GHC.ForeignPtr

base-4.16.0.0: Basic libraries

Copyright | (c) The University of Glasgow 1992-2003
License | see libraries/base/LICENSE
Maintainer | cvs-ghc@haskell.org
Stability | internal
Portability | non-portable (GHC extensions)
Safe Haskell | Unsafe
Language | Haskell2010

GHC.ForeignPtr

Contents

- Types
- Create
- Add Finalizers
- Conversion
- Control over lifetype
- Finalization
- Commentary

Description

GHC's implementation of the `ForeignPtr` data type.

Synopsis

- data ForeignPtr a = ForeignPtr Addr# ForeignPtrContents
- data ForeignPtrContents
  - = PlainForeignPtr !(IORef Finalizers)
  - | FinalPtr
  - | MallocPtr (MutableByteArray# RealWorld) !(IORef Finalizers)
  - | PlainPtr (MutableByteArray# RealWorld)
- data Finalizers
  - = NoFinalizers
  - | CFinalizers (Weak# ())
  - | HaskellFinalizers [IO ()]
- type FinalizerPtr a = FunPtr (Ptr a -> IO ())
- type FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ())
- newForeignPtr_ :: Ptr a -> IO (ForeignPtr a)
- mallocForeignPtr :: Storable a => IO (ForeignPtr a)
- mallocPlainForeignPtr :: Storable a => IO (ForeignPtr a)
- mallocForeignPtrBytes :: Int -> IO (ForeignPtr a)
- mallocPlainForeignPtrBytes :: Int -> IO (ForeignPtr a)
- mallocForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a)
- mallocPlainForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a)
- newConcForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a)
- addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO ()
- addForeignPtrFinalizerEnv :: FinalizerEnvPtr env a -> Ptr env -> ForeignPtr a -> IO ()
- addForeignPtrConcFinalizer :: ForeignPtr a -> IO () -> IO ()
- unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a
- castForeignPtr :: ForeignPtr a -> ForeignPtr b
- plusForeignPtr :: ForeignPtr a -> Int -> ForeignPtr b
- withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b
- unsafeWithForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b
- touchForeignPtr :: ForeignPtr a -> IO ()
- finalizeForeignPtr :: ForeignPtr a -> IO ()

# Types

data ForeignPtr a Source #

The type `ForeignPtr` represents references to objects that are maintained in a foreign language, i.e., that are not part of the data structures usually managed by the Haskell storage manager. The essential difference between `ForeignPtr`s and vanilla memory references of type `Ptr a` is that the former may be associated with finalizers. A finalizer is a routine that is invoked when the Haskell storage manager detects that - within the Haskell heap and stack - there are no more references left that are pointing to the `ForeignPtr`. Typically, the finalizer will, then, invoke routines in the foreign language that free the resources bound by the foreign object.

The `ForeignPtr` is parameterised in the same way as `Ptr`. The type argument of `ForeignPtr` should normally be an instance of class `Storable`.

