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From: stevesu@copper.UUCP (Steve Summit)
Newsgroups: net.lang.c
Subject: Jumping into blocks is a no-no (was: Re: An amusing piece of code)
Message-ID: <275@copper.UUCP>
Date: Sat, 12-Apr-86 17:26:22 EST
Article-I.D.: copper.275
Posted: Sat Apr 12 17:26:22 1986
Date-Received: Mon, 14-Apr-86 01:45:30 EST
References: <1370@ism780c.UUCP> <360@hadron.UUCP>
Organization: Tektronix, Inc., Beaverton, OR
Lines: 56

In article <360@hadron.UUCP>, jsdy@hadron.UUCP (Joseph S. D. Yao) writes:
> One major problem is that this code jumps into blocks
> [the effect of case C: and case D: ].  This is something that X3J11
> warns about.  I thought K&R did, too; but I can't find it.  It is
> generally a bad practice, although most compilers seem to allow it
> without too much trouble.

I used to worry about jumps into blocks, too, but I'm afraid
I've really gotten into the habit of using them.  Just how bad
are they?  It's pretty easy arrange that simpleminded, one-pass
code generators generate loops that can be safely jumped into.
I understand that this sort of thing makes an optimizing code
generator's life miserable, but since so much existing code jumps
into blocks, I would guess that optimizing compiler writers have
to worry about it anyway.

Most people agree that goto's are pretty acceptable for handling
error conditions.  For instance:

	if(stbuf.st_uid != getuid())
		{
nope:		printf("I don't think you want to delete this file.\n");
		return;
		}

	if(strcmp(name, "vmunix") == 0)
		goto nope;

	unlink(name);

	return;

(Of course, this usage is not a jump into a loop, which would be
more worrisome.)  Given that the jumped-into block terminates
with a return, it's quite easy to eliminate the jump into a
block, but only at the expense of a _s_e_c_o_n_d goto:

	if(stbuf.st_uid != getuid())
		goto nope;

	if(strcmp(name, "vmunix") == 0)
		goto nope;

	unlink(name);

	return;

nope:	printf("I don't think you want to delete this file.\n");
	return;

What do people think?  It seems to me that if gotos are legal at
all, they might as well be guaranteed to work for jumping into
blocks.  How strong is the "warning" in X3J11 that Joe mentions?

                                         Steve Summit
                                         tektronix!copper!stevesu