爲什麼AudioFileReadPacketData的參數ioNumBytes會導致崩潰？

Question

我有一些非常奇怪的東西（至少對我來說，但我是一個noob）。

UInt32 numBytesReadFromFile; 
OSStatus err = AudioFileReadPacketData(
            audioFile, // The audio file whose audio packets you want to read. 
            NO, // is cache set? 
            &numBytesReadFromFile, // On output, the number of bytes of audio data that were read from the audio file. 
            (AudioStreamPacketDescription *)_packetDescriptions, // array of packet descriptions of data that was read from the audio file (for CBR null) 
            currentPacket, // the next packet to be read into buffer 
            &numPackets, // number of actually read buffers 
            _audioQueueBuffer->mAudioData 
            ); 

AudioFileReadPacketData從音頻文件讀取數據並將其放入緩衝區。

所以我的問題是關於參數numBytesReadFromFile。 Apple writes

numBytesReadFromFile：輸出時，從音頻文件中讀取的音頻數據的字節數。

到目前爲止好。 Apple在上面的示例代碼中聲明瞭numBytesReadFromFile，但對於我來說，這行代碼崩潰了！我得到一個EXC BAD ACCESS。

UInt32 numBytesReadFromFile; 

我需要聲明numBytesReadFromFile這樣，一切工作正常：

UInt32 numBytesReadFromFile = 2048; // 2048 = size of my buffer 

但是這個崩潰也

UInt32 numBytesReadFromFile = 12 
UInt32 numBytesReadFromFile = sizeof(UInt32) 

但這並不

UInt32 numBytesReadFromFile = 1021; // a random number 

我不是很有經驗enced C程序員，但據我所知，我通過聲明numBytesReadFromFile來保留一些內存，並且audiofilereadpacketdata方法將其數據寫入變量的地址。如果我錯了，請糾正我。

那麼它爲什麼會崩潰？我想我沒有解決真正的問題。

我的假設是我有某種多線程問題。當我準備隊列時，我在主線程中調用AudioFileReadPacketData並聲明

UInt32 numBytesReadFromFile; 

工作正常。我開始播放音頻，並調用一個回調函數，它在音頻隊列的內部後臺線程上調用AudioFilereadPacketData，併發生上述錯誤。如果我的假設是正確的，有人能夠更詳細地向我解釋這個問題，因爲我沒有多線程經驗。

謝謝。

Answer 1

參數ioNumBytes到AudioFileReadPacketData是輸入/輸出參數。 documentation說：

輸入時，outBuffer參數的大小，以字節爲單位。輸出時，實際讀取的字節數爲 。

您將看到輸入和輸出值的區別，如果字節 尺寸爲您在ioNumPackets請求報文的數量 參數比你在outBuffer 參數傳遞的緩衝區大小小。在這種情況下，此參數的輸出值爲 ，小於其輸入值。

調用該函數時的值決定將多少數據寫入緩衝區。如果您發佈的代碼是正確的，numBytesReadFromFile永遠不會初始化爲_audioQueueBuffer->mAudioData的大小，並且程序崩潰，因爲它試圖將未確定數量的數據寫入_audioQueueBuffer->mAudioData。嘗試在函數調用之前設置參數：

UInt32 numBytesReadFromFile = _audioQueueBuffer->mAudioDataByteSize; 
OSStatus err = AudioFileReadPacketData(
            audioFile, // The audio file whose audio packets you want to read. 
            NO, // is cache set? 
            &numBytesReadFromFile, // On output, the number of bytes of audio data that were read from the audio file. 
            (AudioStreamPacketDescription *)_packetDescriptions, // array of packet descriptions of data that was read from the audio file (for CBR null) 
            currentPacket, // the next packet to be read into buffer 
            &numPackets, // number of actually read buffers 
            _audioQueueBuffer->mAudioData 
            );