3.1 輸入函數 input

Shwneo原創首發CSDN技術專欄，轉載請註明出處

在壓縮包裏提供的minimad.c示例參考中給出的input回調函數如下：

/* * This is the input callback. The purpose of this callback is to (re)fill * the stream buffer which is to be decoded. In this example, an entire file * has been mapped into memory, so we just call mad_stream_buffer() with the * address and length of the mapping. When this callback is called a second * time, we are finished decoding. */ static enum mad_flow input(void *data, struct mad_stream *stream) { struct buffer *buffer = data; if (!buffer->length) return MAD_FLOW_STOP; mad_stream_buffer(stream, buffer->start, buffer->length); buffer->length = 0; return MAD_FLOW_CONTINUE; }

其中，buffer的類型定義如下：

/* * This is a private message structure. A generic pointer to this structure * is passed to each of the callback functions. Put here any data you need * to access from within the callbacks. */ struct buffer { unsigned char const *start; unsigned long length; };

這個回調函數的原型有兩個參數，一個是用戶自定義消息指針data，用於輸入用戶消息（可以包含任意消息，但消息類型必須定義明確）；另一個是輸入流結構mad_stream，用於輸出填充好的輸入流，簡言之就是將mp3文件的內存鏡像傳遞給mad_stream，這個mad_stream就是mad_decoder中sync結構的stream成員。

在上面的代碼中，input回調函數調用了mad_stream_buffer()函數將文件的內存鏡像映射給了mad_stream。mad_stream_buffer ()函數原型如下：

/* * NAME: stream->buffer() * DESCRIPTION: set stream buffer pointers */ void mad_stream_buffer(struct mad_stream *stream, unsigned char const *buffer, unsigned long length)

第一個參數stream是個輸出參數，將input回調函數的第二個虛參數直接傳遞就行；第二個和第三個參數分別指定mp3文件內存鏡像的起始地址和長度，對於MSVC++，可以這樣獲取這兩個參數的值：

CFile file(FILE_PATH); long length=file.GetLength(); char * file_map=(char*)malloc(length); UINT Byte_Readed=1; while (Byte_Readed) { Byte_Readed=file.Read(file_map,length); }

除此之外在input這個回調函數裏我們可以做解碼開始之前的任何工作比如讀取並顯示mp3的ID3信息，初始化回放增益等。