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The exit function
GNU coding standards specify that you should always use exit (or
return) within your main function. (See Style.)
You can use the exit function to terminate a program at any
point, no matter how many function calls have been made. Before it
terminates the program, it calls a number of other functions that
perform tidy-up duties such as closing open files.
exit is called with a return code, like this:
exit(0);
In the example above, the return code is 0. Any program that
calls your program can read the return code from your program. The
return code is like a return value from another function that is not
main; in fact, most of the time you can use the return
command within your main, instead of exit.
Conventionally, a return code of 0 specifies that your program
has ended normally and all is well. (You can remember this as "zero
errors", although for technical reasons, you cannot use the number of
errors your program found as the return code. See Style.) A return
code other than 0 indicates that some sort of error has occurred.
If your code terminates when it encounters an error, use exit,
and specify a non-zero return code.