|
| 1 | +Interfacing with C/C++ Libraries |
| 2 | +================================ |
| 3 | + |
| 4 | +C Foreign Function Interface |
| 5 | +---------------------------- |
| 6 | + |
| 7 | +`CFFI <https://cffi.readthedocs.org/en/latest/>`_ provides a simple to use |
| 8 | +mechanism for interfacing with C from both CPython and PyPy. It supports two |
| 9 | +modes: an inline ABI compatibility mode (example provided below), which allows |
| 10 | +you to dynamically load and run functions from executable modules (essentially |
| 11 | +exposing the same functionality as LoadLibrary or dlopen), and an API mode, |
| 12 | +which allows you to build C extension modules. |
| 13 | + |
| 14 | +ABI Interaction |
| 15 | +~~~~~~~~~~~~~~~ |
| 16 | + |
| 17 | +.. code-block:: python |
| 18 | + :linenos: |
| 19 | +
|
| 20 | + from cffi import FFI |
| 21 | + ffi = FFI() |
| 22 | + ffi.cdef("size_t strlen(const char*);") |
| 23 | + clib = ffi.dlopen(None) |
| 24 | + length = clib.strlen("String to be evaluated.") |
| 25 | + # prints: 23 |
| 26 | + print("{}".format(length)) |
| 27 | +
|
| 28 | +ctypes |
| 29 | +------ |
| 30 | + |
| 31 | +`ctypes <https://docs.python.org/3/library/ctypes.html>`_ is the de facto |
| 32 | +library for interfacing with C/C++ from CPython, and it provides not only |
| 33 | +full access to the native C interface of most major operating systems (e.g., |
| 34 | +kernel32 on Windows, or libc on \*nix), but also provides support for loading |
| 35 | +and interfacing with dynamic libraries, such as DLLs or shared objects at |
| 36 | +runtime. It does bring along with it a whole host of types for interacting |
| 37 | +with system APIs, and allows you to rather easily define your own complex |
| 38 | +types, such as structs and unions, and allows you to modify things such as |
| 39 | +padding and alignment, if needed. It can be a bit crufty to use, but in |
| 40 | +conjunction with the `struct <https://docs.python.org/3.5/library/struct.html>`_ |
| 41 | +module, you are essentially provided full control over how your data types get |
| 42 | +translated into something something usable by a pure C(++) method. |
| 43 | + |
| 44 | +Struct Equivalents |
| 45 | +~~~~~~~~~~~~~~~~~~ |
| 46 | + |
| 47 | +:file:`MyStruct.h` |
| 48 | + |
| 49 | +.. code-block:: c |
| 50 | + :linenos: |
| 51 | +
|
| 52 | + struct my_struct { |
| 53 | + int a; |
| 54 | + int b; |
| 55 | + }; |
| 56 | +
|
| 57 | +:file:`MyStruct.py` |
| 58 | + |
| 59 | +.. code-block:: python |
| 60 | + :linenos: |
| 61 | +
|
| 62 | + import ctypes |
| 63 | + class my_struct(ctypes.Structure): |
| 64 | + _fields_ = [("a", c_int), |
| 65 | + ("b", c_int)] |
| 66 | +
|
| 67 | +SWIG |
| 68 | +---- |
| 69 | + |
| 70 | +`SWIG <http://www.swig.org>`_, though not strictly Python focused (it supports a |
| 71 | +large number of scripting languages), is a tool for generating bindings for |
| 72 | +interpreted languages from C/C++ header files. It is extremely simple to use: |
| 73 | +the consumer simply needs to define an interface file (detailed in the |
| 74 | +tutorial and documentations), include the requisite C/C++ headers, and run |
| 75 | +the build tool against them. While it does have some limits, (it currently |
| 76 | +seems to have issues with a small subset of newer C++ features, and getting |
| 77 | +template-heavy code to work can be a bit verbose), it provides a great deal |
| 78 | +of power and exposes lots of features to Python with little effort. |
| 79 | +Additionally, you can easily extend the bindings SWIG creates (in the |
| 80 | +interface file) to overload operators and built-in methods, effectively re- |
| 81 | +cast C++ exceptions to be catchable by Python, etc. |
| 82 | + |
| 83 | +Example: Overloading __repr__ |
| 84 | +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| 85 | + |
| 86 | +:file:`MyClass.h` |
| 87 | + |
| 88 | +.. code-block:: c++ |
| 89 | + :linenos: |
| 90 | + |
| 91 | + #include <string> |
| 92 | + class MyClass { |
| 93 | + private: |
| 94 | + std::string name; |
| 95 | + public: |
| 96 | + std::string getName(); |
| 97 | + }; |
| 98 | + |
| 99 | +:file:`myclass.i` |
| 100 | + |
| 101 | +.. code-block:: c++ |
| 102 | + :linenos: |
| 103 | + |
| 104 | + %include "string.i" |
| 105 | + |
| 106 | + %module myclass |
| 107 | + %{ |
| 108 | + #include <string> |
| 109 | + #include "MyClass.h" |
| 110 | + %} |
| 111 | + |
| 112 | + %extend MyClass { |
| 113 | + std::string __repr__() |
| 114 | + { |
| 115 | + return $self->getName(); |
| 116 | + } |
| 117 | + } |
| 118 | + |
| 119 | + %include "MyClass.h" |
| 120 | + |
| 121 | + |
| 122 | +Boost.Python |
| 123 | +------------ |
| 124 | + |
| 125 | +`Boost.Python <http://www.boost.org/doc/libs/1_59_0/libs/python/doc/>`_ |
| 126 | +requires a bit more manual work to expose C++ object functionality, but |
| 127 | +it is capable of providing all the same features SWIG does and then some, |
| 128 | +to include providing wrappers to access PyObjects in C++, extracting SWIG- |
| 129 | +wrapper objects, and even embedding bits of Python into your C++ code. |
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