Ensuring that safety requirements are respected is a critical issue for the deployment of hazardous and complex reactive systems. We consider a separate safety channel, called a monitor, that is able to partially observe the system and to trigger safety-ensuring actuations. We address the issue of correctly specifying such a monitor with respect to safety and liveness requirements. Two safety requirement synthesis programs are presented and compared. Based on a formal model of the system and its hazards, they compute a monitor behavior that ensures system safety without unduly compromising system liveness. The first program uses the model-checker NuSMV to check safety requirements. These requirements are automatically generated by a branch-and-bound algorithm. Based on a game theory approach, the second program uses the TIGA extension of UPPAAL to synthesize safety requirements, starting from an appropriately reformulated representation of the problem.