Python CFFI完全指南：C Foreign Function Interface

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"cffi 中文 1.14.pdf" 是一份介绍CFFI (C Foreign Function Interface) 的文档，版本1.14.0，由Armin Rigo、Maciej Fijalkowski和Jairo共同创作。文档涵盖了CFFI的使用、更新历史、安装、概览、详细功能、CFFI接口的参考以及如何编写和分发模块等内容，旨在帮助Python开发者更好地利用CFFI与C语言进行交互，提升程序性能和扩展性。 CFFI是Python中用于与C语言代码交互的接口，它允许Python程序员以接近C语言的方式声明类型和函数，从而能够调用C库、定义C结构体和使用C指针。CFFI支持Python 2和Python 3，可在多种平台上使用，包括Windows、Linux和macOS。文档中详细介绍了CFFI的主要使用方式，如`ffi`和`lib`对象，它们分别用于定义C语言的类型和函数，并调用C库。`ffi.cdef()`用于声明C语言的类型和函数原型，而`ffi.dlopen()`则用于加载动态链接库。`ffibuilder.set_source()`和`ffibuilder.compile()`用于创建独立的C扩展模块，以便在Python代码之外编译C代码。在CFFI中，有两种主要的工作模式：ABI（应用程序二进制接口）模式和API（应用程序编程接口）模式。ABI模式直接调用库的二进制接口，而API模式则通过生成C源代码来调用库，提供更灵活的类型检查和错误处理。 CFFI还支持处理指针、结构体和数组，以及处理Python 3的特性。对于回调函数，CFFI提供了“新式回调”和“旧式回调”的两种实现，其中新式回调更安全，因为它们在CFFI内部处理回调函数的转换。同时，文档还讨论了在PyPy下如何优化内存管理，以及在Windows上处理不同的调用约定问题。此外，CFFI还提供了调试动态加载库的方法，以及从CFFI 0.9向1.0版本升级的指南。对于嵌入式使用CFFI的情况，文档给出了详细的用法示例和注意事项，帮助开发者解决可能遇到的问题。这份文档是学习和使用CFFI的宝贵资源，无论你是初学者还是有经验的Python开发者，都能从中获得关于如何在Python中高效地利用C语言功能的全面知识。

CFFI 文档, 发布 1.14.0

3.9.8 v1.10

• Issue #295: use calloc() directly instead of PyObject_Malloc()+memset() to handle .new() with a

default allocator. Speeds up ffi.new(large-array) where most of the time you never touch most of

the array.

• Some OS/X build xes (”only with Xcode but without CLT”).

• Improve a couple of error messages: when getting mismatched versions of c and its backend; and

when calling functions which cannot be called with lib because an argument is a struct that is ”too

complicated” (and not a struct pointer, which always works).

• Add support for some unusual compilers (non-msvc, non-gcc, non-icc, non-clang)

• Implemented the remaining cases for ffi.from_buffer. Now all buer/memoryview objects can be

passed. The one remaining check is against passing unicode strings in Python 2. (They support the

buer interface, but that gives the raw bytes behind the UTF16/UCS4 storage, which is most of the

times not what you expect. In Python 3 this has been xed and the unicode strings don’t support the

memoryview interface any more.)

• The C type _Bool or bool now converts to a Python boolean when reading, instead of the content of

the byte as an integer. The potential incompatibility here is what occurs if the byte contains a value

dierent from 0 and 1. Previously, it would just return it; with this change, CFFI raises an exception

in this case. But this case means ”undened behavior” in C; if you really have to interface with a

library relying on this, don’t use bool in the CFFI side. Also, it is still valid to use a byte string as

initializer for a bool[], but now it must only contain \x00 or \x01. As an aside, ffi.string() no

longer works on bool[] (but it never made much sense, as this function stops at the rst zero).

• ffi.buffer is now the name of c’s buer type, and ffi.buffer() works like before but is the

constructor of that type.

