The 1990 ISO C standard’s most sweeping change to the language is the function prototype borrowed from the C++ language. By specifying the number and types of parameters for each function, every regular compile gets the benefits of argument and parameter checking (similar to those of lint) for each function call, while arguments are automatically converted (just as with an assignment) to the type expected by the function. The 1990 ISO C standard includes rules that govern the mixing of old- and new-style function declarations since there are many, many lines of existing C code that could and should be converted to use prototypes.
The 1999 ISO C standard made old-style function declarations obsolete.