User defined callback function, to get the Beat position in seconds.

void CALLBACK yourBpmBeatProc(
    DWORD chan,
    double beatpos,
    void *user


chanChannel handle that the BASS_FX_BPM_BeatCallbackSet or BASS_FX_BPM_BeatDecodeGet has applied to
beatposThe exact beat position in seconds
userThe user instance data given when BASS_FX_BPM_BeatCallbackSet or BASS_FX_BPM_BeatDecodeGet was called

To filter out false beat positions, users first will have to find a BPM of a song, using one of the BPM detection functions, e.g: BASS_FX_BPM_DecodeGet
When you know the BPM, you can calculate the approximate duration that should be between beat positions, e.g:

stream_length = 330 seconds (5 minutes 30 seconds)
stream_bpm = 140 (beats per minute)
duration_between_beats = stream_bpm / stream_length = 140 / 330 = 0.4242 sec

If beat position returned by callback functions doesn't fit in duration_between_beats, then it's probably some harmonic sound or a false beat, so you can ignore it.
That way you can also detect and map all beats, including the 1st one.

A very simple way to count the BPM in real-time, using only 2 beats.

double prevBeatPos = 0.0f;     // previous beat position in seconds
double bpm = 0.0f;             // the bpm

void CALLBACK BeatProcGetBPM(DWORD handle, double beatpos, void *user)
    if (beatpos != prevBeatPos)
        bpm = 60.0f / (beatpos - prevBeatPos);     // calculate the bpm

    prevBeatPos = beatpos;    // save current beat position

Get the detection progress in percents:

float progress = 100.f * (beatpos - startpos) / (endpos - startpos);

See also
BASS_FX_BPM_BeatCallbackSet, BASS_FX_BPM_BeatDecodeGet