int RingBuffer_trywrite(RingBuffer* rb, char c) {
...
rb->writeptr += tmp; // Здесь...
...
}
int RingBuffer_tryread(RingBuffer* rb, char* c) {
...
rb->readptr += tmp; // и здесь.
...
}

char* tmp = rb->writeptr + CHAR_SIZE;

rb->writeptr += tmp;

rb->writeptr = tmp;