C Function Declaration (Prototypes)
A function declaration, or prototype, tells the compiler a function's name, return type, and parameter types before it's actually defined or used. This allows you to call a function before its full definition appears in the file.
Prototypes are especially important when functions call each other, or when definitions live in separate source files and are shared via header files.
returnType functionName(paramTypes);Why prototypes are needed
The C compiler processes code from top to bottom, so if you call a function before it's defined, the compiler needs a prototype to know its signature and check your call for correctness.
Declaration vs definition
A declaration just states the function's signature and ends with a semicolon (no body). A definition provides the full body and does the actual work.
#include <stdio.h>
int add(int a, int b);
int main() {
printf("%d\n", add(2, 3));
return 0;
}
int add(int a, int b) {
return a + b;
}5The prototype lets main() call add() even though add() is defined afterward.
// math_utils.h
int square(int n);(no output — header file)Header files typically contain only function prototypes, shared across multiple .c files.
Key points
- A prototype declares a function's signature without its body.
- Prototypes let you call functions before they're fully defined.
- Header files (.h) commonly store prototypes shared across files.
- A prototype and its definition must have matching signatures.