The flag can be a pointer variable, a constant variable or a common variable. Generally speaking, flag is used as a marker to mark the state of an event, and this state needs you to give it meaning. And its value is a variable of "true" (.T.) or "false" (.F.) to indicate whether certain conditions are met or whether an event occurs. The flag set in front can be used to control the action of the program later. C language is a process-oriented abstract general programming language, which is widely used in the underlying development. Programming language is a language used to write computer programs. Language is based on a set of symbols and rules. According to the rules, the sum of symbol strings composed of symbols is language. In programming languages, these tag strings are programs.