dup, dup2 - duplicate an existing file descriptor
dup2(int oldd, int newd);
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(3). 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
and newd in any way. Thus if newd and oldd are duplicate
an open file, read(2), write(2) and lseek(2) calls all move
pointer into the file, and append mode, non-blocking I/O and
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 closeon-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, it is first deallocated as if
call had been done first. When newd equals oldd, dup2()
without affecting the close-on-exec flag.
The value -1 is returned if an error occurs in either call.
variable errno indicates the cause of the error.
dup() and dup2() fail if:
[EBADF] oldd or newd is not a valid active descriptor.
[EMFILE] Too many descriptors are active.
accept(2), close(2), fcntl(2), open(2), pipe(2), socket(2),
dup() and dup2() are expected to conform to IEEE Std
OpenBSD 3.6 June 4, 1993
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