is specified. If this
descriptor is already in use, the descriptor is first deallocated as if
a close(2) call had been done first.
The value -1 is returned if an error occurs in either call. The exter-
nal variable errno indicates the cause of the error.
Dup and dup2 fail if:
EBADF Oldfd or newfd is not a valid active descriptor
EMFILE Too many descriptors are active.
accept(2), open(2), close(2), fcntl(2), pipe(2), socket(2), socket-
Dup and dup2 are expected to conform to IEEE Std 1003.1-1988 (POSIX).
GNO 16 January 1997 DUP(2)
Man(1) output converted with