In C, could the program change the inputted spaces into underscores?

I'm new at using the C program.:) I would want to know would the program be able to replace the spaces into underscores? For example: if I inputted John Doe for the name, is there a syntax that would change John Doe into John_Doe instead? Because if there are spaces in the inputted name, I keep getting infinite loops.:)) Thanks to those who will answer correctly