Validity of an fd?

[Guy Harris]

>three quick methods:

A fourth method, which should work on any "modern" UNIX (defined for
the purposes of this discussion as "any UNIX with 'fcntl'" - which
includes 4.2BSD and later, System III and later, and any system derived
from or providing compatibility with either or both of the above):

	#include <fcntl.h>
	#include <errno.h>

	extern int errno;	/* BSD doesn't declare it in <errno.h> */

	int
	isopen4(int fd)
	{
		return !(fcntl(fd, F_GETFD, 0) == -1 && errno == EBADF);
	}

which should be faster than the "fstat" one (since it doesn't have to
get the attributes of the file), doesn't do any "dup"ing of file
descriptors (and thus has no funny side effects), and doesn't depend on
something that, according to the manual page, shouldn't work.

(If you don't have a C compiler that supports prototypes, make the
obvious change to the beginning of "isopen4"....)