CODEWITHSUNDEEP
ccastingc-preprocessor
Matthias Waelti
Matthias Waelti

Reputation: 41

C Typecasts in #define directives

I've seen many #defines for literals using a type cast. As example #define THE_ANSWER ((uint8_t) 42).

Until now, I can hardly imagine a situation where this really matters.

Can someone give me an example where a #define directive without a type cast leads to an "unexpected" behaviour. Maybe there is a good example for desktop environment and one for embedded or microcontroller environment.

Thank you for your suggestions.

Upvotes: 1

Views: 1007

Answers (1)

nullp0tr
nullp0tr

Reputation: 485

One example is multiplication. Say you have the following:

#define KILO 1024

what happens if you later on do the following?

unsigned long long val = KILO * KILO * KILO * KILO * KILO * ...

you might think that the multiplication would get evaluated with the type unsigned long long, but in reality all those multiplications happen on int. So if your directive doesn't have a typecast, or better yet it isn't 1024L you might end up with unexpected behavior because of an int overflow, even though your variable's type can actually hold the result.

Upvotes: 1

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