This paper presents an efficient software technique, control flow error detection through assertions (CEDA), for online detection of control flow errors. Extra instructions are automatically embedded into the program at compile time to continuously update run-time signatures and to compare them against pre-assigned values. The novel method of computing run-time signatures results in a huge reduction in the performance overhead, as well as the ability to deal with complex programs and the capability to detect subtle control flow errors. The widely used C compiler, GCC, has been modified to implement CEDA, and the SPEC benchmark programs were used as the target to compare with earlier techniques. Fault injection experiments were used to evaluate the fault detection capabilities. Based on a new comparison metric, method efficiency, which takes into account both error coverage and performance overhead, CEDA is found to be much better than previously proposed methods.