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From: jsdy@hadron.UUCP.UUCP
Newsgroups: mod.computers.vax
Subject: Re: main() and entry points in C
Message-ID: <8607150515.AA00312@hadron.UUCP>
Date: Tue, 15-Jul-86 01:15:13 EDT
Article-I.D.: hadron.8607150515.AA00312
Posted: Tue Jul 15 01:15:13 1986
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Newsgroups: net.lang.c
Subject: Re: Request for Comments
Summary: Combatting nonsense!

In article <870@bu-cs.UUCP> bzs@bu-cs.UUCP (Barry Shein) writes:
>Any comments? This is taken from INFO-VAX (mod.computers.vax):
>Path: bu-cs!harvard!caip!think!nike!styx!YALE.ARPA!LEICHTER-JERRY
>From: LEICHTER-JERRY@YALE.ARPA
>Subject: Re: main() and entry points in C
>Date: Thu, 26-Jun-86 08:50:52 EDT
>
>		...  Thus, _start == NULL.  Most users will never see this,
>but an implementer of _start() would.

If some implementation of C uses _start instead of start as an
entry point, they are asking for trouble.  I have never seen a
C-accessible symbol as an entry.  The entry point, start, used
to be almost always 0; but it isn't ever 0, now, in the latest
releases of AT&T System V Unix.

>    --  still going to bet that the first routine WON'T end up as the entry
>	point?
>    I won't bet on anything if the loader isn't invoked properly....
>That gets to the crux of things:  The "proper" way to invoke the loader is
>undocumented - you must use cc.  How then do you deal with a program written
>in multiple languages?  Basically, you ask a wizard....

On many versions of Unix, it is documented that properly written
multiple-language modules can be compiled together by appropriately
calling the compiler:
	cc myc.c myftn.o ...		or
	f77 myftn.f myc.o ...		or even
	f77 myftn.f myc.c ...

>		..  Since the "entry point is the first routine" IS, in fact,
>documented - even if only for wizards! - supporting it couldn't hurt....

You claim this is a documented undocumented feature?  It isn't
even true.  Specifically,
	never_called() { printf("Hi.  Joe is wrong.\n"); }
	main() { printf("Hello world.\n"); }
will never call me a liar.  Even if you are talking about
straight link-loaded objects (with no header to call _main()),
several common loaders allow one to specify entry points
other than 0.  These include, but are not! limited to, the
AT&T System V ld (again); the CULC Fortran IV Plus linker;
and the DEC VMS linker.

The C header, which cc puts at the head of the compiled objects,
contains the entry point -- which is not at location 0 under all
versions of Unix!  This header moves the argc, argv, and envp to
a location that main() will understand when called as a function;
then calls main() as a function; then calls exit() as a function
whose argument is main()'s return value; and if this doesn't exit,
typically tries to perform an exit system call itself, and then
(in desperation) a halt.

I should say it TYPICALLY does all this.  If it ain't documented
(and especially if it's not in the standard), it ain't guaranteed,
so don't bet the lunch money, Mildred.
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}
			jsdy@hadron.COM (not yet domainised)