将类文件对象传递给 ctypes 回调

Question

将类文件对象传递给 ctypes 回调

我正在尝试使用 LibVLC Python 绑定来播放内存流（Python 3.4、Windows 7、LibVLC 3.x）。最终，我的目标是将数据馈送到 BytesIO 实例中，然后 VLC 将从中读取和播放。但目前，我决定编写一个快速脚本来尝试从文件流中读取。这是代码和回溯 - 说我对 ctypes 很陌生是轻描淡写的，所以有人知道我做错了什么吗？

import ctypes
import io
import sys
import time

import vlc

MediaOpenCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.POINTER(ctypes.c_void_p), ctypes.POINTER(ctypes.c_uint64))
MediaReadCb = ctypes.CFUNCTYPE(ctypes.c_ssize_t, ctypes.c_void_p, ctypes.c_char_p, ctypes.c_size_t)
MediaSeekCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.c_uint64)
MediaCloseCb = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_void_p)


def media_open_cb(opaque, data_pointer, size_pointer):
    data_pointer.value = opaque
    size_pointer.contents.value = sys.maxsize
    return 0


def media_read_cb(opaque, buffer, length):
    stream = ctypes.cast(opaque, ctypes.py_object).value
    new_data = stream.read(length.contents)
    buffer.contents.value = new_data
    return len(new_data)


def media_seek_cb(opaque, offset):
    stream = ctypes.cast(opaque, ctypes.py_object).value
    stream.seek(offset)
    return 0


def media_close_cb(opaque):
    stream = ctypes.cast(opaque, ctypes.py_object).value
    stream.close()


callbacks = {
    'open': MediaOpenCb(media_open_cb),
    'read': MediaReadCb(media_read_cb),
    'seek': MediaSeekCb(media_seek_cb),
    'close': MediaCloseCb(media_close_cb)
}


def main(path):
    stream = open(path, 'rb')
    instance = vlc.Instance()
    player = instance.media_player_new()
    media = instance.media_new_callbacks(callbacks['open'], callbacks['read'], callbacks['seek'], callbacks['close'], ctypes.byref(ctypes.py_object(stream)))
    player.set_media(media)
    player.play()

    while True:
        time.sleep(1)


if __name__ == '__main__':
    try:
        path = sys.argv[1]
    except IndexError:
        print('Usage: {0} <path>'.format(__file__))
        sys.exit(1)

    main(path)

[02f87cb0] imem demux error: Invalid get/release function pointers
Traceback (most recent call last):
  File "_ctypes/callbacks.c", line 234, in 'calling callback function'
  File "memory_stream.py", line 21, in media_read_cb
    stream = ctypes.cast(opaque, ctypes.py_object).value
ValueError: PyObject is NULL

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重复上面的回溯，直到我杀死程序。

Answer 1

Mun*_*sen 5

一个 NoneType 被传递给 media_read_cb，如回溯所示。代码中的问题似乎是 media_open_cb 函数。如果在 media_new_callbacks 函数中将此回调替换为 None ，则不会调用它，并且将使用适当的不透明指针调用 media_read_cb 。

这样做的原因对我来说有点模糊。如果 open_cb 设置为 None，vlc 将调用其默认 open_cb，然后默认情况下将 size_pointer 设置为 maxsize 并将 data_pointer 设置为 opaque（与您的函数相同）。显然，在设置指针值时，您的代码中出现了错误。我不知道如何解决这个问题，因为我也是 ctypes 的新手。

当我运行您的代码时：

media = instance.media_new_callbacks(None, callbacks['read'], callbacks['seek'], callbacks['close'], ctypes.byref(ctypes.py_object(stream)))

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media_read_cb 被成功调用。但是，python 然后在以下位置崩溃：

stream = ctypes.cast(opaque, ctypes.py_object).value

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我也不知道如何解决这个问题，但有一个解决方法。您可以将流变量设置为全局变量，这样您就可以自己保留指针而不是依赖 ctypes 的东西。

写入缓冲区似乎也不起作用，因为缓冲区作为字符串传递给 media_read_cb。由于字符串在 python 中是不可变的，因此失败。解决方法是将 CFUNCTYPE 更改为包含 ctypes.POINTER 到 c_char 而不是普通的 c_char_p（python 中的字符串）。然后，您可以通过迭代使用流中的字节填充内存区域。

