# Reactis Simulator ```{epigraph} Reactis simulator extends traditional debugging with advanced error-detection and testing. ``` Reactis Simulator provides an environment in which program execution can be carefully controlled and monitored. Simulator's basic features are similar to traditional source-level debuggers: you can single-step through individual source statements, set breakpoints, and display data values. ```{figure} images/simulator-hover-info.png --- width: 60% name: simulator-hover-info-fig --- Coverage metrics and a C macro expansion displayed during a Simulator session. ``` However, Simulator also has a number of unique features which make it more powerful than a traditional debugger. Runtime error detection. : Simulator instantly detects a host of runtime errors, including memory errors, uninitialized variable accesses, and integer overflows. When a runtime error occurs, execution is interrupted, unlike most C environments, in which execution proceeds using a corrupted data value. This is discussed in more detail in [Finding and Fixing Runtime Errors](runtime-errors.md). Test replay. : Simulator provides the ability to replay tests, so that any point in the execution sequence leading to an error can be examined. Execution can be reversed to the beginning of a step at any point, making it is easy to go back and check the value of a variable at an earlier point in time. Graphical display of coverage metrics. : Simulator highlights source code according to coverage status, so that uncovered statements, conditions, decisions and MC/DC targets can be quickly found and uncovered code regions visualized. In {numref}`Figure {number}`, code containing uncovered targets is displayed in red. A thin red overline indicates a decision which has never been true, and a thin red underline indicates a decision which has never evaluated to false. Similarly, a thick red overline indicates a condition which has never been true, and a thick red underline indicates a condition which has never evaluated to false. Macro expansion visualization. : Simulator provides the ability to view C preprocessor macro-expansions while browsing the source code. When a macro is hovered over with the mouse, its expansion is displayed. In {numref}`Figure {number}`, the user is hovering on M_NOTINIT, which is a macro whose expansion is 0 (a single character string). Test suite tuning. : Simulator gives you the capability to fine tune automatically-generated test suites. Additional tests can be added, and existing tests can be modified or extended with additional steps.