python - 如何在 Cython 的新型缓冲区对象中包装 C 指针和长度？

Question

我正在用 Cython 编写 Python 2.7 扩展模块。 我如何创建一个 Python 对象来实现新式缓冲区接口(interface)，该接口(interface)包装了 C 库提供给我的内存块？内存块只是一串字节，而不是结构或多维数组。我得到了一个 const void * 指针和一个长度，以及有关指针保持有效时间的一些详细信息。

我无法复制内存——这会降低我的应用程序的性能。

对于旧式缓冲区对象，我可以简单地使用 PyBuffer_FromMemory()，但我似乎找不到一种类似的简单方法来生成新式缓冲区对象。

我是否必须创建自己的类来实现缓冲区接口(interface)？或者 Cython 是否提供了一种简单的方法来做到这一点？

我读过 Unicode and Passing Strings和 Typed Memoryviews来自 Cython 文档的页面，但该文档不精确且不是很完整，并且没有看起来与我想要做的相似的示例。

这是我尝试过的 (test.pyx):

from libc.stdlib cimport malloc
from libc.string cimport memcpy

## pretend that this function is in some C library and that it does
## something interesting.  (this function is unrelated to the problem
## I'm experiencing -- this is just an example function that returns a
## chunk of memory that I want to wrap in an object that follows the
## new buffer protocol.)
cdef void dummy_function(const void **p, size_t *l):
    cdef void *tmp = malloc(17)
    memcpy(tmp, "some test\0 bytes", 17)
    p[0] = tmp
    l[0] = 17

cpdef getbuf():
    cdef const void *cstr
    cdef size_t l
    dummy_function(&cstr, &l)

    ## error: test.pyx:21:20: Invalid base type for memoryview slice: void
    #cdef const void[:] ret = cstr[:l]

    ## error: test.pyx:24:9: Assignment to const 'ret'
    #cdef const char[:] ret = cstr[:l]

    ## error: test.pyx:27:27: Cannot convert 'void const *' to memoryviewslice
    #cdef char[:] ret = cstr[:l]

    ## this next attempt cythonizes, but raises an exception:
    ## $ python -c 'import test; test.getbuf()'
    ## Traceback (most recent call last):
    ##   File "<string>", line 1, in <module>
    ##   File "test.pyx", line 15, in test.getbuf (test.c:1411)
    ##   File "test.pyx", line 38, in test.getbuf (test.c:1350)
    ##   File "stringsource", line 614, in View.MemoryView.memoryview_cwrapper (test.c:6763)
    ##   File "stringsource", line 321, in View.MemoryView.memoryview.__cinit__ (test.c:3309)
    ## BufferError: Object is not writable.
    cdef char[:] ret = (<const char *>cstr)[:l]

    ## this raises the same exception as above
    #cdef char[:] ret = (<char *>cstr)[:l]

    return ret

Answer 1

您可以定义一个 extension type通过定义 __getbuffer__ 和 __releasebuffer__ 实现缓冲协议(protocol) special methods .例如:

from cpython.buffer cimport PyBuffer_FillInfo
from libc.stdlib cimport free, malloc
from libc.string cimport memcpy

cdef void dummy_function(const void **p, size_t *l):
    cdef void *tmp = malloc(17)
    memcpy(tmp, "some test\0 bytes", 17)
    p[0] = tmp
    l[0] = 17

cdef void free_dummy_data(const void *p, size_t l, void *arg):
    free(<void *>p)

cpdef getbuf():
    cdef const void *p
    cdef size_t l
    dummy_function(&p, &l)
    return MemBuf_init(p, l, &free_dummy_data, NULL)

ctypedef void dealloc_callback(const void *p, size_t l, void *arg)

cdef class MemBuf:
    cdef const void *p
    cdef size_t l
    cdef dealloc_callback *dealloc_cb_p
    cdef void *dealloc_cb_arg

    def __getbuffer__(self, Py_buffer *view, int flags):
        PyBuffer_FillInfo(view, self, <void *>self.p, self.l, 1, flags)
    def __releasebuffer__(self, Py_buffer *view):
        pass

    def __dealloc__(self):
        if self.dealloc_cb_p != NULL:
            self.dealloc_cb_p(self.p, self.l, self.dealloc_cb_arg)

# Call this instead of constructing a MemBuf directly.  The __cinit__
# and __init__ methods can only take Python objects, so the real
# constructor is here.  See:
# https://mail.python.org/pipermail/cython-devel/2012-June/002734.html
cdef MemBuf MemBuf_init(const void *p, size_t l,
                        dealloc_callback *dealloc_cb_p,
                        void *dealloc_cb_arg):
    cdef MemBuf ret = MemBuf()
    ret.p = p
    ret.l = l
    ret.dealloc_cb_p = dealloc_cb_p
    ret.dealloc_cb_arg = dealloc_cb_arg
    return ret

通过上面的代码(名为 test.pyx)，您将获得以下行为:

$ python -c 'import test; print repr(memoryview(test.getbuf()).tobytes())'
'some test\x00 bytes\x00'

不知道有没有更简单的方法。