C: better way to do sizeof(((SomeStruct *) 0)->some_member)?

Question

I want to get the size of a specific member in a struct.

sizeof(((SomeStruct *) 0)->some_member) works for me but I feel like there might be a nicer way to do it.

I could #define SIZEOF_ELEM(STRUCT, ELEM) sizeof(((STRUCT *) 0)->ELEM) and then use SIZEOF_ELEM(SomeStruct, some_member), but I wonder whether there is already something better built-in.

My specific use-case is in hsc2hs (Haskell C bindings).

pokeArray (plusPtr context (#offset AVFormatContext, filename)) .
  take (#size ((AVFormatContext *) 0)->filename) .
  (++ repeat '\NUL') $ filename

Answer 1

What you've got is about as clean as it gets if you can't guarantee you have a variable to dereference. (If you can, then use just sizeof(var.member) or sizeof(ptr->member), of course, but this won't work in some contexts where a compile-time constant is needed.)

Once upon a long, long time ago (circa 1990), I ran into a compiler that had 'offsetof' defined using the base address 0, and it crashed. I worked around the problem by hacking <stddef.h> to use 1024 instead of 0. But you should not run into such problems now.