代码 GetGDALDriverManager()->AutoLoadDrivers(); 包含了两部分：
首先获得GDALDriverManager的singleton对象的指针，这点之前已经说明过，采用DCLP是个错误用法，不过可以通过下面的方法规避：
永远只在main函数内部单线程调用一次GDALAllRegister, 在其他线程尚未创建之前，singleton对象已经被创建出来
然后运行void GDALDriverManager::AutoLoadDrivers() 函数。这是本次分析的主要内容。注释写的不错，很容易就理解了该函数的逻辑。
/**
* rief Auto-load GDAL drivers from shared libraries.
*
* This function will automatically load drivers from shared libraries. It
* searches the "driver path" for .so (or .dll) files that start with the
* prefix "gdal_X.so". It then tries to load them and then tries to call a
* function within them called GDALRegister_X() where the 'X' is the same as
* the remainder of the shared library basename ('X' is case sensitive), or
* failing that to call GDALRegisterMe().
*
* There are a few rules for the driver path. If the GDAL_DRIVER_PATH
* environment variable it set, it is taken to be a list of directories to
* search separated by colons on UNIX, or semi-colons on Windows. Otherwise
* the /usr/local/lib/gdalplugins directory, and (if known) the
* lib/gdalplugins subdirectory of the gdal home directory are searched on
* UNIX and $(BINDIR)gdalplugins on Windows.
*/
该函数在driver path中查找文件名为gdal_X.dll或者gdal_X.so的动态库文件。如果找到，就加载该动态库，并调用其中的GDALResiter_X()函数，每隔动态库都应该实现这个函数。X代表库的名称。
driver path可以通过设置环境变量GDAL_DRIVER_PATH来获得，path之间通过,或者;分隔。 如果没有设定环境变量，就使用默认查找路径：/usr/local/lib/gdalplugins,
并且：
1. 如果Unix下有gdal home 目录的话，还会找该目录下的lib/gdalplugins子目录，
2. 如果windows下会用$(BINDDIR)gdalplugins 作为查找路径。这里$(BINDIR)值得是当前进程所在目录。
下面是全部代码：
void GDALDriverManager::AutoLoadDrivers()
{
char **papszSearchPath = NULL;
const char *pszGDAL_DRIVER_PATH =
CPLGetConfigOption( "GDAL_DRIVER_PATH", NULL );
/* -------------------------------------------------------------------- */
/* Where should we look for stuff? */
/* -------------------------------------------------------------------- */
if( pszGDAL_DRIVER_PATH != NULL )
{
#ifdef WIN32
papszSearchPath =
CSLTokenizeStringComplex( pszGDAL_DRIVER_PATH, ";", TRUE, FALSE );
#else
papszSearchPath =
CSLTokenizeStringComplex( pszGDAL_DRIVER_PATH, ":", TRUE, FALSE );
#endif
}
else
{
#ifdef GDAL_PREFIX
papszSearchPath = CSLAddString( papszSearchPath,
#ifdef MACOSX_FRAMEWORK
GDAL_PREFIX "/PlugIns");
#else
GDAL_PREFIX "/lib/gdalplugins" );
#endif
#else
char szExecPath[1024];
if( CPLGetExecPath( szExecPath, sizeof(szExecPath) ) )
{
char szPluginDir[sizeof(szExecPath)+50];
strcpy( szPluginDir, CPLGetDirname( szExecPath ) );
strcat( szPluginDir, "\gdalplugins" );
papszSearchPath = CSLAddString( papszSearchPath, szPluginDir );
}
else
{
papszSearchPath = CSLAddString( papszSearchPath,
"/usr/local/lib/gdalplugins" );
}
#endif
#ifdef MACOSX_FRAMEWORK
#define num2str(x) str(x)
#define str(x) #x
papszSearchPath = CSLAddString( papszSearchPath,
"/Library/Application Support/GDAL/"
num2str(GDAL_VERSION_MAJOR) "."
num2str(GDAL_VERSION_MINOR) "/PlugIns" );
#endif
if( strlen(GetHome()) > 0 )
{
papszSearchPath = CSLAddString( papszSearchPath,
CPLFormFilename( GetHome(),
#ifdef MACOSX_FRAMEWORK
"/Library/Application Support/GDAL/"
num2str(GDAL_VERSION_MAJOR) "."
num2str(GDAL_VERSION_MINOR) "/PlugIns", NULL ) );
#else
"lib/gdalplugins", NULL ) );
#endif
}
}
/* -------------------------------------------------------------------- */
/* Scan each directory looking for files starting with gdal_ */
/* -------------------------------------------------------------------- */
for( int iDir = 0; iDir < CSLCount(papszSearchPath); iDir++ )
{
char **papszFiles = CPLReadDir( papszSearchPath[iDir] );
for( int iFile = 0; iFile < CSLCount(papszFiles); iFile++ )
{
char *pszFuncName;
const char *pszFilename;
const char *pszExtension = CPLGetExtension( papszFiles[iFile] );
void *pRegister;
#if ( defined( _WIN32 ) && ( defined(DEBUG) || defined(_DEBUG) ) )
if( !EQUALN(papszFiles[iFile],"debug_gdal_",11) )
continue;
#else
if( !EQUALN(papszFiles[iFile],"gdal_",5) )
continue;
#endif
if( !EQUAL(pszExtension,"dll")
&& !EQUAL(pszExtension,"so")
&& !EQUAL(pszExtension,"dylib") )
continue;
pszFuncName = (char *) CPLCalloc(strlen(papszFiles[iFile])+20,1);
#if ( defined( _WIN32 ) && ( defined(DEBUG) || defined(_DEBUG) ) )
CPLString sName = CPLGetBasename(papszFiles[iFile]) + 11;
#else
CPLString sName = CPLGetBasename(papszFiles[iFile]) + 5;
#endif
sprintf( pszFuncName, "GDALRegister_%s", sName.c_str() );
pszFilename =
CPLFormFilename( papszSearchPath[iDir],
papszFiles[iFile], NULL );
pRegister = CPLGetSymbol( pszFilename, pszFuncName );
if( pRegister == NULL )
{
sName.toupper();
sprintf( pszFuncName, "GDALRegister_%s", sName.c_str() );
pRegister = CPLGetSymbol( pszFilename, pszFuncName );
if( pRegister == NULL )
{
strcpy( pszFuncName, "GDALRegisterMe" );
pRegister = CPLGetSymbol( pszFilename, pszFuncName );
}
}
if( pRegister != NULL )
{
CPLDebug( "GDAL", "Auto register %s using %s.",
pszFilename, pszFuncName );
((void (*)()) pRegister)();
}
CPLFree( pszFuncName );
}
CSLDestroy( papszFiles );
}
CSLDestroy( papszSearchPath );
}
那个查找目录树的算法算不上糟糕，不过用惯了boost和newlisp的我，看到不禁皱眉头。太不优雅了。一帮C程序员在写C++代码，循环之后还要用CSLFree和CSLDestory 释放内存，有string不用。就算不用string，也可以自己写个简单的class啊。
很明显，GDAL的源代码并没有随着C++语言的发展而进步，让我不仅想起了ACE。
调试一遍后，发现我的GDAL代码在这里没有加载任何驱动。就是我想要的结果。：）