XBMC分析系列文章：
本文我们分析XBMC中视频播放器（dvdplayer）中的文件头部分。文件头部分里包含的是封装Dll用到的头文件。由于文件头种类很多，不可能一一分析，因此还是以ffmpeg文件头为例进行分析。
XBMC中文件头部分文件目录结构如下图所示。
在这里我们看一下封装AVCodec和AVFormat结构体的头文件，分别是DllAvCodec.h和DllAvFormat.h。
DllAvFormat.h内容如下。其中包含了2个主要的类：DllAvFormatInterface和DllAvFormat。
其中DllAvFormatInterface是一个纯虚类，里面全是纯虚函数。
DllAvFormat中包含很多已经定义过的宏，稍后我们分析一下这些宏的含义。
/* * 雷霄骅 * leixiaohua1020@126.com * 中国传媒大学/数字电视技术 * */ //接口的作用class DllAvFormatInterface{public: virtual ~DllAvFormatInterface() {} virtual void av_register_all_dont_call(void)=0; virtual void avformat_network_init_dont_call(void)=0; virtual void avformat_network_deinit_dont_call(void)=0; virtual AVInputFormat *av_find_input_format(const char *short_name)=0; virtual void avformat_close_input(AVFormatContext **s)=0; virtual int av_read_frame(AVFormatContext *s, AVPacket *pkt)=0; virtual void av_read_frame_flush(AVFormatContext *s)=0; virtual int av_read_play(AVFormatContext *s)=0; virtual int av_read_pause(AVFormatContext *s)=0; virtual int av_seek_frame(AVFormatContext *s, int stream_index, int64_t timestamp, int flags)=0;#if (!defined USE_EXTERNAL_FFMPEG) && (!defined TARGET_DARWIN) virtual int avformat_find_stream_info_dont_call(AVFormatContext *ic, AVDictionary **options)=0;#endif virtual int avformat_open_input(AVFormatContext **ps, const char *filename, AVInputFormat *fmt, AVDictionary **options)=0; virtual AVIOContext *avio_alloc_context(unsigned char *buffer, int buffer_size, int write_flag, void *opaque, int (*read_packet)(void *opaque, uint8_t *buf, int buf_size), int (*write_packet)(void *opaque, uint8_t *buf, int buf_size), offset_t (*seek)(void *opaque, offset_t offset, int whence))=0; virtual AVInputFormat *av_probe_input_format(AVProbeData *pd, int is_opened)=0; virtual AVInputFormat *av_probe_input_format2(AVProbeData *pd, int is_opened, int *score_max)=0; virtual int av_probe_input_buffer(AVIOContext *pb, AVInputFormat **fmt, const char *filename, void *logctx, unsigned int offset, unsigned int max_probe_size)=0; virtual void av_dump_format(AVFormatContext *ic, int index, const char *url, int is_output)=0; virtual int avio_open(AVIOContext **s, const char *filename, int flags)=0; virtual int avio_close(AVIOContext *s)=0; virtual int avio_open_dyn_buf(AVIOContext **s)=0; virtual int avio_close_dyn_buf(AVIOContext *s, uint8_t **pbuffer)=0; virtual offset_t avio_seek(AVIOContext *s, offset_t offset, int whence)=0; virtual int avio_read(AVIOContext *s, unsigned char *buf, int size)=0; virtual void avio_w8(AVIOContext *s, int b)=0; virtual void avio_write(AVIOContext *s, const unsigned char *buf, int size)=0; virtual void avio_wb24(AVIOContext *s, unsigned int val)=0; virtual void avio_wb32(AVIOContext *s, unsigned int val)=0; virtual void avio_wb16(AVIOContext *s, unsigned int val)=0; virtual AVFormatContext *avformat_alloc_context(void)=0; virtual int avformat_alloc_output_context2(AVFormatContext **ctx, AVOutputFormat *oformat, const char *format_name, const char *filename) = 0; virtual AVStream *avformat_new_stream(AVFormatContext *s, AVCodec *c)=0; virtual AVOutputFormat *av_guess_format(const char *short_name, const char *filename, const char *mime_type)=0; virtual int avformat_write_header (AVFormatContext *s, AVDictionary **options)=0; virtual int av_write_trailer(AVFormatContext *s)=0; virtual int av_write_frame (AVFormatContext *s, AVPacket *pkt)=0;#if defined(AVFORMAT_HAS_STREAM_GET_R_FRAME_RATE) virtual