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Re: [vox-tech] Nifty macro expansion bug
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Re: [vox-tech] Nifty macro expansion bug



Peter Jay Salzman wrote:
On Thu 18 Jan 07, 4:14 PM, Bill Kendrick <nbs@sonic.net> said:

So at work, a coworker noticed an issue with the following code:

for (i = 0; i < n; i++)
myFREE(foo[i]);

which went away when he wrapped it in braces:

for (i = 0; i < n; i++)
{
myFREE(foo[i]);
}

Figured maybe it was a weird compiler bug. Being author of the myFREE()
code -- which happens to be a macro -- I realized immediately what I did
wrong. (And continue to be annoyed with C syntax ;) )

See, myFREE() is actually a macro that looks like this:

#define myFREE(x) if ((x) != NULL) FREE(x); x = NULL;

Two statements... hence the need for {} around the call to the macro.
(Or, more sensibly, including {}s around what the macro expands to.)

That was a neat catch. :)

I ran into something similar, but more complicated, last week. An
expression that should've executed didn't.

I looked at the output of "cl.exe -E" (similar to gcc -E) which displays
what the compiler "sees" after the preprocessor does its thing. From this
view, it was easy to see that what looked like:


if ( some condition )
die( some error message )
else
do_something();


was arriving at the compiler as:


if ( some condition )
if ( lots of stuff )
print error message with lots of stuff.
else
do_something();

So the code needs to look like:

if ( some condition ) {
die( some error message )
} else {
do_something();
}


Pete

This is why Lisp has those parentheses...   ;-P
And don't get me started about Lisp macros versus the C preprocessor.
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