• ffi.addressof(lib, "name") now works also in in-line mode, not only in out-of-line mode. This is

useful for taking the address of global variables.

• Issue #255: cdata objects of a primitive type (integers, oats, char) are now compared and ordered

by value. For example, <cdata 'int' 42> compares equal to 42 and <cdata 'char' b'A'> compares

equal to b'A'. Unlike C, <cdata 'int' -1> does not compare equal to ffi.cast("unsigned int",

-1): it compares smaller, because -1 < 4294967295.

• PyPy: ffi.new() and ffi.new_allocator()() did not record ”memory pressure”, causing the GC

to run too infrequently if you call ffi.new() very often and/or with large arrays. Fixed in PyPy 5.7.

• Support in ffi.cdef() for numeric expressions with + or -. Assumes that there is no overow; it

should be xed rst before we add more general support for arbitrary arithmetic on constants.

12 Chapter 3. 有什么新变化

CFFI 文档, 发布 1.14.0

3.9.9 v1.9

• Structs with variable-sized arrays as their last eld: now we track the length of the array after ffi.

new() is called, just like we always tracked the length of ffi.new("int[]", 42). This lets us detect

out-of-range accesses to array items. This also lets us display a better repr(), and have the total size

returned by ffi.sizeof() and ffi.buffer(). Previously both functions would return a result based

on the size of the declared structure type, with an assumed empty array. (Thanks andrew for starting

this refactoring.)

• Add support in cdef()/set_source() for unspecied-length arrays in typedefs: typedef int

foo_t[...];. It was already supported for global variables or structure elds.

• I turned in v1.8 a warning from cffi/model.py into an error: 'enum xxx' has no

values explicitly defined: refusing to guess which integer type it is meant to be

(unsigned/signed, int/long). Now I’m turning it back to a warning again; it seems that guessing

that the enum has size int is a 99%-safe bet. (But not 100%, so it stays as a warning.)

• Fix leaks in the code handling FILE * arguments. In CPython 3 there is a remaining issue that is

hard to x: if you pass a Python le object to a FILE * argument, then os.dup() is used and the new

le descriptor is only closed when the GC reclaims the Python le object—and not at the earlier time

when you call close(), which only closes the original le descriptor. If this is an issue, you should

avoid this automatic convertion of Python le objects: instead, explicitly manipulate le descriptors

and call fdopen() from C (...via c).

3.9.10 v1.8.3

• When passing a void * argument to a function with a dierent pointer type, or vice-versa, the cast

occurs automatically, like in C. The same occurs for initialization with ffi.new() and a few other

places. However, I thought that char * had the same property—but I was mistaken. In C you get

the usual warning if you try to give a char * to a char ** argument, for example. Sorry about the

confusion. This has been xed in CFFI by giving for now a warning, too. It will turn into an error in

a future version.

3.9.11 v1.8.2

• Issue #283: xed ffi.new() on structures/unions with nested anonymous structures/unions, when

there is at least one union in the mix. When initialized with a list or a dict, it should now behave more

closely like the { } syntax does in GCC.

3.9.12 v1.8.1

• CPython 3.x: experimental: the generated C extension modules now use the ”limited API”, which

means that, as a compiled .so/.dll, it should work directly on any version of CPython >= 3.2. The

3.9. 旧版本 13

CFFI 文档, 发布 1.14.0

name produced by distutils is still version-specic. To get the version-independent name, you can

rename it manually to NAME.abi3.so, or use the very recent setuptools 26.

• Added ffi.compile(debug=...), similar to python setup.py build --debug but defaulting to True

if we are running a debugging version of Python itself.

3.9.13 v1.8

• Removed the restriction that ffi.from_buffer() cannot be used on byte strings. Now you can get a

char * out of a byte string, which is valid as long as the string object is kept alive. (But don’t use it

to modify the string object! If you need this, use bytearray or other ocial techniques.)

