How can I print the result of sizeof() at compile time in C?

Question

How can I print the result of sizeof() at compile time in C?

For now I am using a static assert (home brewed based on other web resources) to compare the sizeof() result to various constants. While this works... it is far from elegant or fast. I can also create an instance of the variable/struct and look in the map file but this is also less elegant and fast than a direct call/command/operator. Further, this is an embedded project using multiple cross-compilers... so building and loading a sample program to the target and then reading out a value is even more of a hassle than either of the above.

In my case (old GCC), #warning sizeof(MyStruct) does not actually interpret sizeof() before printing the warning.

Answer 1

I was mucking around looking for similar functionality when I stumbled on this:

Is it possible to print out the size of a C++ class at compile-time?

Which gave me the idea for this:

char (*__kaboom)[sizeof( YourTypeHere )] = 1;

Which results in the following warning in VS2015:

warning C4047: 'initializing': 'DWORD (*)[88]' differs in levels of indirection from 'int'

where 88 in this case would be the size you're looking for.

Super hacky, but it does the trick. Probably a couple years too late, but hopefully this will be useful to someone.

I haven't had a chance to try with gcc or clang yet, but I'll try to confirm whether or not it works if someone doesn't get to it before me.

Edit: Works out of the box for clang 3.6

The only trick I could get to work for GCC was abusing -Wformat and having the macro define a function like the following:

void kaboom_print( void )
{
    printf( "%d", __kaboom );
}

Which will give you a warning like:

...blah blah blah... argument 2 has type 'char (*)[88]'

Slightly more gross than the original suggestion, but maybe someone who knows gcc a bit better can think of a better warning to abuse.