將文件類對象傳遞給ctypes回調

Question

我試圖使用LibVLC Python綁定來播放內存流（Python 3.4，Windows 7，LibVLC 3.x）。最終，我的目標是將數據提供給一個BytesIO實例，然後VLC將讀取並播放它。但目前，我決定修改一個快速腳本來嘗試從文件流中讀取數據。這裏的代碼和回溯 - 說我對ctypes很陌生會是輕描淡寫，所以有人知道我做錯了什麼？

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 

上述追蹤被重複，直到我殺死程序。

Answer 1

根據回溯指示，將一個NoneType傳遞給media_read_cb。 代碼中的問題似乎是media_open_cb函數。 如果在media_new_callbacks函數中將此回調替換爲None，則不會調用它，並且將使用適當的不透明指針調用media_read_cb。

對此我有點晦澀難懂。如果open_cb設置爲None，vlc將調用其默認的open_cb，然後默認將size_pointer設置爲maxsize和data_pointer爲不透明（與您的函數相同）。顯然，在設置指針的值時，代碼中的某些內容會出錯。我不知道如何解決這個問題，因爲我也是ctypes的新手。

當我運行你的代碼：

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

的media_read_cb被成功地調用。然而，python然後崩潰在：

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

我不知道如何解決這個問題，但有一個解決方法。你可以將流變量設置爲全局變量，所以你自己保持指針，而不是依賴於ctypes的東西。

寫入緩衝區也似乎不起作用，因爲緩衝區作爲字符串傳遞給media_read_cb。由於字符串在python中是不可變的，因此失敗。解決方法是更改​​CFUNCTYPE以將ctypes.POINTER包含爲c_char，而不是簡單的c_char_p（python中的字符串）。然後，您可以通過迭代使用流中的字節填充內存區域。

應用這些變化，你的代碼看起來是這樣的：

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) 

而且它成功地運行！

當然，不是使用全局變量，而是將所有這些封裝在python類中。

編輯：我想出瞭如何適當地設置data_pointer。這裏是代碼：

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)