AVRational av_stream_get_r_frame_rate(const AVStream *s)=0;#endif};//封装的Dll，继承了DllDynamic，以及接口class DllAvFormat : public DllDynamic, DllAvFormatInterface{ DECLARE_DLL_WRAPPER(DllAvFormat, DLL_PATH_LIBAVFORMAT) LOAD_SYMBOLS() DEFINE_METHOD0(void, av_register_all_dont_call) DEFINE_METHOD0(void, avformat_network_init_dont_call) DEFINE_METHOD0(void, avformat_network_deinit_dont_call) DEFINE_METHOD1(AVInputFormat*, av_find_input_format, (const char *p1)) DEFINE_METHOD1(void, avformat_close_input, (AVFormatContext **p1)) DEFINE_METHOD1(int, av_read_play, (AVFormatContext *p1)) DEFINE_METHOD1(int, av_read_pause, (AVFormatContext *p1)) DEFINE_METHOD1(void, av_read_frame_flush, (AVFormatContext *p1)) DEFINE_FUNC_ALIGNED2(int, __cdecl, av_read_frame, AVFormatContext *, AVPacket *) DEFINE_FUNC_ALIGNED4(int, __cdecl, av_seek_frame, AVFormatContext*, int, int64_t, int) DEFINE_FUNC_ALIGNED2(int, __cdecl, avformat_find_stream_info_dont_call, AVFormatContext*, AVDictionary **) DEFINE_FUNC_ALIGNED4(int, __cdecl, avformat_open_input, AVFormatContext **, const char *, AVInputFormat *, AVDictionary **) DEFINE_FUNC_ALIGNED2(AVInputFormat*, __cdecl, av_probe_input_format, AVProbeData*, int) DEFINE_FUNC_ALIGNED3(AVInputFormat*, __cdecl, av_probe_input_format2, AVProbeData*, int, int*) DEFINE_FUNC_ALIGNED6(int, __cdecl, av_probe_input_buffer, AVIOContext *, AVInputFormat **, const char *, void *, unsigned int, unsigned int) DEFINE_FUNC_ALIGNED3(int, __cdecl, avio_read, AVIOContext*, unsigned char *, int) DEFINE_FUNC_ALIGNED2(void, __cdecl, avio_w8, AVIOContext*, int) DEFINE_FUNC_ALIGNED3(void, __cdecl, avio_write, AVIOContext*, const unsigned char *, int) DEFINE_FUNC_ALIGNED2(void, __cdecl, avio_wb24, AVIOContext*, unsigned int) DEFINE_FUNC_ALIGNED2(void, __cdecl, avio_wb32, AVIOContext*, unsigned int) DEFINE_FUNC_ALIGNED2(void, __cdecl, avio_wb16, AVIOContext*, unsigned int) DEFINE_METHOD7(AVIOContext *, avio_alloc_context, (unsigned char *p1, int p2, int p3, void *p4, int (*p5)(void *opaque, uint8_t *buf, int buf_size), int (*p6)(void *opaque, uint8_t *buf, int buf_size), offset_t (*p7)(void *opaque, offset_t offset, int whence))) DEFINE_METHOD4(void, av_dump_format, (AVFormatContext *p1, int p2, const char *p3, int p4)) DEFINE_METHOD3(int, avio_open, (AVIOContext **p1, const char *p2, int p3)) DEFINE_METHOD1(int, avio_close, (AVIOContext *p1)) DEFINE_METHOD1(int, avio_open_dyn_buf, (AVIOContext **p1)) DEFINE_METHOD2(int, avio_close_dyn_buf, (AVIOContext *p1, uint8_t **p2)) DEFINE_METHOD3(offset_t, avio_seek, (AVIOContext *p1, offset_t p2, int p3)) DEFINE_METHOD0(AVFormatContext *, avformat_alloc_context) DEFINE_METHOD4(int, avformat_alloc_output_context2, (AVFormatContext **p1, AVOutputFormat *p2, const char *p3, const char *p4)) DEFINE_METHOD2(AVStream *, avformat_new_stream, (AVFormatContext *p1, AVCodec *p2)) DEFINE_METHOD3(AVOutputFormat *, av_guess_format, (const char *p1, const char *p2, const char *p3)) DEFINE_METHOD2(int, avformat_write_header , (AVFormatContext *p1, AVDictionary **p2)) DEFINE_METHOD1(int, av_write_trailer, (AVFormatContext *p1)) DEFINE_METHOD2(int, av_write_frame , (AVFormatContext *p1, AVPacket *p2))#if defined(AVFORMAT_HAS_STREAM_GET_R_FRAME_RATE) DEFINE_METHOD1(AVRational, av_stream_get_r_frame_rate, (const AVStream *p1))#endif