Constructors

ForeignPtr Addr# ForeignPtrContents |

#### Instances

Instances details

Data a => Data (ForeignPtr a) Source # | Since: base-4.8.0.0
Instance details Defined in Data.Data Methods gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> ForeignPtr a -> c (ForeignPtr a) Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c (ForeignPtr a) Source # toConstr :: ForeignPtr a -> Constr Source # dataTypeOf :: ForeignPtr a -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c (ForeignPtr a)) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c (ForeignPtr a)) Source # gmapT :: (forall b. Data b => b -> b) -> ForeignPtr a -> ForeignPtr a Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> ForeignPtr a -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> ForeignPtr a -> r Source # gmapQ :: (forall d. Data d => d -> u) -> ForeignPtr a -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> ForeignPtr a -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> ForeignPtr a -> m (ForeignPtr a) Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> ForeignPtr a -> m (ForeignPtr a) Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> ForeignPtr a -> m (ForeignPtr a) Source #
Show (ForeignPtr a) Source # | Since: base-2.1
Instance details Defined in GHC.ForeignPtr Methods showsPrec :: Int -> ForeignPtr a -> ShowS Source # show :: ForeignPtr a -> String Source # showList :: [ForeignPtr a] -> ShowS Source #
Eq (ForeignPtr a) Source # | Since: base-2.1
Instance details Defined in GHC.ForeignPtr Methods (==) :: ForeignPtr a -> ForeignPtr a -> Bool Source # (/=) :: ForeignPtr a -> ForeignPtr a -> Bool Source #
Ord (ForeignPtr a) Source # | Since: base-2.1
Instance details Defined in GHC.ForeignPtr Methods compare :: ForeignPtr a -> ForeignPtr a -> Ordering Source # (<) :: ForeignPtr a -> ForeignPtr a -> Bool Source # (<=) :: ForeignPtr a -> ForeignPtr a -> Bool Source # (>) :: ForeignPtr a -> ForeignPtr a -> Bool Source # (>=) :: ForeignPtr a -> ForeignPtr a -> Bool Source # max :: ForeignPtr a -> ForeignPtr a -> ForeignPtr a Source # min :: ForeignPtr a -> ForeignPtr a -> ForeignPtr a Source #

data ForeignPtrContents Source #

Controls finalization of a `ForeignPtr`, that is, what should happen if the `ForeignPtr` becomes unreachable. Visually, these data constructors are appropriate in these scenarios:

                              Memory backing pointer is
                               GC-Managed   Unmanaged
    Finalizer functions are: +------------+-----------------+
                    Allowed  | MallocPtr  | PlainForeignPtr |
                             +------------+-----------------+
                 Prohibited  | PlainPtr   | FinalPtr        |
                             +------------+-----------------+

Constructors

PlainForeignPtr !(IORef Finalizers) | The pointer refers to unmanaged memory that was allocated by a foreign function (typically using `malloc`). The finalizer frequently calls the C function `free` or some variant of it.
FinalPtr | The pointer refers to unmanaged memory that should not be freed when the `ForeignPtr` becomes unreachable. Functions that add finalizers to a `ForeignPtr` throw exceptions when the `ForeignPtr` is backed by `PlainPtr`Most commonly, this is used with `Addr#` literals. See Note [Why FinalPtr]. Since: base-4.15
MallocPtr (MutableByteArray# RealWorld) !(IORef Finalizers) | The pointer refers to a byte array. The `MutableByteArray#` field means that the `MutableByteArray#` is reachable (by GC) whenever the `ForeignPtr` is reachable. When the `ForeignPtr` becomes unreachable, the runtime's normal GC recovers the memory backing it. Here, the finalizer function intended to be used to `free()` any ancillary *unmanaged* memory pointed to by the `MutableByteArray#`. See the `zlib` library for an example of this use. Invariant: The `Addr#` in the parent `ForeignPtr` is an interior pointer into this `MutableByteArray#`. Invariant: The `MutableByteArray#` is pinned, so the `Addr#` does not get invalidated by the GC moving the byte array. Invariant: A `MutableByteArray#` must not be associated with more than one set of finalizers. For example, this is sound:
    incrGood :: ForeignPtr Word8 -> ForeignPtr Word8
    incrGood (ForeignPtr p (MallocPtr m f)) = ForeignPtr (plusPtr p 1) (MallocPtr m f)
But this is unsound:
    incrBad :: ForeignPtr Word8 -> IO (ForeignPtr Word8)
    incrBad (ForeignPtr p (MallocPtr m _)) = do
      f <- newIORef NoFinalizers
      pure (ForeignPtr p (MallocPtr m f))
PlainPtr (MutableByteArray# RealWorld) | The pointer refers to a byte array. Finalization is not supported. This optimizes `MallocPtr` by avoiding the allocation of a `MutVar#` when it is known that no one will add finalizers to the `ForeignPtr`. Functions that add finalizers to a `ForeignPtr` throw exceptions when the `ForeignPtr` is backed by `PlainPtr`. The invariants that apply to `MallocPtr` apply to `PlainPtr` as well.

data Finalizers Source #

Functions called when a `ForeignPtr` is finalized. Note that C finalizers and Haskell finalizers cannot be mixed.

