c语言中getopt的返回

Question

I am so confused about the return of getopt. When does it return (-1)?

When I assign it to an int variable, it returns (-1) if I don't write any options in the terminal window. Whereas it returns the first option character in ascii even if I write more than one option.

But when using it without assigning, it returns (-1) if I write more than one option.

And concerning the attached images, how did it type the error message although the value equals -1?

The Statments

The results

Answer 1

In your example with 2 arguments getopt will return a different value each time you will call it. The first time it will return 'a' then it will return 'b' and the last time it has no more options to read from and will return -1

It should be handled within a loop like this https://www.tutorialspoint.com/getopt-function-in-c-to-parse-command-line-arguments