IOProc question

I want to play multiple files using MCI with a custom IOProc.

All examples i saw are like the following:

Code:

---

LRESULT CALLBACK IOProc(LPMMIOINFO lpMMIOInfo, UINT uMessage, LPARAM lParam1, LPARAM lParam2)
{
        static BOOL alreadyOpened = FALSE;

        switch (uMessage) {
                case MMIOM_OPEN:
                        if (alreadyOpened)
                                return 0;

                        alreadyOpened = TRUE;

                        lpMMIOInfo->lDiskOffset = 0;
                        return 0;

                case MMIOM_CLOSE:
                        return 0;

                case MMIOM_READ:
                        memcpy((void *)lParam1, lpData+lpMMIOInfo->lDiskOffset, lParam2);
                        lpMMIOInfo->lDiskOffset += lParam2;
                        return (lParam2);

                case MMIOM_SEEK:
                        switch (lParam2) {
                                case SEEK_SET:
                                        lpMMIOInfo->lDiskOffset = lParam1;
                                        break;

                                case SEEK_CUR:
                                        lpMMIOInfo->lDiskOffset += lParam1;

                                case SEEK_END:
                                        lpMMIOInfo->lDiskOffset = fileSize - 1 - lParam1;
                                        break;
                        }
                        return lpMMIOInfo->lDiskOffset;
                default:
                        return -1;
        }
}

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Now the problem is the above method uses an external lpData (MMIOM_READ case) which is the buffer in which a file is loaded. If I want to play multiple files, say I have an array of buffers, how can I determine in the IOProc which file is being played?

I see that in the MMIOM_READ case lParam1 is the destination buffer to be played by the system and lParam2 is the length to be copied into lParam1.

Thank you.