A function is a group of statements that together perform a task.
Every C program has at least one
function, which is main().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.
The C standard library provides numerous built-in functions that your program can call. For example,
strcat() to concatenate two strings, memcpy() to copy one memory location to another location, and
many more functions. A function can also be referred as a method or a sub-routine or a procedure, etc.
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.
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 −
int max(int num1, int num2) {
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
While creating a C function, you give a definition of what the function has to do. To use a function, you
will have to call that function to perform the defined task.
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 −
Live Demo
#include <stdio.h>
int max(int num1, int num2);
int main () {
int a = 100;
int b = 200;
int ret;
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 −
Sr.No. Call Type & Description
1 Call by value
This method copies the actual value of an argument into the formal parameter of the function. In this
case, changes made to the parameter inside the function have no effect on the argument.
2 Call by reference
This method copies the address of an argument into the formal parameter. Inside the function, the
address is used to access the actual argument used in the call. This means that changes made to the
parameter affect the argument.
By default, C 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.
BASIS FOR COMPARISON DEFINITION DECLARATION
Basic Determines the value stored in Specifies the name and type of
variable, function or class. variable, function, class, etc.
Memory allocation Occurs Does not take place.
Repetition Statements cannot be defined Redeclaration can be easily
again if once it is already possible.
defined.
Scope Duration is determined Visibility is specified
