Declaration and Definition

When a function is defined at any place in the program then it is called function definition. At the time of definition of a function actual logic is implemented with-in the function.

A function declaration does not have any body and they just have their interfaces.

A function declaration is usually declared at the top of a C source file, or in a separate header file.

A function declaration is sometime called function prototype or function signature. For the above Demo() function which returns an integer, and takes two parameters a function declaration will be as follows:

int Demo( int par1, int par2);

Passing Parameters to a Function
There are two ways to pass parameters to a function:

• Pass by Value: mechanism is used when you don't want to change the value of passed parameters. When parameters are passed by value then functions in C create copies of the passed in variables and do required processing on these copied variables.

• Pass by Reference mechanism is used when you want a function to do the changes in passed parameters and reflect those changes back to the calling function. In this case only addresses of the variables are passed to a function so that function can work directly over the addresses.

Here are two programs to understand the difference: First example is for Pass by value:

#include /* function declaration goes here.*/ void swap( int p1, int p2 ); int main() { int a = 10; int b = 20; printf("Before: Value of a = %d and value of b = %d\n", a, b ); swap( a, b ); printf("After: Value of a = %d and value of b = %d\n", a, b ); } void swap( int p1, int p2 ) { int t; t = p2; p2 = p1; p1 = t; printf("Value of a (p1) = %d and value of b(p2) = %d\n", p1, p2 ); }

Here is the result produced by the above example. Here the values of a and b remain unchanged before calling swap function and after calling swap function.

Before: Value of a = 10 and value of b = 20 Value of a (p1) = 20 and value of b(p2) = 10 After: Value of a = 10 and value of b = 20

Following is the example which demonstrate the concept of pass by reference

#include /* function declaration goes here.*/ void swap( int *p1, int *p2 ); int main() { int a = 10; int b = 20; printf("Before: Value of a = %d and value of b = %d\n", a, b ); swap( &a, &b ); printf("After: Value of a = %d and value of b = %d\n", a, b ); } void swap( int *p1, int *p2 ) { int t; t = *p2; *p2 = *p1; *p1 = t; printf("Value of a (p1) = %d and value of b(p2) = %d\n", *p1, *p2 ); }

Here is the result produced by the above example. Here the values of a and b are changes after calling swap function.

Before: Value of a = 10 and value of b = 20 Value of a (p1) = 20 and value of b(p2) = 10 After: Value of a = 20 and value of b = 10