Constructors

NoFinalizers | No finalizer. If there is no intent to add a finalizer at any point in the future, consider `FinalPtr` or `PlainPtr` instead since these perform fewer allocations.
CFinalizers (Weak# ()) | Finalizers are all C functions.
HaskellFinalizers [IO ()] | Finalizers are all Haskell functions.

type FinalizerPtr a = FunPtr (Ptr a -> IO ()) Source #

A finalizer is represented as a pointer to a foreign function that, at finalisation time, gets as an argument a plain pointer variant of the foreign pointer that the finalizer is associated with.

Note that the foreign function must use the `ccall` calling convention.

type FinalizerEnvPtr env a = FunPtr (Ptr env -> Ptr a -> IO ()) Source #

# Create

newForeignPtr_ :: Ptr a -> IO (ForeignPtr a) Source #

Turns a plain memory reference into a foreign pointer that may be associated with finalizers by using `addForeignPtrFinalizer`.

mallocForeignPtr :: Storable a => IO (ForeignPtr a) Source #

Allocate some memory and return a `ForeignPtr` to it. The memory will be released automatically when the `ForeignPtr` is discarded.

`mallocForeignPtr` is equivalent to

       do { p <- malloc; newForeignPtr finalizerFree p }

although it may be implemented differently internally: you may not assume that the memory returned by `mallocForeignPtr` has been allocated with `malloc`.

GHC notes: `mallocForeignPtr` has a heavily optimised implementation in GHC. It uses pinned memory in the garbage collected heap, so the `ForeignPtr` does not require a finalizer to free the memory. Use of `mallocForeignPtr` and associated functions is strongly recommended in preference to `newForeignPtr` with a finalizer.

mallocPlainForeignPtr :: Storable a => IO (ForeignPtr a) Source #

Allocate some memory and return a `ForeignPtr` to it. The memory will be released automatically when the `ForeignPtr` is discarded.

GHC notes: `mallocPlainForeignPtr` has a heavily optimised implementation in GHC. It uses pinned memory in the garbage collected heap, as for mallocForeignPtr. Unlike mallocForeignPtr, a ForeignPtr created with mallocPlainForeignPtr carries no finalizers. It is not possible to add a finalizer to a ForeignPtr created with mallocPlainForeignPtr. This is useful for ForeignPtrs that will live only inside Haskell (such as those created for packed strings). Attempts to add a finalizer to a ForeignPtr created this way, or to finalize such a pointer, will throw an exception.

mallocForeignPtrBytes :: Int -> IO (ForeignPtr a) Source #

This function is similar to `mallocForeignPtr`, except that the size of the memory required is given explicitly as a number of bytes.

mallocPlainForeignPtrBytes :: Int -> IO (ForeignPtr a) Source #

This function is similar to `mallocForeignPtrBytes`, except that the internally an optimised ForeignPtr representation with no finalizer is used. Attempts to add a finalizer will cause an exception to be thrown.

mallocForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a) Source #

This function is similar to `mallocForeignPtrBytes`, except that the size and alignment of the memory required is given explicitly as numbers of bytes.

mallocPlainForeignPtrAlignedBytes :: Int -> Int -> IO (ForeignPtr a) Source #

This function is similar to `mallocForeignPtrAlignedBytes`, except that the internally an optimised ForeignPtr representation with no finalizer is used. Attempts to add a finalizer will cause an exception to be thrown.

newConcForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a) Source #

Turns a plain memory reference into a foreign object by associating a finalizer - given by the monadic operation - with the reference. The storage manager will start the finalizer, in a separate thread, some time after the last reference to the `ForeignPtr` is dropped. There is no guarantee of promptness, and in fact there is no guarantee that the finalizer will eventually run at all.

