How can I use "sizeof" in a preprocessor macro?

Question

Is there any way to use a sizeof in a preprocessor macro?

For example, there have been a ton of situations over the years in which I wanted to do something like:

#if sizeof(someThing) != PAGE_SIZE #error Data structure doesn't match page size #endif 

The exact thing I'm checking here is completely made up - the point is, I often like to put in these types of (size or alignment) compile-time checks to guard against someone modifying a data-structure which could misalign or re-size things which would break them.

Needless to say - I don't appear to be able to use a sizeof in the manner described above.

Answer 1

There are several ways of doing this. Following snippets will produce no code if sizeof(someThing) equals PAGE_SIZE; otherwise they will produce a compile-time error.

1. C11 way

Starting with C11 you can use static_assert (requires #include <assert.h>).

Usage:

static_assert(sizeof(someThing) == PAGE_SIZE, "Data structure doesn't match page size"); 

2. Custom macro

If you just want to get a compile-time error when sizeof(something) is not what you expect, you can use following macro:

#define BUILD_BUG_ON(condition) ((void)sizeof(char[1 - 2*!!(condition)])) 

Usage:

BUILD_BUG_ON( sizeof(someThing) != PAGE_SIZE ); 

This article explains in details why it works.

3. MS-specific

On Microsoft C++ compiler you can use C_ASSERT macro (requires #include <windows.h>), which uses a trick similar to the one described in section 2.

Usage:

C_ASSERT(sizeof(someThing) == PAGE_SIZE); 

Answer 2

Is there anyway to use a "sizeof" in a pre-processor macro?

No. The conditional directives take a restricted set of conditional expressions; sizeof is one of the things not allowed.

Preprocessing directives are evaluated before the source is parsed (at least conceptually), so there aren't any types or variables yet to get their size.

However, there are techniques to getting compile-time assertions in C (for example, see this page).