BEGIN_METHOD_RESOLVE() RESOLVE_METHOD_RENAME(av_register_all, av_register_all_dont_call) RESOLVE_METHOD_RENAME(avformat_network_init, avformat_network_init_dont_call) RESOLVE_METHOD_RENAME(avformat_network_deinit, avformat_network_deinit_dont_call) RESOLVE_METHOD(av_find_input_format) RESOLVE_METHOD(avformat_close_input) RESOLVE_METHOD(av_read_frame) RESOLVE_METHOD(av_read_play) RESOLVE_METHOD(av_read_pause) RESOLVE_METHOD(av_read_frame_flush) RESOLVE_METHOD(av_seek_frame) RESOLVE_METHOD_RENAME(avformat_find_stream_info, avformat_find_stream_info_dont_call) RESOLVE_METHOD(avformat_open_input) RESOLVE_METHOD(avio_alloc_context) RESOLVE_METHOD(av_probe_input_format) RESOLVE_METHOD(av_probe_input_format2) RESOLVE_METHOD(av_probe_input_buffer) RESOLVE_METHOD(av_dump_format) RESOLVE_METHOD(avio_open) RESOLVE_METHOD(avio_close) RESOLVE_METHOD(avio_open_dyn_buf) RESOLVE_METHOD(avio_close_dyn_buf) RESOLVE_METHOD(avio_seek) RESOLVE_METHOD(avio_read) RESOLVE_METHOD(avio_w8) RESOLVE_METHOD(avio_write) RESOLVE_METHOD(avio_wb24) RESOLVE_METHOD(avio_wb32) RESOLVE_METHOD(avio_wb16) RESOLVE_METHOD(avformat_alloc_context) RESOLVE_METHOD(avformat_alloc_output_context2) RESOLVE_METHOD(avformat_new_stream) RESOLVE_METHOD(av_guess_format) RESOLVE_METHOD(avformat_write_header) RESOLVE_METHOD(av_write_trailer) RESOLVE_METHOD(av_write_frame)#if defined(AVFORMAT_HAS_STREAM_GET_R_FRAME_RATE) RESOLVE_METHOD(av_stream_get_r_frame_rate)#endif END_METHOD_RESOLVE() /* dependencies of libavformat */ DllAvCodec m_dllAvCodec; // DllAvUtil loaded implicitely by m_dllAvCodecpublic: void av_register_all() { CSingleLock lock(DllAvCodec::m_critSection); av_register_all_dont_call(); } int avformat_find_stream_info(AVFormatContext *ic, AVDictionary **options) { CSingleLock lock(DllAvCodec::m_critSection); return avformat_find_stream_info_dont_call(ic, options); } virtual bool Load() { if (!m_dllAvCodec.Load()) return false; bool loaded = DllDynamic::Load(); CSingleLock lock(DllAvCodec::m_critSection); if (++m_avformat_refcnt == 1 && loaded) avformat_network_init_dont_call(); return loaded; } virtual void Unload() { CSingleLock lock(DllAvCodec::m_critSection); if (--m_avformat_refcnt == 0 && DllDynamic::IsLoaded()) avformat_network_deinit_dont_call(); DllDynamic::Unload(); }protected: static int m_avformat_refcnt;};#endif 这些宏的含义如下：
DEFINE_METHOD0(result, name) 定义一个方法（不包含参数）DEFINE_METHOD1(result, name, args) 定义一个方法（1个参数）DEFINE_METHOD2(result, name, args) 定义一个方法（2个参数）DEFINE_METHOD3(result, name, args) 定义一个方法（3个参数）DEFINE_METHOD4(result, name, args) 定义一个方法（4个参数）以此类推...DEFINE_FUNC_ALIGNED0(result, linkage, name) 定义一个方法（不包含参数）DEFINE_FUNC_ALIGNED1(result, linkage, name, t1) 定义一个方法（1个参数）DEFINE_FUNC_ALIGNED2(result, linkage, name, t1, t2) 定义一个方法（2个参数）以此类推...可以看一下这些宏的定义。看了一会，感觉宏的定义太多了，好乱。在这里仅举一个例子：RESOLVE_METHOD
#define RESOLVE_METHOD(method) \ if (!m_dll->ResolveExport( #method , & m_##method##_ptr )) \ return false;
从定义中可以看出，调用了m_dll的方法ResolveExport()。但是在DllAvFormat中并没有m_dll变量。实际上m_dll位于DllAvFormat的父类DllDynamic里面。
DllAvFormat继承了DllDynamic。DllDynamic是用于加载Dll的类。我们可以看一下它的定义：
//Dll动态加载类class DllDynamic{public: DllDynamic(); DllDynamic(const CStdString& strDllName); virtual ~DllDynamic(); virtual bool Load();//加载 virtual void Unload();//卸载 virtual bool IsLoaded() const { return m_dll!=NULL; }//是否加载 bool CanLoad(); bool EnableDelayedUnload(bool bOnOff); bool SetFile(const CStdString& strDllName);//设置文件 const CStdString &GetFile() const { return m_strDllName; }protected: virtual bool ResolveExports()=0; virtual bool LoadSymbols() { return false; } bool m_DelayUnload; LibraryLoader* m_dll; CStdString m_strDllName;}; 其中有一个变量LibraryLoader* m_dll。是用于加载Dll的。
