How are program side-effects handled in replay mode?

Any side-effects a program has on the rest of the system do not occur when in replay mode. For example, if a program issues a printf, then when the printf is encountered in record mode, text is displayed on stdout. However, when the printf is replayed, no output is generated. As another example, if a file is unlinked, in record mode the file will be really unlinked from the file-system; when in replay mode, the program will have no interaction with the operating system. Instead, UDB will return in replay mode whatever result the unlink system-call returned to the debuggee when it executed in record mode.