Simple #define:d macro doesn't work in C

Question

I'm doing an introductory course to C programming, and we've just started defining macros. I'm supposed to define the macro CHECK that prints msg (a string) if cond is false, however, it doesn't print the msg to the stdout stream, whether cond is true or false. Any ideas?

1     #define CHECK(cond, msg) if(!cond) fprintf(stdout, "%s", msg)

I'd bet you are passing it something like `CHECK(a == b, "a is not b");`, right? in which case that will expand to `if (!a == b)`, and then operator precedence ruins your day… that's why you **always unconditionally parenthesize** all your macro arguments. — The Paramagnetic Croissant, Mar 14 '15 at 11:45
maybe take cond in parenthesis `#define CHECK(cond, msg) if(!(cond)) fprintf(stdout, "%s", msg)` — David Szalai, Mar 14 '15 at 11:45
Your macro definition doesn't work because the line starts with a `1`. See http://sscce.org for how to ask a “my program doesn't work” question. — Pascal Cuoq, Mar 14 '15 at 11:46
Thank you @TheParamagneticCroissant, makes complete sense but didn't come to think of that. — Adrian, Mar 14 '15 at 13:21
Also, just don't write macros that change control flow in a unexpected way. Your macro is flawed in multiple ways. — Jens Gustedt, Mar 14 '15 at 14:40
@Adrian hopefully this makes you prefer functions to macros enough so that after completing this task, you won't make macros where an (inline) function would do. — The Paramagnetic Croissant, Mar 14 '15 at 15:15

score 8 · Answer 1 · edited May 23 '17 at 12:07

8

You should wrap your function-like macro like this:

#define CHECK(cond, msg)                   \
    do {                                   \
        if (!(cond)) {                     \
             fprintf(stdout, "%s", msg);   \
        }                                  \
    } while (0)

Explanation: C multi-line macro: do/while(0) vs scope block

edited May 23 '17 at 12:07

Community

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answered Mar 14 '15 at 11:55

Marco

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Nearly perfect. Just wrap any occurencies of `cond` and `msg` in parentheses: `(cond) ... (msg)` – alk Mar 14 '15 at 12:05
You are right, thanks! I changed my answer accordingly. – Marco Mar 14 '15 at 12:37

score 0 · Answer 2 · answered Mar 14 '15 at 11:47

0

try wrapping it in {}:

 #define CHECK(cond, msg) {if(!cond) fprintf(stdout, "%s", msg)}

plus , make sure you really don't start with '1'

answered Mar 14 '15 at 11:47

David Haim

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This will lead to surprising behavior: `if (x) CHECK(cond, msg); else func();` won't work because of the extra semicolon. You have to use `do { ... } while (0)`. – Dietrich Epp Mar 14 '15 at 11:48

Simple #define:d macro doesn't work in C

2 Answers2