Why use function prototypes?

Question

Why use function prototypes in C? It seems sort of redundant because we already declare the function name, argument types, and return type in the definition. Do the prototypes h

Answer 1

With the declaration of a function the compiler can check the consistent use of parameters and return value, and can compile the code even if the function is not implemented in this module. If the function is only declared but not implemented in the respective module, this gap will be closed by the linker, not the compiler.

It's similar to declaring extern variables. If you'd define them, the memory for them would be allocated multiple times. That's why you should never define variables in h-files, but declare them there. Including the h-file would result in multiple allocations of memory.