How to get C compiler #error if a sizeof(struct ...) not equal to a given number? [duplicate]

Question

How to get C compile time #error if a sizeof(struct ...) not equal to a given number?

The question is from programming course, where I'd like to avoid to run miss-sized binary code.

(The sizeof operator, as we know, doesn't work in #if .. #endif directive.)

eerorika · Accepted Answer · 2019-03-02 18:06:15Z

5

How to get C compile time #error if a sizeof(struct ...) not equal to a given number?

You cannot, because the pre-processor knows nothing about sizes of types.

You can however static_assert:

static_assert(sizeof(T) == N, "T must have size N")

In C, the keyword is _Static_assert, also available through macro static_assert in <assert.h>.

answered Mar 2, 2019 at 16:38

eerorika

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Lightness Races in Orbit · Accepted Answer · 2019-03-02 16:38:22Z

4

Don't. You already explained why.

In modern C++ you can write:

static_assert(sizeof(T) == 42);

Although it is better to write code that doesn't care what the size of T is.

answered Mar 2, 2019 at 16:38

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Hasitha Amarathunga · Accepted Answer · 2019-03-02 18:12:10Z

1

#include <assert.h>
//T should have size 10
static_assert(sizeof(T) == 10)

It's available only the latest C compiler

answered Mar 2, 2019 at 18:12

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Detail: _Static_assert available since C11. Latest C is C18