Storable tuples and what is 'alignment'?

[Alastair Reid]

Andrew J Bromage <ajb@spamcop.net> writes:
> This number is called the "alignment", and a good rule of thumb for
> computing it is:

>  instance Storable a where alignment a = sizeOf a `min` machine_word_size

The way we calculate it in GHC and Hugs is:

  #define offsetof(ty,field) ((size_t)((char *)&((ty *)0)->field - (char *)(ty *)0))

  print("%d", offsetof(struct { char c; $1 ty;},ty));

The simplest way of getting this value into some Haskell code is to
use hsc2hs (comes with GHC).  Something like this should do you:

instance Storable Foo where
  alignment _ = #const offsetof(struct { char c; $1 foo_ty;}
  sizeOf    _ = #const sizeof(foo_ty)
  peek = ...
  poke = ...

See

  http://haskell.org/ghc/docs/latest/html/users_guide/hsc2hs.html

for more info about hsc2hs.

-- 
Alastair Reid                 alastair@reid-consulting-uk.ltd.uk  
Reid Consulting (UK) Limited  http://www.reid-consulting-uk.ltd.uk/alastair/