NAME
dup, dup2 —
duplicate an existing file
descriptor
LIBRARY
library “libc”
SYNOPSIS
#include
<unistd.h>
int
dup(int
oldd);
int
dup2(int
oldd, int
newd);
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(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 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 and oldd ≠
newd, the descriptor is first deallocated as if a
close(2) call had been used. If oldd is not a
valid descriptor, then newd is not closed. If
oldd == newd and
oldd is a valid descriptor, then
dup2() is successful, and does nothing.
RETURN VALUES
The value -1 is returned if an error occurs in either call. The external variable errno indicates the cause of the error.
ERRORS
Dup() and dup2()
fail if:
- [
EBADF] - Oldd or newd is not a valid active descriptor
- [
EMFILE] - Too many descriptors are active.
SEE ALSO
accept(2), close(2), fcntl(2), getdtablesize(2), open(2), pipe(2), socket(2), socketpair(2), dup3(3)
STANDARDS
The dup() and
dup2() function calls are expected to conform to
IEEE Std 1003.1-1990 (“POSIX.1”).
HISTORY
A dup() and a
dup2() function call appeared in
Version 7 AT&T UNIX.