Note that references from a finalizer do not necessarily prevent another object from being finalized. If A's finalizer refers to B (perhaps using `touchForeignPtr`, then the only guarantee is that B's finalizer will never be started before A's. If both A and B are unreachable, then both finalizers will start together. See `touchForeignPtr` for more on finalizer ordering.

# Add Finalizers

addForeignPtrFinalizer :: FinalizerPtr a -> ForeignPtr a -> IO () Source #

This function adds a finalizer to the given foreign object. The finalizer will run before all other finalizers for the same object which have already been registered.

addForeignPtrFinalizerEnv :: FinalizerEnvPtr env a -> Ptr env -> ForeignPtr a -> IO () Source #

Like `addForeignPtrFinalizer` but the finalizer is passed an additional environment parameter.

addForeignPtrConcFinalizer :: ForeignPtr a -> IO () -> IO () Source #

This function adds a finalizer to the given `ForeignPtr`. The finalizer will run before all other finalizers for the same object which have already been registered.

This is a variant of `addForeignPtrFinalizer`, where the finalizer is an arbitrary `IO` action. When it is invoked, the finalizer will run in a new thread.

NB. Be very careful with these finalizers. One common trap is that if a finalizer references another finalized value, it does not prevent that value from being finalized. In particular, `Handle`s are finalized objects, so a finalizer should not refer to a `Handle` (including `stdout`, `stdin`, or `stderr`).

# Conversion

unsafeForeignPtrToPtr :: ForeignPtr a -> Ptr a Source #

This function extracts the pointer component of a foreign pointer. This is a potentially dangerous operations, as if the argument to `unsafeForeignPtrToPtr` is the last usage occurrence of the given foreign pointer, then its finalizer(s) will be run, which potentially invalidates the plain pointer just obtained. Hence, `touchForeignPtr` must be used wherever it has to be guaranteed that the pointer lives on - i.e., has another usage occurrence.

To avoid subtle coding errors, hand written marshalling code should preferably use `withForeignPtr` rather than combinations of `unsafeForeignPtrToPtr` and `touchForeignPtr`. However, the latter routines are occasionally preferred in tool generated marshalling code.

castForeignPtr :: ForeignPtr a -> ForeignPtr b Source #

This function casts a `ForeignPtr` parameterised by one type into another type.

plusForeignPtr :: ForeignPtr a -> Int -> ForeignPtr b Source #

Advances the given address by the given offset in bytes.

The new `ForeignPtr` shares the finalizer of the original, equivalent from a finalization standpoint to just creating another reference to the original. That is, the finalizer will not be called before the new `ForeignPtr` is unreachable, nor will it be called an additional time due to this call, and the finalizer will be called with the same address that it would have had this call not happened, *not* the new address.

Since: base-4.10.0.0

# Control over lifetype

withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b Source #

This is a way to look at the pointer living inside a foreign object. This function takes a function which is applied to that pointer. The resulting `IO` action is then executed. The foreign object is kept alive at least during the whole action, even if it is not used directly inside. Note that it is not safe to return the pointer from the action and use it after the action completes. All uses of the pointer should be inside the `withForeignPtr` bracket. The reason for this unsafeness is the same as for `unsafeForeignPtrToPtr` below: the finalizer may run earlier than expected, because the compiler can only track usage of the `ForeignPtr` object, not a `Ptr` object made from it.

This function is normally used for marshalling data to or from the object pointed to by the `ForeignPtr`, using the operations from the `Storable` class.

unsafeWithForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b Source #

This is similar to `withForeignPtr` but comes with an important caveat: the user must guarantee that the continuation does not diverge (e.g. loop or throw an exception). In exchange for this loss of generality, this function offers the ability of GHC to optimise more aggressively.