应用这些更改，您的代码如下所示：

import ctypes
import io
import sys
import time

import vlc

MediaOpenCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.POINTER(ctypes.c_void_p), ctypes.POINTER(ctypes.c_uint64))
MediaReadCb = ctypes.CFUNCTYPE(ctypes.c_ssize_t, ctypes.c_void_p, ctypes.POINTER(ctypes.c_char), ctypes.c_size_t)
MediaSeekCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.c_uint64)
MediaCloseCb = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_void_p)

stream=None

def media_open_cb(opaque, data_pointer, size_pointer):
    data_pointer.value = opaque
    size_pointer.contents.value = sys.maxsize
    return 0


def media_read_cb(opaque, buffer, length):
    new_data = stream.read(length)
    for i in range(len(new_data)):
        buffer[i]=new_data[i]
    return len(new_data)


def media_seek_cb(opaque, offset):
    stream.seek(offset)
    return 0


def media_close_cb(opaque):
    stream.close()


callbacks = {
    'open': MediaOpenCb(media_open_cb),
    'read': MediaReadCb(media_read_cb),
    'seek': MediaSeekCb(media_seek_cb),
    'close': MediaCloseCb(media_close_cb)
}

def main(path):
    global stream
    stream = open(path, 'rb')
    instance = vlc.Instance('-vvv')
    player = instance.media_player_new()
    media = instance.media_new_callbacks(None, callbacks['read'], callbacks['seek'], callbacks['close'], ctypes.byref(ctypes.py_object(stream)))
    player.set_media(media)
    player.play()

    while True:
        time.sleep(1)

if __name__ == '__main__':
    try:
        path = sys.argv[1]
    except IndexError:
        print('Usage: {0} <path>'.format(__file__))
        sys.exit(1)

    main(path)

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它成功运行！

当然，与其使用全局变量，不如将所有这些都包装在一个 python 类中。

编辑：我想出了如何适当地设置 data_pointer 。这是代码：

import ctypes
import io
import sys
import time

import vlc

MediaOpenCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.POINTER(ctypes.c_void_p), ctypes.POINTER(ctypes.c_uint64))
MediaReadCb = ctypes.CFUNCTYPE(ctypes.c_ssize_t, ctypes.c_void_p, ctypes.POINTER(ctypes.c_char), ctypes.c_size_t)
MediaSeekCb = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_void_p, ctypes.c_uint64)
MediaCloseCb = ctypes.CFUNCTYPE(ctypes.c_void_p, ctypes.c_void_p)


def media_open_cb(opaque, data_pointer, size_pointer):
    data_pointer.contents.value = opaque
    size_pointer.contents.value = sys.maxsize
    return 0


def media_read_cb(opaque, buffer, length):
    stream=ctypes.cast(opaque,ctypes.POINTER(ctypes.py_object)).contents.value
    new_data = stream.read(length)
    for i in range(len(new_data)):
        buffer[i]=new_data[i]
    return len(new_data)


def media_seek_cb(opaque, offset):
    stream=ctypes.cast(opaque,ctypes.POINTER(ctypes.py_object)).contents.value
    stream.seek(offset)
    return 0


def media_close_cb(opaque):
    stream=ctypes.cast(opaque,ctypes.POINTER(ctypes.py_object)).contents.value
    stream.close()


callbacks = {
    'open': MediaOpenCb(media_open_cb),
    'read': MediaReadCb(media_read_cb),
    'seek': MediaSeekCb(media_seek_cb),
    'close': MediaCloseCb(media_close_cb)
}

def main(path):
    stream = open(path, 'rb')
    instance = vlc.Instance()
    player = instance.media_player_new()
    media = instance.media_new_callbacks(callbacks['open'], callbacks['read'], callbacks['seek'], callbacks['close'], ctypes.cast(ctypes.pointer(ctypes.py_object(stream)), ctypes.c_void_p))
    player.set_media(media)
    player.play()

    while True:
        time.sleep(1)

if __name__ == '__main__':
    try:
        path = sys.argv[1]
    except IndexError:
        print('Usage: {0} <path>'.format(__file__))
        sys.exit(1)

    main(path)

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