Re: ick, ick -- grossness in linux kernel

From: Ben Liblit (liblit@EECS.Berkeley.EDU)
Date: 10/09/00 

Message-ID: <39E24E12.D870AB@cs.berkeley.edu>
Date: Mon, 09 Oct 2000 16:00:34 -0700
From: Ben Liblit <liblit@EECS.Berkeley.EDU>
Subject: Re: ick, ick -- grossness in linux kernel

> Anyone know why this code gets wrapped in a useless do loop?

Let me guess: the code you found came from a macro expansion, right?

This is a standard trick when you want a macro that expands into
multiple statements.  Suppose I want a macro "AB(x)" that executes two
statements: a(x) followed by b(x).

Here's one approach that doesn't work:

	#define AB(x)  a(x); b(x);

	if (p)
	  AB(z);	/* oops!  b(z) is outside of conditional */

Here's another approach that also doesn't work:

	#define AB(x)  { a(x); b(x); }

	if (p)
	  AB(z);
	else		/* oops!  extra semicolon before "else" */
	  foo();

Here's the approach that works:

	#define AB(x)  do { a(x); b(x); } while (0)

	if (p)
	  AB(z);
	else
	  foo();

It's an ugly trick, but a popular one just the same.

This archive was generated by hypermail 2b30 : 11/04/02 PST