# NBEP 4: Defining C callbacks

Author:

  Antoine Pitrou

Date:

  April 2016

Status:

  Draft

Interfacing with some native libraries (for example written in C or C++) can necessitate writing native callbacks to provide business logic to the library. Some Python-facing libraries may also provide the alternative of passing a ctypes-wrapped native callback instead of a Python callback for better performance. A simple example is the `scipy.integrate` package where the user passes the function to be integrated as a callback.

Users of those libraries may want to benefit from the performance advantage of running purely native code, while writing their code in Python. This proposal outlines a scheme to provide such a functionality in Numba.

## Basic usage

We propose adding a new decorator, `@cfunc`, importable from the main package. This decorator allows defining a callback as in the following example:

    from numba import cfunc
    from numba.types import float64

    # A callback with the C signature `double(double)`

    @cfunc(float64(float64), nopython=True)
    def integrand(x):
        return 1 / x

The `@cfunc` decorator returns a “C function” object holding the resources necessary to run the given compiled function (for example its LLVM module). This object has several attributes and methods:

- the `ctypes` attribute is a ctypes function object representing the native function.

- the `address` attribute is the address of the native function code, as an integer (note this can also be computed from the `ctypes` attribute).

- the `native_name` attribute is the symbol under which the function can be looked up inside the current process.

- the `inspect_llvm()` method returns the IR for the LLVM module in which the function is compiled. It is expected that the `native_name` attribute corresponds to the function’s name in the LLVM IR.

The general signature of the decorator is `cfunc(signature, **options)`.

The `signature` must specify the argument types and return type of the function using Numba types. In contrary to `@jit`, the return type cannot be omitted.

The `options` are keyword-only parameters specifying compilation options. We are expecting that the standard `@jit` options (`nopython`, `forceobj`, `cache`) can be made to work with `@cfunc`.

### Calling from Numba-compiled functions

While the intended use is to pass a callback’s address to foreign C code expecting a function pointer, it should be made possible to call the C callback from a Numba-compiled function.

## Passing array data

Native platform ABIs as used by C or C++ don’t have the notion of a shaped array as in Numpy. One common solution is to pass a raw data pointer and one or several size arguments (depending on dimensionality). Numba must provide a way to rebuild an array view of this data inside the callback.

    from numba import cfunc, carray
    from numba.types import float64, CPointer, void, intp

    # A callback with the C signature `void(double *, double *, size_t)`

    @cfunc(void(CPointer(float64), CPointer(float64), intp))
    def invert(in_ptr, out_ptr, n):
        in_ = carray(in_ptr, (n,))
        out = carray(out_ptr, (n,))
        for i in range(n):
            out[i] = 1 / in_[i]

The `carray` function takes `(pointer, shape, dtype)` arguments (`dtype` being optional) and returns a C-layout array view over the data pointer, with the given shape and dtype. pointer must be a ctypes pointer object (not a Python integer). The array’s dimensionality corresponds to the shape tuple’s length. If dtype is not given, the array’s dtype corresponds to the pointer’s pointee type.

The `farray` function is similar except that it returns a F-layout array view.

## Error handling

There is no standard mechanism in C for error reporting. Unfortunately, Numba currently doesn’t handle `try..except` blocks, which makes it more difficult for the user to implement the required error reporting scheme. The current stance of this proposal is to let users guard against invalid arguments where necessary, and do whatever is required to inform the caller of the error.

Based on user feedback, we can later add support for some error reporting schemes, such as returning an integer error code depending on whether an exception was raised, or setting `errno`.

## Deferred topics

### Ahead-of-Time compilation

This proposal doesn’t make any provision for AOT compilation of C callbacks. It would probably necessitate a separate API (a new method on the `numba.pycc.CC` object), and the implementation would require exposing a subset of the C function object’s functionality from the compiled C extension module.

### Opaque data pointers

Some libraries allow passing an opaque data pointer (`void *`) to a user-provided callback, to provide any required context for execution of the callback. Taking advantage of this functionality would require adding specific support in Numba, for example the ability to do generic conversion from `types.voidptr` and to take the address of a Python-facing `jitclass` instance.