 | Haskell Core Libraries (base package) | Parent | Contents | Index |
| Foreign.ForeignPtr |
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| newForeignPtr :: Ptr a -> IO () -> IO (ForeignPtr a) | ||
| addForeignPtrFinalizer :: ForeignPtr a -> IO () -> IO () | ||
| withForeignPtr :: ForeignPtr a -> (Ptr a -> IO b) -> IO b | ||
| foreignPtrToPtr :: ForeignPtr a -> Ptr a | ||
| touchForeignPtr :: ForeignPtr a -> IO () | ||
| castForeignPtr :: ForeignPtr a -> ForeignPtr b |
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 finalisers. A finaliser 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 finaliser 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. | |||
| Instances | |||
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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 unsafety is the same as for foreignPtrToPtr 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 function extracts the pointer component of a foreign pointer. This is a potentially dangerous operations, as if the argument to foreignPtrToPtr is the last usage occurence of the given foreign pointer, then its finaliser(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 foreignPtrToPtr and touchForeignPtr. However, the later routines are occasionally preferred in tool generated marshalling code.
This function ensures that the foreign object in question is alive at the given place in the sequence of IO actions. In particular withForeignPtr does a touchForeignPtr after it executes the user action.
This function can be used to express liveness dependencies between ForeignPtrs: for example, if the finalizer for one ForeignPtr touches a second ForeignPtr, then it is ensured that the second ForeignPtr will stay alive at least as long as the first. This can be useful when you want to manipulate interior pointers to a foreign structure: you can use touchForeignObj to express the requirement that the exterior pointer must not be finalized until the interior pointer is no longer referenced.