96 lines
4.2 KiB
Markdown
96 lines
4.2 KiB
Markdown
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# Low-Level Wrappers
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### Unwrapped Native Handles
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Some native libraries work with handles to large amounts of data at once, making
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it undesirable to marshal data back and forth from Clojure, both because it's
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not necessary to work with the data in Clojure directly, or also because of the
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high (de)serialization costs associated with marshaling. In cases like these,
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unwrapped native handles are desirable.
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The functions `make-downcall` and `make-varargs-factory` are also provided to
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create raw function handles.
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```clojure
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(def raw-strlen (ffi/make-downcall "strlen" [::mem/c-string] ::mem/long))
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(raw-strlen (mem/serialize "hello" ::mem/c-string))
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;; => 5
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```
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With raw handles, the argument types are expected to exactly match the types
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expected by the native function. For primitive types, those are primitives. For
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pointers, that is `MemorySegment`, and for composite types like structs and
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unions, that is also `MemorySegment`. `MemorySegment` comes from the
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`java.lang.foreign` package.
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In addition, when a raw handle returns a composite type represented with a
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`MemorySegment`, it requires an additional first argument, a `SegmentAllocator`,
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which can be acquired with `arena-allocator` to get one associated with a
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specific arena. The returned value will live until that arena is released.
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In addition, function types can be specified as being raw, in the following
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manner:
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```clojure
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[::ffi/fn [::mem/int] ::mem/int :raw-fn? true]
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```
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Clojure functions serialized to this type will have their arguments and return
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value exactly match the types specified and will not perform any serialization
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or deserialization at their boundaries.
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One important caveat to consider when writing wrappers for performance-sensitive
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functions is that the convenience macro `defcfn` that coffi provides will
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already perform no serialization or deserialization on primitive arguments and
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return types, so for functions with only primitive argument and return types
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there is no performance reason to choose unwrapped native handles over the
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convenience macro.
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### Manual (De)Serialization
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Coffi uses multimethods to dispatch to (de)serialization functions to enable
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code that's generic over the types it operates on. However, in cases where you
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know the exact types that you will be (de)serializing and the multimethod
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dispatch overhead is too high a cost, it may be appropriate to manually handle
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(de)serializing data. This will often be done paired with [Unwrapped Native
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Handles](#unwrapped-native-handles).
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Convenience functions are provided to both read and write all primitive types
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and addresses, including byte order.
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As an example, when wrapping a function that returns an array of big-endian
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floats, the following code might be used.
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``` clojure
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;; int returns_float_array(float **arr)
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(def ^:private returns-float-array* (ffi/make-downcall "returns_float_array" [::mem/pointer] ::mem/int))
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;; void releases_float_array(float *arr)
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(def ^:private release-floats* (ffi/make-downcall "releases_float_array" [::mem/pointer] ::mem/void))
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(defn returns-float-array
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[]
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(with-open [arena (mem/confined-arena)]
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;; float *out_floats;
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;; int num_floats = returns_float_array(&out_floats);
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(let [out-floats (mem/alloc mem/pointer-size arena)
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num-floats (returns-float-array* out-floats)
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floats-addr (mem/read-address out-floats)
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floats-slice (mem/reinterpret floats-addr (unchecked-multiply-int mem/float-size num-floats))]
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;; Using a try/finally to perform an operation when the stack frame exits,
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;; but not to try to catch anything.
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(try
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(loop [floats (transient [])
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index 0]
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(if (>= index num-floats)
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(persistent! floats)
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(recur (conj! floats (mem/read-float floats-slice
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(unchecked-multiply-int index mem/float-size)
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mem/big-endian))
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(unchecked-inc-int index))))
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(finally
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(release-floats* floats-addr))))))
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```
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The above code manually performs all memory operations rather than relying on
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coffi's dispatch. This will be more performant, but because multimethod overhead
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is usually relatively low, it's recommended to use the multimethod variants for
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convenience in colder functions.
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