Richard Hansen Richard Hansen - 3 months ago 51
Python Question

How to wrap a C pointer and length in a new-style buffer object in Cython?

I'm writing a Python 2.7 extension module in Cython. How do I create a Python object implementing the new-style buffer interface that wraps a chunk of memory given to me by a C library? The chunk of memory is just a string of bytes, not a structure or multidimensional array. I'm given a

const void *
pointer and a length, and some details about how long the pointer stays valid.

I can't copy the memory—that would kill performance for my application.

With the old-style buffer objects I could simply use
PyBuffer_FromMemory()
, but I can't seem to find a similarly easy way to produce a new-style buffer object.

Do I have to create my own class that implements the buffer interface? Or does Cython provide an easy way to do this?

I've read the Unicode and Passing Strings and Typed Memoryviews pages from the Cython documentation, but the documentation is imprecise and not very complete and there are no examples that look similar to what I want to do.

Here's what I've tried (
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

You can define an extension type that implements the buffer protocol by defining the __getbuffer__ and __releasebuffer__ special methods. For example:

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

With the above (named test.pyx) you get the following behavior:

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

I don't know if there's an easier way.

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