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

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.

Solution 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);

Solution 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).

Solution 3:

I know it's a late answer, but to add on to Mike's version, here's a version we use that doesn't allocate any memory. I didn't come up with the original size check, I found it on the internet years ago and unfortunately can't reference the author. The other two are just extensions of the same idea.

Because they are typedef's, nothing is allocated. With the __LINE__ in the name, it's always a different name so it can be copied and pasted where needed. This works in MS Visual Studio C compilers, and GCC Arm compilers. It does not work in CodeWarrior, CW complains about redefinition, not making use of the __LINE__ preprocessor construct.

//Check overall structure size
typedef char p__LINE__[ (sizeof(PARS) == 4184) ? 1 : -1];

//check 8 byte alignment for flash write or similar
typedef char p__LINE__[ ((sizeof(PARS) % 8) == 0) ? 1 : 1];

//check offset in structure to ensure a piece didn't move
typedef char p__LINE__[ (offsetof(PARS, SUB_PARS) == 912) ? 1 : -1];

Solution 4:

I know this thread is really old but...

My solution:

extern char __CHECK__[1/!(<<EXPRESSION THAT SHOULD COME TO ZERO>>)];

As long as that expression equates to zero, it compiles fine. Anything else and it blows up right there. Because the variable is extern'd it will take up no space, and as long as no-one references it (which they won't) it won't cause a link error.

Not as flexible as the assert macro, but I couldn't get that to compile in my version of GCC and this will... and I think it will compile just about anywhere.

Solution 5:

The existing answers just show how to achieve the effect of "compile-time assertions" based on the size of a type. That may meet the OP's needs in this particular case, but there are other cases where you really need a preprocessor conditional based on the size of a type. Here's how to do it:

Write yourself a little C program like:

/* you could call this sizeof_int.c if you like... */
#include <stdio.h>
/* 'int' is just an example, it could be any other type */
int main(void) { printf("%zd", sizeof(int); }

Compile that. Write a script in your favorite scripting language, which runs the above C program and captures its output. Use that output to generate a C header file. For example, if you were using Ruby, it might look like:

sizeof_int = `./sizeof_int`
File.open('include/sizes.h','w') { |f| f.write(<<HEADER) }
/* COMPUTER-GENERATED, DO NOT EDIT BY HAND! */
#define SIZEOF_INT #{sizeof_int}
/* others can go here... */
HEADER

Then add a rule to your Makefile or other build script, which will make it run the above script to build sizes.h.

Include sizes.h wherever you need to use preprocessor conditionals based on sizes.

Done!

(Have you ever typed ./configure && make to build a program? What configure scripts do is basically just like the above...)