1, meaning: flag is a flag bit, usually used as a boolean value to indicate whether a state or condition is true. By setting or clearing flags, programs can make conditional judgments when performing certain operations, thus realizing different control structures or processing logics.

2. Usage: The use of flag needs to pay attention to initialization and update to ensure that its value is always accurate and reliable. Flag can also be used in signal transmission, synchronization and other aspects, and has a wide range of application values. In a word, flag is a common programming skill in C language, which is of great help to improve the readability, maintainability and reliability of the program.