Chapter 31
Foreign.Marshal.Alloc

The module Foreign.Marshal.Alloc provides operations to allocate and deallocate blocks of raw memory (i.e., unstructured chunks of memory outside of the area maintained by the Haskell storage manager). These memory blocks are commonly used to pass compound data structures to foreign functions or to provide space in which compound result values are obtained from foreign functions.

If any of the allocation functions fails, a value of nullPtr is produced. If free or reallocBytes is applied to a memory area that has been allocated with alloca or allocaBytes, the behaviour is undefined. Any further access to memory areas allocated with alloca or allocaBytes, after the computation that was passed to the allocation function has terminated, leads to undefined behaviour. Any further access to the memory area referenced by a pointer passed to realloc, reallocBytes, or free entails undefined behaviour.

All storage allocated by functions that allocate based on a size in bytes must be sufficiently aligned for any of the basic foreign types that fits into the newly allocated storage. All storage allocated by functions that allocate based on a specific type must be sufficiently aligned for that type. Array allocation routines need to obey the same alignment constraints for each array element.

31.1 Memory allocation

31.1.1 Local allocation

alloca :: Storable a => (Ptr a -> IO b) -> IO b

alloca f executes the computation f, passing as argument a pointer to a temporarily allocated block of memory sufficient to hold values of type a.

The memory is freed when f terminates (either normally or via an exception), so the pointer passed to f must not be used after this.

allocaBytes :: Int -> (Ptr a -> IO b) -> IO b

allocaBytes n f executes the computation f, passing as argument a pointer to a temporarily allocated block of memory of n bytes. The block of memory is sufficiently aligned for any of the basic foreign types that fits into a memory block of the allocated size.

The memory is freed when f terminates (either normally or via an exception), so the pointer passed to f must not be used after this.

31.1.2 Dynamic allocation

malloc :: Storable a => IO (Ptr a)

Allocate a block of memory that is sufficient to hold values of type a. The size of the area allocated is determined by the sizeOf method from the instance of Storable for the appropriate type.

The memory may be deallocated using free or finalizerFree when no longer required.

mallocBytes :: Int -> IO (Ptr a)

Allocate a block of memory of the given number of bytes. The block of memory is sufficiently aligned for any of the basic foreign types that fits into a memory block of the allocated size.

The memory may be deallocated using free or finalizerFree when no longer required.

realloc :: Storable b => Ptr a -> IO (Ptr b)

Resize a memory area that was allocated with malloc or mallocBytes to the size needed to store values of type b. The returned pointer may refer to an entirely different memory area, but will be suitably aligned to hold values of type b. The contents of the referenced memory area will be the same as of the original pointer up to the minimum of the original size and the size of values of type b.

If the argument to realloc is nullPtr, realloc behaves like malloc.

reallocBytes :: Ptr a -> Int -> IO (Ptr a)

Resize a memory area that was allocated with malloc or mallocBytes to the given size. The returned pointer may refer to an entirely different memory area, but will be sufficiently aligned for any of the basic foreign types that fits into a memory block of the given size. The contents of the referenced memory area will be the same as of the original pointer up to the minimum of the original size and the given size.

If the pointer argument to reallocBytes is nullPtr, reallocBytes behaves like malloc. If the requested size is 0, reallocBytes behaves like free.

free :: Ptr a -> IO ()

Free a block of memory that was allocated with malloc, mallocBytes, realloc, reallocBytes, Foreign.Marshal.Utils.new or any of the newX functions in Foreign.Marshal.Array or Foreign.C.String.

finalizerFree :: FinalizerPtr a

A pointer to a foreign function equivalent to free, which may be used as a finalizer (cf Foreign.ForeignPtr.ForeignPtr) for storage allocated with malloc, mallocBytes, realloc or reallocBytes.