I've written a function to do some lexical analysis as following: Code: struct Token* GetToken(FILE* stream, int* line, int* column); Now in another project I want to write an expression calculator. The first step is to cut the expression into tokens. So I want to use my GetToken() method. But the ploblem is, my expression is stored in a string, just as "char expression;". How can my function GetToken() work on a string?