C Function Definition, Declaration and Syntax

A function in C language is a collection of statements or we can say sentences that work together to complete a goal. The main() function is a must for any C program, even the simplest programs can declare extra functions.
Your code can be broken up into several functions. It is up to you how to split your code into several functions, but logically each function should carry out a certain task.

The name, return type, and parameters of a function in C language are disclosed to the compiler in the function declaration in C. The actual function body is provided by a function definition in C language.
Your program can call a variety of built-in functions from the C standard library. For example, strcat() joins two strings together, memcpy() copies data from one memory region to another, and many other methods.
In addition to these names, a function in C language may also be referred to as a method, subroutine, process, etc.

Use of the Function in C language

A C program’s code may easily be split up into numerous distinct functions. It is entirely up to us how we split the given code into the many possible functions. However, the split must be systematically done so that each function can perform and can function is able to do a certain task.
The function declaration in C provides the compiler with information about the function’s name, arguments, and return type. To put it another way, the function definition in C assists in giving us the real body of any function we need in a program or piece of code.

Functions are also known as subroutines, methods, and procedures in computer programs. Users of the C standard library have access to a number of built-in functions that they can utilize in the relevant programs. For combining two strings, we may use the strcat() method. We can also use the memcpy() function to copy data from one location in memory to another.

Defining a Function

The following is the general format for a function definition in C programming language:

return_type function_name( parameter list ) {
   body of the function

A function definition in C consists of a function body and a function header. The components of a function are listed below:

  • Return Type − Values can be returned by functions. The data type of the value the function returns is indicated by the return type. Some functions carry out the required tasks without giving a value back. The term void is used as the return type in this case.
  • Function Name − This is the function’s official name. The function signature is made up of the function name and the argument list.
  • Parameters − A placeholder is similar to a parameter. The argument is sent a value when a function is called. It is known as an actual parameter or argument to refer to this value. The type, order, and quantity of a function’s parameters are described in the parameter list. The presence of arguments is optional, a function may not have any.
  • Function Body − A series of statements that define function in C is the function’s purpose i.e. found in the function body.

The max function’s source code is provided below (). The largest value between the two arguments, num1 and num2, is returned by this function−

/* function returning the max between two numbers */
int max(int num1, int num2) {

   /* local variable declaration */
   int result;

   if (num1 > num2)
      result = num1;
      result = num2;

   return result; 

Function Declarations in C language

A function declaration in C provides information to the compiler on the name and method of calling a function. The function’s actual body can be specified independently.

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);

Only their type is necessary when defining a function; hence, the following declaration is likewise acceptable:

int max(int, int);

Types of Functions in C language

The types of functions in C language has functions that are of the following types:

  • User-defined Functions – These are the kinds of functions that can be written using the C programming language and used repeatedly. This function makes any large program less complicated, which optimizes the provided code.
  • Library Functions – These are the functions whose declaration occurs in the header files of C, such as floor(), ceil(), outs(), gets(), printf(), scanf(), etc.

Function Arguments

A function must define the variables that happen to accept the values of the parameters whenever it wants to use them. These variables are referred to as the formal parameters of the given function.
The formal parameters of our given function operate just like any other local variables. When they enter a function, these arguments are formed. When it leaves after that, it is destroyed.
There are two methods to deliver these parameters to a particular function during the calling of that function:

Type of Call Description of Call Type
Call By Reference The formal address of the specified argument is copied into the parameter using the Call by Reference technique. Accessing the actual parameter used in this call is made easier inside of this method due to the usage of the address. It implies that any modifications made to the parameter will inevitably have an impact on the given argument.
Call By Value The formal function’s parameter is copied using the Call by Value technique from the real value of the given argument. In this case, the modifications made to the parameter (which is present inside the function) have no impact at all on the available argument.

Call by Value is the default method for sending arguments in C programming. In general, it implies that we are unable to modify the parameters used to call a function using the code that is already present there.

Return Value/Statement of Function

In C programming, no function may ever return the value of another function. We can utilize the void in the form of a return type if we don’t need to return the value that is provided in any function. Let’s examine a C function example that doesn’t execute the return of a value from the accessible function.

Let us look at an example of a C function that has no return value:

void chocolate(){
printf(“chocolate c”);

Any of the data types, including char, long, int, etc., must be used if we wish to utilize one of the available functions to return a result. As a result, the return type only depends on the value that the available function must return. We’ll examine a C function example that uses the supplied function to return an int value.

Example of a C function with the int return value,

int get(){
return 20;

The data type in the example above is int, and the value we are attempting to return is 20. If we want to utilize a floating point value as our return value (for instance, 64.5, 7,9, 27,61, etc.), we must use the float as the method’s return type.

float get(){
return 83.7;

Here, we have to call a function so that we get the function’s value.

Advantages of Using Function in C Programming

The following benefits are provided to us by the functions of the C programming language:

  • Making use of the functions makes it simple to avoid repeatedly writing the same code or logic in any application.
  • Any program can have as many instances of invoking C functions as it wants. And we can do that from anywhere in the provided software.
  • If we break a huge C program up into several functions, we can follow it quite simply.
  • In C language, the primary achievements of the C functions is reusability.
  • However, keep in mind that in a C program, invoking a function always adds overhead.

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