Function Call

FUNCTION CALL

The function call statement invokes the function. When a function is invoked the compiler jumps to the called function to execute the statements that are part of that function. Once the called function is executed, the program control passes back to the calling function.

Function call statement has the following syntax:

function_name (variablel, variable2, ...) ;

When the function declaration is present before the function call, the compiler can check if the correct number and type of arguments are used in the function call and the returned value, if any, is being used reasonably.

Function definitions are often placed in separate header files which can be included in other C source files that wish to use these functions. For example, the header file stdio.h contains the definition of scanf and printf functions. We simply include this header file and call these functions without worrying about the code to implement their functionality.

Note

List of variables used in function call is known as actual parameter list. The actual parameter list may contain variable names, expressions, or constants.

Points to Remember While Callingt Functions

The following points are to be kept in mind while calling functions:

• Function name and the number and type of arguments in the function call must be same as that given in the function declaration and function header of the function definition.

Programming Tip: A logical error will be generated if the arguments in the function call are placed in the wrong order.

• If the parameters passed to a function are more than what it is specified to accept then the extra arguments will be discarded.

• If the parameters passed to a function are less than what it is specified to accept then the unmatched arguments will be initialized to some garbage value.

• Names (and not the types) of variables in function declaration, function call, and header of function definition may vary.

• If the data type of the argument passed does not match with that specified in the function declaration then either the unmatched argument will be initialized to some be garbage value or a compile time error will be generated.

• Arguments may be passed in the form of expressions to the called function. In such cases, arguments are first evaluated and converted to the type of formal parameter and then the body of the function gets executed.

• The parameter list must be separated with commas.

• If the return type of the function is not void, then the value returned by the called function may be assigned to some variable as shown in the following statement.

variable_name = function_name (variablel, variable2, ...);

Let us now try writing a program using function.

1. Write a program to add two integers using functions.

#include <stdio.h>

// FUNCTION DECLARATION

int sum (int a, int b);

int main()

{

int num1, num2, total =0;

printf("\n Enter the first number: ");

scanf("%d", &num1);

printf("\n Enter the second number: ");

scanf("%d", &num2);

total = sum (num1, num2);

// FUNCTION CALL

printf("\n Total = %d", total);

return 0;

}

// FUNCTION DEFINITION

int sum (int a, int b) // FUNCTION HEADER

{ // FUNCTION BODY

int result;

result= a + b;

return result;

}

Output

Enter the first number: 20

Enter the second number: 30

Total = 50

The variables declared within the function and its parameters are local to that function. The programmer may use same names for variables in other functions. This eliminates the need for thinking and keeping unique names for variables declared in all the functions in the program.

Programming Tip: It is an error to use void as the return type when the function is expected to return a value to the calling function.

In the sum() function, we have declared a variable result just like any other variable. Variables declared within a function are called automatic local variables because of two reasons.

• First, they are local to the function. So, their effect (in terms of scope, lifetime, or accessibility) is limited to the function. Any change made to these variables is visible only in that function.

• Second, they are automatically created whenever the function is called and cease to exist after control exits the function.

Note