C Function Declaration and Definition
Function Declaration and Definition
You have already learned from the previous chapters that you can create and call a function in the following way:
Example
// Create a function
void myFunction() {
printf("I just got executed!");
}
int main() {
myFunction(); // call the function
return 0;
}
Try it Yourself »
void myFunction() {
printf("I just got executed!");
}
int main() {
myFunction(); // call the function
return 0;
}
A function consist of two parts:
- Declaration: the function's name, return type, and parameters (if any)
- Definition: the body of the function (code to be executed)
void myFunction() { // declaration
// the body of the function (definition)
}
// the body of the function (definition)
}
For code optimization, it is recommended to separate the declaration and the definition of the function.
You will often see C programs that have function declaration above main(),
and function definition below main().
This will make the code better organized and easier to read:
Example
// Function declaration
void myFunction();
// The main method
int main() {
myFunction(); // call the function
return 0;
}
// Function definition
void myFunction() {
printf("I just got executed!");
}
Try it Yourself »
void myFunction();
// The main method
int main() {
myFunction(); // call the function
return 0;
}
// Function definition
void myFunction() {
printf("I just got executed!");
}
What About Parameters
If we use the example from the function parameters chapter regarding parameters and return values:
Example
int myFunction(int x, int y) {
return x + y;
}
int main() {
int result = myFunction(5, 3);
printf("Result is = %d", result);
return 0;
}
// Outputs 8 (5 + 3)
Try it Yourself »
return x + y;
}
int main() {
int result = myFunction(5, 3);
printf("Result is = %d", result);
return 0;
}
// Outputs 8 (5 + 3)
It is considered good practice to write it like this instead:
Example
// Function declaration
int myFunction(int x, int y);
// The main method
int main() {
int result = myFunction(5, 3); // call the function
printf("Result is = %d", result);
return 0;
}
// Function definition
int myFunction(int x, int y) {
return x + y;
}
Try it Yourself »
int myFunction(int x, int y);
// The main method
int main() {
int result = myFunction(5, 3); // call the function
printf("Result is = %d", result);
return 0;
}
// Function definition
int myFunction(int x, int y) {
return x + y;
}
Functions Calling Other Functions
As long as you declare functions first, it is also possible to use functions to call other functions:
Example
Use one function to call another function:
// Declare two functions, myFunction and myOtherFunction
void myFunction();
void myOtherFunction();
int main() {
myFunction(); // call myFunction (from main)
return 0;
}
// Define myFunction
void myFunction() {
printf("Some text in myFunction\n");
myOtherFunction(); // call myOtherFunction (from myFunction)
}
// Define myOtherFunction
void myOtherFunction() {
printf("Hey! Some text in myOtherFunction\n");
}
Try it Yourself »
void myFunction();
void myOtherFunction();
int main() {
myFunction(); // call myFunction (from main)
return 0;
}
// Define myFunction
void myFunction() {
printf("Some text in myFunction\n");
myOtherFunction(); // call myOtherFunction (from myFunction)
}
// Define myOtherFunction
void myOtherFunction() {
printf("Hey! Some text in myOtherFunction\n");
}