Typically the return value is used as a exit status code (I prefer to use the exit () function myself). So that, if your program is called by another program or a shell/batch script, the calling entity can react to the exit status appropriately.
To explain this a little more clearly, assume your program parses a large file and inserts the parsed data into a database. Your program is called by a user clicking a button in some other program. How does the user know if your program successfully completed the task? Or, if it was not completed successfully, why?
The easiest way to tell what happened after your program has completed is to check the returned exit status code. Most of the time these codes are defined by you. For instance, 0 if it completed okay, 1 if it could not connect to the database, 2 if it deadlocked with another user in the database, 3 if it couldn 't find the file to parse, etc. Then, using these codes, the calling program can print the appropriate message to the user 's screen, or use some sort of logic that will attempt to get your program to complete successfully.
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