We derive a unifying framework for the a posteriori error analysis of control constrained linear-quadratic optimal control problems. We consider finite element discretizations with discretized and nondiscretized control. A fundamental error equivalence drastically simplifies the a posteriori error analysis for optimal control problems. It basically remains to apply error estimators for the linear state and adjoint problem. We give several examples, including stabilized discretizations, and investigate the quality of the estimators and the performance of the adaptive iteration by selected numerical experiments.