Function in C

A function declaration tells the compiler about a function’s name, return type, and parameters. A function definition provides the actual body of the function.

Defining a Function

The general form of a function definition in C programming language is as follows −

return_type function_name( parameter list ) 
{
 body of the function 
}

A function definition in C programming consists of a function header and a function body. Here are all the parts of a function −

• Return Type − A function may return a value. The return_type is the data type of the value the function returns. Some functions perform the desired operations without returning a value. In this case, the return_type is the keyword void.
• Function Name − This is the actual name of the function. The function name and the parameter list together constitute the function signature.
• Parameters − A parameter is like a placeholder. When a function is invoked, you pass a value to the parameter. This value is referred to as actual parameter or argument. The parameter list refers to the type, order, and number of the parameters of a function. Parameters are optional; that is, a function may contain no parameters.
• Function Body − The function body contains a collection of statements that define what the function does.

Example

Given below is the source code for a function called max(). This function takes two parameters num1 and num2 and returns the maximum value between the two −

/* function returning the max between two numbers */ 
int max(int num1, int num2) { 
/* local variable declaration */ 
int result;
 if (num1 > num2)
 result = num1;
 else 
result = num2; 
return result;
 }

Function Declarations

A function declaration tells the compiler about a function name and how to call the function. The actual body of the function can be defined separately.

A function declaration has the following parts −

return_type function_name( parameter list );

For the above defined function max(), the function declaration is as follows −

int max(int num1, int num2);

Parameter names are not important in function declaration only their type is required, so the following is also a valid declaration −

int max(int, int);

Function declaration is required when you define a function in one source file and you call that function in another file. In such case, you should declare the function at the top of the file calling the function.

Calling a Function

When a program calls a function, the program control is transferred to the called function. A called function performs a defined task and when its return statement is executed or when its function-ending closing brace is reached, it returns the program control back to the main program.

To call a function, you simply need to pass the required parameters along with the function name, and if the function returns a value, then you can store the returned value. For example −

#include <stdio.h> /* function declaration */ 
int max(int num1, int num2);
 int main () { /* local variable definition */
 int a = 100; 
int b = 200; 
int ret; /* calling a function to get max value */ 
ret = max(a, b);
 printf( "Max value is : %d\n", ret );
 return 0;
 }
 /* function returning the max between two numbers */ 
int max(int num1, int num2) {
 /* local variable declaration */ 
int result; 
if (num1 > num2)
 result = num1; 
else result = num2;
 return result; 
}

We have kept max() along with main() and compiled the source code. While running the final executable, it would produce the following result −

Max value is : 200

Function Arguments

If a function is to use arguments, it must declare variables that accept the values of the arguments. These variables are called the formal parameters of the function. Formal parameters behave like other local variables inside the function and are created upon entry into the function and destroyed upon exit.

While calling a function, there are two ways in which arguments can be passed to a function

Function call by Value

By default, C programming uses call by value to pass arguments. In general, it means the code within a function cannot alter the arguments used to call the function. Consider the function swap() definition as follows.

/* function definition to swap the values */
void swap(int x, int y) {

   int temp;

   temp = x; /* save the value of x */
   x = y;    /* put y into x */
   y = temp; /* put temp into y */
  
   return;
}

Now, let us call the function swap() by passing actual values as in the following example

#include <stdio.h> /* function declaration */ 
void swap(int x, int y);
 int main () { /* local variable definition */ 
int a = 100;
 int b = 200; 
printf("Before swap, value of a : %d\n", a );
 printf("Before swap, value of b : %d\n", b );
 /* calling a function to swap the values */ 
swap(a, b);
printf("After swap, value of a : %d\n", a ); 
printf("After swap, value of b : %d\n", b );
 return 0; 
}
 void swap(int x, int y) { 
int temp; 
temp = x; 
/* save the value of x */ 
x = y; 
/* put y into x */
 y = temp; 
/* put temp into y */
 return; 
}

Let us put the above code in a single C file, compile and execute it, it will produce the following result −

Before swap, value of a : 100
Before swap, value of b : 200
After swap, value of a : 100
After swap, value of b : 200

It shows that there are no changes in the values, though they had been changed inside the function.

To pass a value by reference, argument pointers are passed to the functions just like any other value. So accordingly you need to declare the function parameters as pointer types as in the following function swap(), which exchanges the values of the two integer variables pointed to, by their arguments.

/* function definition to swap the values */
 void swap(int *x, int *y) {
 int temp;
temp = *x;
 /* save the value at address x */ 
*x = *y; 
/* put y into x */
 *y = temp; /* put temp into y */ 
return;
 }

Let us now call the function swap() by passing values by reference as in the following example:

#include <stdio.h>
 int main () { 
/* local variable definition */ 
int a = 100;
 int b = 200; 
printf("Before swap, value of a : %d\n", a );
 printf("Before swap, value of b : %d\n", b ); /* calling a function to swap the values */ 
swap(&a, &b); 
printf("After swap, value of a : %d\n", a );
 printf("After swap, value of b : %d\n", b ); 
return 0; 
} 
void swap(int *x, int *y) {
 int temp;
 temp = *x;
 /* save the value of x */ 
*x = *y; 
/* put y into x */ 
*y = temp;
/* put temp into y */
 return; 
}

Let us put the above code in a single C file, compile and execute it, to produce the following result −

Before swap, value of a : 100
Before swap, value of b : 200
After swap, value of a : 200
After swap, value of b : 100

It shows that the change has reflected outside the function as well, unlike call by value where the changes do not reflect outside the function.