|Copyright||(c) The FFI task force 2001|
- class Storable a where
The member functions of this class facilitate writing values of primitive types to raw memory (which may have been allocated with the above mentioned routines) and reading values from blocks of raw memory. The class, furthermore, includes support for computing the storage requirements and alignment restrictions of storable types.
Memory addresses are represented as values of type
, for some
a which is an instance of class
Storable. The type argument to
Ptr helps provide some valuable type safety in FFI code (you can't
mix pointers of different types without an explicit cast), while
helping the Haskell type system figure out which marshalling method is
needed for a given pointer.
All marshalling between Haskell and a foreign language ultimately
boils down to translating Haskell data structures into the binary
representation of a corresponding data structure of the foreign
language and vice versa. To code this marshalling in Haskell, it is
necessary to manipulate primitive data types stored in unstructured
memory blocks. The class
Storable facilitates this manipulation on
all types for which it is instantiated, which are the standard basic
types of Haskell, the fixed size
Int types (
Int64), the fixed size
Word types (
StablePtr, all types from Foreign.C.Types,
as well as
Computes the storage requirements (in bytes) of the argument. The value of the argument is not used.
Computes the alignment constraint of the argument. An
x is fulfilled by any address divisible
x. The value of the argument is not used.
Read a value from a memory area regarded as an array
of values of the same kind. The first argument specifies
the start address of the array and the second the index into
the array (the first element of the array has index
0). The following equality holds,
peekElemOff addr idx = IOExts.fixIO $ \result -> peek (addr `plusPtr` (idx * sizeOf result))
Note that this is only a specification, not necessarily the concrete implementation of the function.
Write a value to a memory area regarded as an array of values of the same kind. The following equality holds:
pokeElemOff addr idx x = poke (addr `plusPtr` (idx * sizeOf x)) x
Read a value from a memory location given by a base address and offset. The following equality holds:
peekByteOff addr off = peek (addr `plusPtr` off)
Write a value to a memory location given by a base address and offset. The following equality holds:
pokeByteOff addr off x = poke (addr `plusPtr` off) x
Read a value from the given memory location.
Note that the peek and poke functions might require properly
aligned addresses to function correctly. This is architecture
dependent; thus, portable code should ensure that when peeking or
poking values of some type
a, the alignment
a, as given by the function
alignment is fulfilled.
Write the given value to the given memory location. Alignment
restrictions might apply; see