Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ucbvax!jade!eris!mwm
From: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Newsgroups: comp.lang.c
Subject: Re: Pragmas
Message-ID: <6477@jade.BERKELEY.EDU>
Date: 7 Jan 88 09:16:04 GMT
References: <17196@topaz.rutgers.edu> <1620005@hpcilzb.HP.COM> <5724@ccv.bbn.COM>
Sender: usenet@jade.BERKELEY.EDU
Reply-To: mwm@eris.BERKELEY.EDU (Mike (My watch has windows) Meyer)
Organization: Missionaria Phonibalonica
Lines: 49

In article <5724@ccv.bbn.COM> kgregory@ccv.bbn.com (Keith D. Gregory) writes:
<So, to all the people "who really know" . . . is there any good reason for
<#pragma's existance (I don't consider the above mentioned reason to be "good")

I don't know about "a good reason for #pragma's existence," but I *do*
know of a case where they were put to good use.

AmigaDOS supports shared libraries. The interface is a jump table into
the library, with a pointer to the table being stored in a variable of
a known name. In assembly, the calling sequence looks like:

	move.l	<args>,<registers>	; arguments
	movea.l	SomeLibrary,a0		; address table
	jsr	offset(a0)		; and go there

To use these things from C (vital - the OS interface looks like shared
libraries), it's normally done the same way as Unix: a stub routine in
assembler that takes the C arguments, puts them in the right
registers, and then jmps/jsrs to the real routine.

The latest version of the Lattice C compiler has a pragma for dealing
with shared libraries. It basically looks like:

#pragma	routinename libraryname offset	registers

This makes it generate an inline-call as the above, rather than a call
to a C stub that does the above. This means you get faster code, and
possibly slightly smaller code (depends on the details of code
generation & calling sequences I haven't looked at closely - going to
find out, though).

What makes the useage "good" is that when you include the header file
with the prototypes for the functions in a library, you get pragmas
for all the functions, too. This means that I can use the feature, and
the code will work on compilers that don't support this feature
without change. Or if they support it in another (though similar) way,
it should work.

I can't see a good way to do this without something similar to
pragmas. Wiring the names into the compiler means you can't get your
own libraries done that way (and is probably much nastier than the
above) without changing the compiler. Not good.

	<mike
--
Estant assis, de nuit secrette estude,			Mike Meyer
Seul, repose sur la selle d'airain,   			mwm@berkeley.edu
Flambe exigue, sortant de solitude,   			ucbvax!mwm
Fait proferer qui n'est a croire vain.			mwm@ucbjade.BITNET