What's wrong with pass-by-value structs? Yes, I'm aware that long ago there was a problem on SPARCs where Sun Studio and GCC handled struct value returns differently, but not pass-by-value. ABIs can and do define how to do pass by value of values of struct types.

The bits of C that need to be avoided in APIs are:

  - bitfields unless no bitfield crosses a
    byte boundary
  
  - struct fields of enum types (because the
    size of the enum type is implementation
    specific)

The spec should fix this, damnit.