MIDIBUF_SIZE
count = MIDIBUF_SIZE - mb->start;
count = MIDIBUF_SIZE - MIDIBUF_END(mb);
#define MIDIBUF_MASK (MIDIBUF_SIZE - 1)
unsigned char data[MIDIBUF_SIZE];
#define MIDIBUF_AVAIL(buf) (MIDIBUF_SIZE - (buf)->used)
#define MIDIBUF_ISFULL(buf) ((buf)->used >= MIDIBUF_SIZE)