如何多次使用ctypes recv_into C缓冲区？ [英] How do I use ctypes to recv_into a C buffer multiple times?

问题描述

我有一个Python库，该库使用 ctypes 向C库注册回调。回调的签名为：

I have a Python library that is using ctypes to register a callback with a C library. The signature of the callback is:

CFUNCTYPE(c_int_32, c_uint_32, POINTER(c_byte), POINTER(c_size_t))

在这种情况下，第三个参数是指向C库分配的字节数组的指针，而第四个参数是它的长度。

In this case, the third argument is a pointer to a byte array allocated by the C library, and the fourth argument is its length.

我想通过调用 socket.recv_into 。更重要的是，我想完全填充这个字节数组：也就是说，如果 recv_into 返回的字节数少于该函数的第四个参数，那么我'想再次调用 recv_into 。

I would like to populate the byte array with data from the network by calling socket.recv_into. More importantly, I would like to completely fill this byte array: that is, if recv_into returns fewer bytes than the fourth argument of this function then I'd like to call recv_into again.

假设我的函数如下：

def callback(first, second, buffer, size):
    total_read = 0
    while total_read < size[0]:
        total_read += some_socket.recv_into(buffer, size[0] - total_read)
        # XXX: What happens here???

我的问题是：如何操纵 buffer的值以确保每次对 recv_into 的调用都附加到缓冲区，而不是覆盖以前写入的数据？

My question is: how can I manipulate the value of buffer to ensure that each call to recv_into appends to the buffer, rather than overwriting previously-written data?

推荐答案

以注释为基础，无需中间强制转换：

Building on the comments, without intermediate casting:

# assuming we know OFFSET (where to start copying in the buffer)
# and AMOUNT (how many bytes we will read)
tmp_buf = (ctypes.c_char * size).from_address(ctypes.addressof(buffer.contents))
mview = memoryview(tmp_buf)[OFFSET:AMOUNT]
_ = sock.recv_into(mview, AMOUNT)

buffer.contents 返回指向缓冲区的指针，因此可以使用 ctypes

buffer.contents returns the pointer to the buffer, so its address can be extracted using ctypes

这对我来说很简单，它通过简单的C代码将预分配的缓冲区传递到回调中，该回调已在python代码中注册。

This worked for me in a simple C code that passes a pre-allocated buffer into a callback, that is registered in the python code.

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