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From: guy@gorodish.UUCP
Newsgroups: news.software.b,comp.lang.c
Subject: Re: News for Xenix on PC AT ?
Message-ID: <17435@sun.uucp>
Date: Mon, 27-Apr-87 14:27:45 EDT
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Posted: Mon Apr 27 14:27:45 1987
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Xref: utgpu news.software.b:532 comp.lang.c:1858

>Signal() returns a pointer to a function; in the event of an error, -1 is
>returned, not 0.  As far as I know, all other functions that return a
>pointer, return 0 on error.  This seems to imply that 0 may be a valid
>function address;

No, you're inferring (incorrectly) that 0 may be a valid function address.

>not being familiar with UNIX internals, I can't say for certain, but other
>O/S's load programs such that they regard their starting address as 0.

As UNIX is a portable operating system, it is not meaningful to ask
whether UNIX loads programs such that they regard their starting
address as 0.  Some implementations do, some don't.  (Ours doesn't.)

However, just because a UNIX implementation loads a program such that
it regards its starting address as 0 does NOT mean that the address
of any function known to C within that program is necessarily 0.  In
fact, a C implementation is obliged to do whatever is necessary to
ensure that no function in a program will have the address 0; a
"nop" or a "br .+N" at location 0 should be sufficient.

>On the other hand, if 0 cannot be a valid function address, why does
>signal not return 0 on error?  I would be curious to know.

Because:

	1) other system calls return -1 on error (including a System
	V shared memory call that returns a pointer);

	2) the special signal value for "this signal is not caught"
	is usually a null pointer, so a call to "signal" that is
	successful can return a null pointer.