Question

I’m trying to write a function to get the Windows-equivalent of HOME. My C skills are rusty, so don’t mind that my example code does not compile. I’m trying to use SHGetKnownFolderPath on Windows Vista and newer, and SHGetFolderPath on Server 2003 and older. Since I expect to encounter users running Windows XP (as it is still the number one deployed version of Windows), I am going to avoid having a reference to SHGetKnownFolderPath in the symbol table (as this would lead to a binary that won’t even load on XP). I know to LoadLibrary() shell32 and to GetProcAddress() from there, but my skills on doing function pointers are, well, crap, to say the least.

When I write features, and they are difficult to handle, I isolate them into an separate example file. The broken example I have so far is:

#include <windows.h>
#include <stdio.h>

// Pointerizing this Vista-and-later call for XP/2000 compat, etc.
typedef HRESULT (WINAPI* lpSHGetKnownFolderPath)(
    REFKNOWNFOLDERID rfid,
    DWORD dwFlags,
    HANDLE hToken,
    PWSTR *ppszPath
) lpSHGetKnownFolderPath;

int main(int argc, char *argv[])
{
    // SHGet(Known)FolderPath() method.
    HMODULE hndl_shell32;
    lpSHGetKnownFolderPath pSHGetKnownFolderPath;
    hndl_shell32 = LoadLibrary("shell32");
    pSHGetKnownFolderPath = GetProcAddress(hndl_shell32, "SHGetKnownFolderPathW");
    if(pSHGetKnownFolderPath != NULL) {

    } else {

    }

}

My question is this: Knowing that I’m doing this wrong, how would I go about doing this right? And an explanation as to how to do it right in the future would be appreciated. Thanks.

Answer 1

Here is a small application that shows how to use LoadLibrary() and GetProcAddress() with advice provided in comments:

#include <windows.h>
#include <stdio.h>
#include <shlobj.h>

/* The name of the function pointer type is
    'lpSHGetKnownFolderPath', no need for
    additional token after ')'. */
typedef HRESULT (WINAPI* lpSHGetKnownFolderPath)(
    REFKNOWNFOLDERID rfid,
    DWORD dwFlags,
    HANDLE hToken,
    PWSTR *ppszPath
);

int main()
{
    HMODULE hndl_shell32;
    lpSHGetKnownFolderPath pSHGetKnownFolderPath;

    /* Always check the return value of LoadLibrary. */
    hndl_shell32 = LoadLibrary("shell32");
    if (NULL != hndl_shell32)
    {
        /* There is no 'SHGetKnownFolderPathW()'.
           You need to cast return value of 'GetProcAddress()'. */
        pSHGetKnownFolderPath = (lpSHGetKnownFolderPath)
            GetProcAddress(hndl_shell32, "SHGetKnownFolderPath");

        if(pSHGetKnownFolderPath != NULL)
        {
            PWSTR user_dir = 0;
            if (SUCCEEDED(pSHGetKnownFolderPath(
                            FOLDERID_Profile,
                            0,
                            NULL,
                            &user_dir)))
            {
                /* Use 'user_dir' - remember to:

                     CoTaskMemFree(user_dir);

                   when no longer required.
                 */
            }
        }
        else
        {
            fprintf(stderr, "Failed to locate function: %d\n",
                GetLastError());
        }

        /* Always match LoadLibrary with FreeLibrary.
           If FreeLibrary() results in the shell32.dll
           being unloaded 'pSHGetKnownFolderPath' is
           no longer valid.
         */
        FreeLibrary(hndl_shell32);
    }
    else
    {
        fprintf(stderr, "Failed to load shell32.dll: %d\n", GetLastError());
    }

    return 0;
}

This was compiled on Windows XP.

Output on Windows XP:

Failed to locate function: 127

where 127 means The specified procedure could not be found.

Output on Windows Vista:

C:\Users\admin