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Calling the DbgHelp Library

Although DbgHelp.dll ships with all versions of Windows, callers should consider using one of the more recent versions of this DLL as found in the Debugging Tools For Windows package. For details on distribution of DbgHelp, see DbgHelp Versions.

When using DbgHelp, the best strategy is to install a copy of the library from the Debugging Tools For Windows package in the application directory logically adjacent to the software that calls it. If Symbol Server and Source Server are also needed, then both SymSrv.dll and SrcSrv.dll must be installed in the same directory as DbgHelp.dll, as DbgHelp will only call these DLLs if they share the same directory with it. (Note that DbgHelp will not call these two DLLs from the standard search path.) This helps prevent the usage of mismatched DLLs; likewise, it also improves security overall.

The following code is extracted from the DbgHelp source. It shows how DbgHelp only loads versions of SymSrv.dll and SrcSrv.dll from the same directory that DbgHelp.dll resides in.



HINSTANCE ghinst;

// For calculating the size of arrays for safe string functions.

#ifndef cch
 #define ccht(Array, EltType) (sizeof(Array) / sizeof(EltType))
 #define cch(Array) ccht(Array, (Array)[0])
#endif

//
// LoadLibrary() a DLL, using the same directory as dbghelp.dll.
//

HMODULE 
LoadDLL(
    __in PCWSTR filename
    )
{
    WCHAR drive[10] = L"";
    WCHAR dir[MAX_PATH + 1] = L"";
    WCHAR file[MAX_PATH + 1] = L"";
    WCHAR ext[MAX_PATH + 1] = L"";
    WCHAR path[MAX_PATH + 1] = L"";
    HMODULE hm;
    
    // Chop up 'filename' into its elements.
    
    _wsplitpath_s(filename, drive, cch(drive), dir, cch(dir), file, cch(file), ext, cch(ext));

    // If 'filename' contains no path information, then get the the path to our module and 
    // use it to create a fully qualified path to the module we are loading.  Then load it.
    
    if (!*drive && !*dir) 
    {
        // ghinst is the HINSTANCE of this module, initialized in DllMain or WinMain
         
        if (GetModuleFileNameW(ghinst, path, MAX_PATH)) 
        {
            _wsplitpath_s(path, drive, cch(drive), dir, cch(dir), NULL, 0, NULL, 0);
            if (*drive || *dir) 
            {
                swprintf_s(path, cch(path), L"%s%s%s%s", drive, dir, file, ext);
                hm = LoadLibrary(path);
                if (hm)
                    return hm;
            }
        }
    }
    else
    {
        // If we wanted to, we could have LoadDLL also support directories being specified
        // in 'filename'.  We could pass the path here.  The result is if no path is specified,
        // the module path is used as above, otherwise the path in 'filename' is specified.
        // But the standard search logic of LoadLibrary is still avoided.
        
        /*
        hm = LoadLibrary(path);
        if (hm)
            return hm;
        */
    }
    
    return 0;
}


After loading these two DLLs, DbgHelp calls GetProcAddress to obtain the functions it needs from them.

Normally, code that calls DbgHelp.dll ensures that the correct version is loaded by installing DbgHelp.dll in the same directory as the application that initiated the current process. If the calling code is in a DLL and does not have access to or knowledge of the location of the initial process, then DbgHelp.dll must be installed alongside the calling DLL and code similar to DbgHelp's LoadDLL should be used.

Related topics

DbgHelp Versions
LoadLibrary
GetProcAddress
GetModuleFileName

 

 

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