The source code is old K&R C, without function declarations.

Don't write code like that, and don't ever modify old K&R C by adding function declarations - unless you like getting into the intricacies and implications of C variable promotion rules, and how they may have changed over the years.

Original K&R C just took all arguments to a function, promoted them so they'd all be the same size, and stuffed them on the stack.

I think, if arguments aren't declared after the first function definition line:

Code:

main(argc, argv) <--defintion
char *argv[];  <-- argument declaration
{
    ....

then the argument implicitly defaults to "int".

Basically, in K&R C all functions are called as variable argument functions with every argument promoted to the same size, and the arguments aren't type-checked. Ever. And argument declarations in the function definitions only tell the function how to interpret the data in the variable passed - whatever that value may be, with, again, no type checking.

A "declaration" is code that tells the compiler what something is - think of it as a customs declaration for a bottle of booze - you're telling customs that you have a bottle of booze somewhere in your luggage, and what it is. It's not the bottle itself.

A "definition" is code that IS the function or variable. It's the bottle itself.

K&R C has pretty much no declarations. No one knows what anything else is. Try making drinks without knowing in advance what's in every bottle of booze...