GHC.Internal.ForeignPtr

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 ForeignPtrs 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.

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        |
                         +------------+-----------------+

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

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 either use the ccall or the capi calling convention.

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

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.

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.

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

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.

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

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.

Turns a plain memory reference into a foreign object by associating a finalizer - given by the monadic operation - with the reference.

When finalization is triggered by GC, 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 for GC-triggered finalization.

When finalization is triggered by explicitly calling finalizeForeignPtr, the finalizer will run immediately on the current Haskell thread.

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.

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.

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

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 finalization is triggered by GC, the finalizer will run in a new thread. When finalization is triggered by explicitly calling finalizeForeignPtr, the finalizer will run immediately on the current Haskell 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, System.IO.Handles are finalized objects, so a finalizer should not refer to a System.IO.Handle (including GHC.Internal.System.IO.stdout, GHC.Internal.System.IO.stdin, or GHC.Internal.System.IO.stderr).

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.

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

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.

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.

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.

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 preceding 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 ForeignPtrs. 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 MVars 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.

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.