Re: do { } while (0) question

On Tue, Aug 01, 2006 at 11:45:53AM +0159, Jiri Slaby wrote:
Peter Zijlstra wrote:
On Tue, 2006-08-01 at 02:03 -0700, Hua Zhong wrote:
#if KILLER == 1
#define MACRO
#else
#define MACRO do { } while (0)
#endif

{
if (some_condition)
MACRO

if_this_is_not_called_you_loose_your_data();
}

How do you want to define KILLER, 0 or 1? I personally choose 0.
Really? Does it compile?

No, and that is the whole point.

The empty 'do {} while (0)' makes the missing semicolon a syntax error.

Bulls^WNope, it was a bad example (we don't want to break the compilation,
just not want to emit a warn or an err).

Your sentence does not make sense, but I'm going to take it as saying
that you disagree that the above will cause a syntax error. Try it:

$ cat t.c
#if KILLER == 1
#define MACRO
#else
#define MACRO do { } while (0)
#endif

void foo(int some_condition)
{
if (some_condition)
MACRO

if_this_is_not_called_you_loose_your_data();
}
$ gcc -O2 -o - -E t.c
# 1 "t.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "t.c"






void foo(int some_condition)
{
if (some_condition)
do { } while (0)

if_this_is_not_called_you_loose_your_data();
}
$ gcc -O2 -o - -S t.c >/dev/null
t.c: In function `foo':
t.c:12: error: parse error before "if_this_is_not_called_you_loose_your_data"
$ gcc -O2 -o - -E t.c -DKILLER
# 1 "t.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "t.c"






void foo(int some_condition)
{
if (some_condition)


if_this_is_not_called_you_loose_your_data();
}
$ gcc -O2 -o - -S t.c -DKILLER >/dev/null
$

Hence, using do { } while (0) has had the desired effect - the missing
semicolon causes a compile error, while the empty macro results in
unintentional successful compilation without warning or error.

--
Russell King
Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/
maintainer of: 2.6 Serial core
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