extending 1003.1 to include sym links

[Moderator, John S. Quarterman]

From: Doug Gwyn <uunet!smoke.brl.mil!gwyn>

In article <580 at longway.TIC.COM> std-unix at uunet.uu.net writes:
>From: andrew at alice.uucp (Andrew Hume)
>the posix 1003.1 standard has cleverly evaded naming of bits
>in the mode field of the stat structure. it does this by
>defining tests (like S_ISDIR(mode) rather than (mode&S_IFMT)==S_IFDIR).
>the question is, how do people deal with sym links? I simply
>added a S_ISLNK macro but would prefer to go with the flow
>if there is one.

I think that would be the most obvious name for such a macro.
Unfortunately, so far as I can tell IEEE Std 1003.1-1988 fails
to stake out portions of macro name space for headers such as
<fcntl.h> (O_*) and <sys/stat.h> (S_*).  (I had thought that it
had, but I can't seem to find such provisions now; 2.8.2's
definition of the action of _POSIX_SOURCE was supposed to
support this.)

>From the point of view of 1003.1, S_ISLNK() is not very useful,
because both stat() and fstat() treat symbolic links as
"transparent", and 1003.1 doesn't mention lstat() which is the
function for which S_ISLNK() has a real use.

Probably the best solution for implementors is along these lines:

	/* <sys/stat.h> */
	...
	#define	_S_IFMT		0170000	/* type of file: */
	#define	_S_IFDIR	0040000	/* directory */
	#define	_S_IFCHR	0020000	/* character special */
	#define	_S_IFBLK	0060000	/* block special */
	#define	_S_IFREG	0100000	/* regular */
	#define	_S_IFLNK	0120000	/* symbolic link */
	#define	_S_IFSOCK	0140000 /* socket */
	#define	_S_IFIFO	0010000	/* fifo */

	#define	S_ISBLK(mode)	(((mode) & _S_IFMT) == _S_IFBLK)
	#define	S_ISCHR(mode)	(((mode) & _S_IFMT) == _S_IFCHR)
	#define	S_ISDIR(mode)	(((mode) & _S_IFMT) == _S_IFDIR)
	#define	S_ISFIFO(mode)	(((mode) & _S_IFMT) == _S_IFIFO)
	#define	S_ISREG(mode)	(((mode) & _S_IFMT) == _S_IFREG)

	#ifndef _POSIX_SOURCE	/* enable "common usage" extensions */

	#define	S_ISLNK(mode)	(((mode) & _S_IFMT) == _S_IFLNK)
	#define	S_ISSOCK(mode)	(((mode) & _S_IFMT) == _S_IFSOCK)

	#define	S_IFMT		_S_IFMT
	#define	S_IFDIR		_S_IFDIR
	#define	S_IFCHR		_S_IFCHR
	#define	S_IFBLK		_S_IFBLK
	#define	S_IFREG		_S_IFREG
	#define	S_IFLNK		_S_IFLNK
	#define	S_IFSOCK	_S_IFSOCK
	#define	S_IFIFO		_S_IFIFO

	#endif	/* !_POSIX_SOURCE */
	...

Volume-Number: Volume 19, Number 18