• PyPy 5.4 can now pass a byte string directly to a char * argument (in older versions, a copy would

be made). This used to be a CPython-only optimization.

3.9.14 v1.7

• ffi.gc(p, None) removes the destructor on an object previously created by another call to ffi.gc()

• bool(ffi.cast("primitive type", x)) now returns False if the value is zero (including -0.0), and

True otherwise. Previously this would only return False for cdata objects of a pointer type when the

pointer is NULL.

• bytearrays: ffi.from_buffer(bytearray-object) is now supported. (The reason it was not sup-

ported was that it was hard to do in PyPy, but it works since PyPy 5.3.) To call a C function

with a char * argument from a buer object—now including bytearrays—you write lib.foo(ffi.

from_buffer(x))

. Additionally, this is now supported:

p[0:length] = bytearray-object

. The

problem with this was that a iterating over bytearrays gives numbers instead of characters. (Now it is

implemented with just a memcpy, of course, not actually iterating over the characters.)

• C++: compiling the generated C code with C++ was supposed to work, but failed if you make use

the bool type (because that is rendered as the C _Bool type, which doesn’t exist in C++).

• help(lib) and help(lib.myfunc) now give useful information, as well as dir(p) where p is a struct

or pointer-to-struct.

3.9.15 v1.6

• .list_types()

• .unpack()

• extern ”Python+C”

• in API mode, lib.foo.__doc__ contains the C signature now. On CPython you can say help(lib.

foo), but for some reason help(lib) (or help(lib.foo) on PyPy) is still useless; I haven’t yet

14 Chapter 3. 有什么新变化

CFFI 文档, 发布 1.14.0

• ffi.init_once() added. See docs.

• dir(lib) now works on libs returned by ffi.dlopen() too.

• Cleaned up and modernized the content of the demo subdirectory in the sources (thanks matti!).

• ffi.new_handle() is now guaranteed to return unique void * values, even if called twice on the same

object. Previously, in that case, CPython would return two cdata objects with the same void *

value. This change is useful to add and remove handles from a global dict (or set) without worrying

about duplicates. It already used to work like that on PyPy. This change can break code that used to

work on CPython by relying on the object to be kept alive by other means than keeping the result of

.new_handle() alive. (The corresponding warning in the docs of ffi.new_handle() has been here

since v0.8!)

3.9.22 v1.3.1

• The optional typedefs (bool, FILE and all Windows types) were not always available from out-of-line

FFI objects.

• Opaque enums are phased out from the cdefs: they now give a warning, instead of (possibly wrongly)

being assumed equal to unsigned int. Please report if you get a reasonable use case for them.

• Some parsing details, notably volatile is passed along like const and restrict. Also, older versions

of pycparser mis-parse some pointer-to-pointer types like char * const *: the ”const” ends up at the

wrong place. Added a workaround.

3.9.23 v1.3.0

• Added .memmove().

• Pull request #64: out-of-line API mode: we can now declare oating-point types with typedef float.

.. foo_t;. This only works if foo_t is a oat or a double, not long double.

• Issue #217: x possible unaligned pointer manipulation, which crashes on some architectures (64-bit,

non-x86).

• Issues #64 and #126: when using set_source() or verify(), the const and restrict keywords are

copied from the cdef to the generated C code; this xes warnings by the C compiler. It also xes corner

cases like typedef const int T; T a; which would previously not consider a as a constant. (The

cdata objects themselves are never const.)

• Win32: support for __stdcall. For callbacks and function pointers; regular C functions still don’t

need to have their calling convention declared.

• Windows: CPython 2.7 distutils doesn’t work with Microsoft’s ocial Visual Studio for Python, and

I’m told this is not a bug. For .compile(), we removed a workaround that was inside c but which

had unwanted side-eects. Try saying import setuptools rst, which patches distutils...

16 Chapter 3. 有什么新变化

剩余98页未读，继续阅读

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