Specifically, applications of the form: `unsafeWithForeignPtr fptr (`forever` something)`

See GHC issue #17760 for more information about the unsoundness behavior that this function can result in.

touchForeignPtr :: ForeignPtr a -> IO () Source #

This function ensures that the foreign object in question is alive at the given place in the sequence of IO actions. However, this comes with a significant caveat: the contract above does not hold if GHC can demonstrate that the code preceeding `touchForeignPtr` diverges (e.g. by looping infinitely or throwing an exception). For this reason, you are strongly advised to use instead `withForeignPtr` where possible.

Also, note that this function should not be used to express dependencies between finalizers on `ForeignPtr`s. For example, if the finalizer for a `ForeignPtr` `F1` calls `touchForeignPtr` on a second `ForeignPtr` `F2`, then the only guarantee is that the finalizer for `F2` is never started before the finalizer for `F1`. They might be started together if for example both `F1` and `F2` are otherwise unreachable, and in that case the scheduler might end up running the finalizer for `F2` first.

In general, it is not recommended to use finalizers on separate objects with ordering constraints between them. To express the ordering robustly requires explicit synchronisation using `MVar`s between the finalizers, but even then the runtime sometimes runs multiple finalizers sequentially in a single thread (for performance reasons), so synchronisation between finalizers could result in artificial deadlock. Another alternative is to use explicit reference counting.

# Finalization

finalizeForeignPtr :: ForeignPtr a -> IO () Source #

Causes the finalizers associated with a foreign pointer to be run immediately. The foreign pointer must not be used again after this function is called. If the foreign pointer does not support finalizers, this is a no-op.

# Commentary

This is a high-level overview of how `ForeignPtr` works. The implementation of `ForeignPtr` must accomplish several goals:

1. Invoke a finalizer once a foreign pointer becomes unreachable.
2. Support augmentation of finalizers, i.e. `addForeignPtrFinalizer`. As a motivating example, suppose that the payload of a foreign pointer is C struct `bar` that has an optionally NULL pointer field `foo` to an unmanaged heap object. Initially, `foo` is NULL, and later the program uses `malloc`, initializes the object, and assigns `foo` the address returned by `malloc`. When the foreign pointer becomes unreachable, it is now necessary to first `free` the object pointed to by `foo` and then invoke whatever finalizer was associated with `bar`. That is, finalizers must be invoked in the opposite order they are added.
3. Allow users to invoke a finalizer promptly if they know that the foreign pointer is unreachable, i.e. `finalizeForeignPtr`.

How can these goals be accomplished? Goal 1 suggests that weak references and finalizers (via `Weak#` and `mkWeak#`) are necessary. But how should they be used and what should their key be? Certainly not `ForeignPtr` or `ForeignPtrContents`. See the warning in GHC.Weak about weak pointers with lifted (non-primitive) keys. The two finalizer-supporting data constructors of `ForeignPtr` have an ``IORef` `Finalizers`` (backed by `MutVar#`) field. This gets used in two different ways depending on the kind of finalizer:

- `HaskellFinalizers`: The first `addForeignPtrConcFinalizer_` call uses `mkWeak#` to attach the finalizer `foreignPtrFinalizer` to the `MutVar#`. The resulting `Weak#` is discarded (see `addForeignPtrConcFinalizer_`). Subsequent calls to `addForeignPtrConcFinalizer_` (goal 2) just add finalizers onto the list in the `HaskellFinalizers` data constructor.
- `CFinalizers`: The first `addForeignPtrFinalizer` call uses `mkWeakNoFinalizer#` to create a `Weak#`. The `Weak#` is preserved in the `CFinalizers` data constructor. Both the first call and subsequent calls (goal 2) use `addCFinalizerToWeak#` to attach finalizers to the `Weak#` itself. Also, see Note [MallocPtr finalizers] for discussion of the key and value of this `Weak#`.

In either case, the runtime invokes the appropriate finalizers when the `ForeignPtr` becomes unreachable.