A clamp in one embodiment includes a flexible base; a bifurcation pivotal about one end of the base; a fastening member pivotal about the other end of the base and including pawls, an opposite projection, and a toothed section formed above; a universal member meshed with the toothed section; a seat rotatably secured to the universal member and adapted to releasably secure to a light; and a flexible ratchet pivotal about the other end of the base. Wrapping the ratchet around a seat post of bicycle by inserting it through the bifurcation and a space between the pawls and one end of the ratchet, pivoting the fastening member toward the seat until one pawl engages the ratchet and the fastening member presses on the base to urge against the seat post, and pivoting the bifurcation to press the bar onto the projection will fasten the seat post.