可以看一DllDynamic中主要的几个函数，就能明白这个类的作用了。
//加载bool DllDynamic::Load(){ if (m_dll) return true; if (!(m_dll=CSectionLoader::LoadDLL(m_strDllName, m_DelayUnload, LoadSymbols()))) return false; if (!ResolveExports()) { CLog::Log(LOGERROR, "Unable to resolve exports from dll %s", m_strDllName.c_str()); Unload(); return false; } return true;} //卸载void DllDynamic::Unload(){ if(m_dll) CSectionLoader::UnloadDLL(m_strDllName); m_dll=NULL;} 可以看看LibraryLoader的定义。LibraryLoader本身是一个纯虚类，具体方法的实现在其子类里面。
//Dll加载类class LibraryLoader{public: LibraryLoader(const char* libraryFile); virtual ~LibraryLoader(); virtual bool Load() = 0; virtual void Unload() = 0; virtual int ResolveExport(const char* symbol, void** ptr, bool logging = true) = 0; virtual int ResolveOrdinal(unsigned long ordinal, void** ptr); virtual bool IsSystemDll() = 0; virtual HMODULE GetHModule() = 0; virtual bool HasSymbols() = 0; char* GetName(); // eg "mplayer.dll" char* GetFileName(); // "special://xbmcbin/system/mplayer/players/mplayer.dll" char* GetPath(); // "special://xbmcbin/system/mplayer/players/" int IncrRef(); int DecrRef(); int GetRef();private: LibraryLoader(const LibraryLoader&); LibraryLoader& operator=(const LibraryLoader&); char* m_sFileName; char* m_sPath; int m_iRefCount;}; LibraryLoader的继承关系如下图所示。
由于自己的操作系统是Windows下的，因此可以看看Win32DllLoader的定义。
//Windows下的Dll加载类class Win32DllLoader : public LibraryLoader{public: class Import { public: void *table; DWORD function; }; Win32DllLoader(const char *dll); ~Win32DllLoader(); virtual bool Load();//加载 virtual void Unload();//卸载 virtual int ResolveExport(const char* symbol, void** ptr, bool logging = true); virtual bool IsSystemDll(); virtual HMODULE GetHModule(); virtual bool HasSymbols();private: void OverrideImports(const CStdString &dll); void RestoreImports(); static bool ResolveImport(const char *dllName, const char *functionName, void **fixup); static bool ResolveOrdinal(const char *dllName, unsigned long ordinal, void **fixup); bool NeedsHooking(const char *dllName); HMODULE m_dllHandle; bool bIsSystemDll; std::vector m_overriddenImports; std::vector m_referencedDlls;}; 其中加载Dll使用Load()，卸载Dll使用Unload()。可以看看这两个函数具体的代码。
//加载bool Win32DllLoader::Load(){ if (m_dllHandle != NULL) return true; //文件路径 CStdString strFileName = GetFileName(); CStdStringW strDllW; g_charsetConverter.utf8ToW(CSpecialProtocol::TranslatePath(strFileName), strDllW, false, false, false); //加载库 m_dllHandle = LoadLibraryExW(strDllW.c_str(), NULL, LOAD_WITH_ALTERED_SEARCH_PATH); if (!m_dllHandle) { LPVOID lpMsgBuf; DWORD dw = GetLastError(); FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS, NULL, dw, 0, (LPTSTR) &lpMsgBuf, 0, NULL ); CLog::Log(LOGERROR, "%s: Failed to load %s with error %d:%s", __FUNCTION__, CSpecialProtocol::TranslatePath(strFileName).c_str(), dw, lpMsgBuf); LocalFree(lpMsgBuf); return false; } // handle functions that the dll imports if (NeedsHooking(strFileName.c_str())) OverrideImports(strFileName); else bIsSystemDll = true; return true;}//卸载void Win32DllLoader::Unload(){ // restore our imports RestoreImports(); //卸载库 if (m_dllHandle) { if (!FreeLibrary(m_dllHandle)) CLog::Log(LOGERROR, "%s Unable to unload %s", __FUNCTION__, GetName()); } m_dllHandle = NULL;}