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DUP(2) BSD System Calls Manual DUP(2)
NAME
dup, dup2 -- duplicate an existing file descriptor
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 num-
bered 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, the descriptor is first deallocated as if a close(2) call had been done first.
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), open(2), close(2), fcntl(2), pipe(2), socket(2), socketpair(2), getdtablesize(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 |
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