Support reading mp4 container structure from file offset

include/mp4v2/file.h

@@ -404,6 +404,18 @@ MP4V2_EXPORT
 MP4FileHandle MP4Read(
     const char* fileName );
 
+/** Generalized version of above MP4Read, supporting reading a MP4 file at
+ *  the specific file offset.
+ *
+ *  @param fileName As for MP4Read
+ *  @param fileOffset seek offset in file that MP4 container structure starts
+ *  at.
+ */
+MP4V2_EXPORT
+MP4FileHandle MP4ReadFromOffset(
+    const char* fileName,
+    int64_t seekOffset);
+
 /** Read an existing mp4 file.
  *
  *  MP4ReadProvider is the first call that should be used when you want to just
@@ -432,4 +444,18 @@ MP4FileHandle MP4ReadProvider(
 
 /** @} ***********************************************************************/
 
+/** Generalized version of above MP4Read, supporting reading a MP4 file at
+ *  the specific file offset.
+ *
+ *  @param fileName As for MP4Read
+ *  @param fileOffset seek offset in file that MP4 container structure starts
+ *  at.
+ */
+
+MP4V2_EXPORT
+MP4FileHandle MP4ReadProviderFromOffset(
+    const char*            fileName,
+    const MP4FileProvider* fileProvider,
+    int64_t seekOffset );
+
 #endif /* MP4V2_FILE_H */

src/mp4.cpp

@@ -89,6 +89,10 @@ const char* MP4GetFilename( MP4FileHandle hFile )
 
 MP4FileHandle MP4Read( const char* fileName )
 {
+      return MP4ReadFromOffset(fileName, 0);
+}
+
+MP4FileHandle MP4ReadFromOffset( const char* fileName, int64_t seekOffset) {
     if (!fileName)
         return MP4_INVALID_FILE_HANDLE;
 
@@ -99,6 +103,7 @@ MP4FileHandle MP4Read( const char* fileName )
     try
     {
         ASSERT(pFile);
+        pFile->SetInitialSeekOffset( seekOffset );
         pFile->Read( fileName, NULL );
         return (MP4FileHandle)pFile;
     }
@@ -117,6 +122,12 @@ MP4FileHandle MP4Read( const char* fileName )
 }
 
 MP4FileHandle MP4ReadProvider( const char* fileName, const MP4FileProvider* fileProvider )
+{
+    return MP4ReadProviderFromOffset(fileName, fileProvider, 0);
+}
+
+MP4FileHandle MP4ReadProviderFromOffset(
+    const char* fileName, const MP4FileProvider* fileProvider, int64_t seekOffset)
 {
     if (!fileName)
         return MP4_INVALID_FILE_HANDLE;
@@ -126,6 +137,7 @@ MP4FileHandle MP4ReadProvider( const char* fileName, const MP4FileProvider* file
         return MP4_INVALID_FILE_HANDLE;
 
     try {
+        pFile->SetInitialSeekOffset( seekOffset );
         pFile->Read( fileName, fileProvider );
         return (MP4FileHandle)pFile;
     }
@@ -142,7 +154,7 @@ MP4FileHandle MP4ReadProvider( const char* fileName, const MP4FileProvider* file
         delete pFile;
     return MP4_INVALID_FILE_HANDLE;
 }
-
+    
 ///////////////////////////////////////////////////////////////////////////////
 
     MP4FileHandle MP4Create (const char* fileName,

src/mp4file.cpp

@@ -59,6 +59,7 @@ void MP4File::Init()
     m_memoryBuffer = NULL;
     m_memoryBufferSize = 0;
     m_memoryBufferPosition = 0;
+    m_initialSeekOffset = 0;
 
     m_numReadBits = 0;
     m_bufReadBits = 0;
@@ -414,7 +415,6 @@ void MP4File::Open( const char* name, File::Mode mode, const MP4FileProvider* pr
 
 void MP4File::ReadFromFile()
 {
-    // ensure we start at beginning of file
     SetPosition(0);
 
     // create a new root atom
@@ -624,6 +624,10 @@ void MP4File::Close(uint32_t options)
     m_file = NULL;
 }
 
+void MP4File::SetInitialSeekOffset(int64_t seekOffset) {
+    m_initialSeekOffset = seekOffset;
+}
+
 void MP4File::Rename(const char* oldFileName, const char* newFileName)
 {
     if( FileSystem::rename( oldFileName, newFileName ))

src/mp4file.h

@@ -91,6 +91,7 @@ class MP4File
     bool CopyClose( const string& copyFileName );
     void Dump( bool dumpImplicits = false );
     void Close(uint32_t flags = 0);
+    void SetInitialSeekOffset(int64_t seekOffset);
 
     bool Use64Bits(const char *atomName);
     void Check64BitStatus(const char *atomName);
@@ -954,6 +955,7 @@ class MP4File
     File*    m_file;
     uint64_t m_fileOriginalSize;
     uint32_t m_createFlags;
+    int64_t m_initialSeekOffset;
 
     MP4Atom*          m_pRootAtom;
     MP4Integer32Array m_trakIds;

src/mp4file_io.cpp

@@ -37,7 +37,7 @@ uint64_t MP4File::GetPosition( File* file )
         file = m_file;
 
     ASSERT( file );
-    return file->position;
+    return file->position - m_initialSeekOffset;
 }
 
 void MP4File::SetPosition( uint64_t pos, File* file )
@@ -53,7 +53,7 @@ void MP4File::SetPosition( uint64_t pos, File* file )
         file = m_file;
 
     ASSERT( file );
-    if( file->seek( pos ))
+    if( file->seek( pos + m_initialSeekOffset ))
         throw new PlatformException( "seek failed", sys::getLastError(), __FILE__, __LINE__, __FUNCTION__ );
 }
 
@@ -66,7 +66,7 @@ uint64_t MP4File::GetSize( File* file )
         file = m_file;
 
     ASSERT( file );
-    return file->size;
+    return file->size  - m_initialSeekOffset;
 }
 
 void MP4File::ReadBytes( uint8_t* buf, uint32_t bufsiz, File* file )