macros and semicolons

[Michael Kahl]

In article <1991Jun24.213932.595 at otago.ac.nz> andrew at otago.ac.nz writes:
>I often get pissed off with the C pre-processor.  Here is one thats been
>getting up my wick for months.
>
>#define SWAP(a, b) {int c;  c = a; a = b; b = c}
>
>if (spam)
>   SWAP(a, b);
>else
>   a++;
>
>[discusses why this doesn't work, asks how to get it to work...]

What I do in cases like this is to surround a "statement-like" macro with
do/while(0), like so:

    #define SWAP(a, b)  do { int c = a; a = b; b = c; } while (0)

This expansion has the right properties:  it becomes a statement when a
semicolon appears after it.  If your compiler is reasonable, there will
be no overhead associated with the fake "loop".
-- 
Michael Kahl, Symantec Corporation
mkahl at world.std.com  -or-  75236.3146 at compuserve.com
Disclaimer:  Keep this quiet; what my employer doesn't know won't get me fired.