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C Function Definition, Declaration and Syntax

Last Updated on April 28, 2023 by Prepbytes

A function in c programming is a group of statements that perform a specific task. While the main() function is required in every C program, additional functions can be declared in even the simplest programs. Breaking up code into multiple functions is at the discretion of the programmer, but it’s important to do so in a logical manner where each function performs a distinct task. This promotes code modularity, reusability, and maintainability.

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. A function in C programming is a block of code that performs a specific task. Functions may be referred to as methods, subroutines, or processes. They encapsulate a set of instructions that can be reused throughout a program and are typically named based on their intended purpose, such as strcat() for string concatenation or memcpy() for memory copying.

Use of the Function in C Language

Function in C programming language allows us to split our code into several functions and it is our responsibility to do so in a structured manner where each function has a specific task to perform. The function declaration in C provides important information to the compiler about the function’s name, arguments, and return type. This information is necessary for the compiler to locate and use the function in our program. On the other hand, the function definition in C provides the actual implementation of the function’s behaviour, allowing us to define the logic that executes when the function is called in our program

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.

Syntax for Defining a Function in C

Here, is the syntax for defining a function in C programming:

return_type function_name( parameter list ) {
   body of the function

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

  • Return Type −The 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 acts as a substitute for a value, while parameters in functions act as placeholders for values that will be passed as arguments when the function is called. The type, order, and number of parameters are specified in the parameter list, while the actual value passed as an argument is known as the actual parameter or argument. A function may or may not have arguments.
  • 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; 

Declaration of Function in C Programming

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

Declaration of function in c 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 Function in C Programming

The types of function in C programming 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

When a function is called the values of the arguments are passed to the function and stored in variables called formal parameters. These formal parameters must be defined in the function’s code before they can be used to perform the desired task. 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
Function in C programming 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 us see a function in C example that doesn’t execute the return of a value from the available function.
Here, is an example of a function in c 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 function in c 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

Here, are the advantages of the using function in c programming:

  • 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.
  • Dividing a large C program into multiple functions improves readability and allows for easier comprehension and modification of the code.
  • In C language, the primary achievement of the C functions is reusability.
  • It is important to note that calling a function in a C program incurs additional overhead, which can impact the program’s performance

In conclusion, the function in c programming is a self-contained block of code that performs a specific task. Function in C can be declared with a return type, a name, and a list of parameters that are passed to the function. Functions in c can be called from other parts of the program, allowing for modular and reusable code. Function in C programming is an essential part they help to organize code, make it easier to read, maintain and reduce the amount of duplicated code. They can also improve the efficiency of a program by allowing common tasks to be performed in a separate function rather than being repeated in multiple places throughout the program.

Frequently Asked Questions(FAQs):

Q1. What is a prototype of function in C programming?
Ans: A prototype function in c is a declaration of the function that specifies its return type, name, and parameters. It is typically placed at the beginning of a source file or header file to inform the compiler about the function’s signature before it is used in the program.

Q2. What is a pointer function in C programming?
Ans: A pointer function in C is a variable that holds the memory address of a function. It allows a program to pass functions as arguments to other functions, and to assign functions to variables. Function pointers are commonly used in C to implement callback functions, where a function is called in response to an event.

Q3. What is a void function in C programming?
Ans: A void function in C is a function that does not return a value. It is typically used to perform a task or operation that does not require a return value, such as printing a message to the screen or modifying a variable passed as a parameter.

Q4. What is the difference between pass-by-value and pass-by-reference in C programming?
Ans: Pass by value means that a copy of the value of a variable is passed to a function as a parameter, and any changes made to the parameter inside the function do not affect the original variable. Pass by reference means that the memory address of a variable is passed to a function as a parameter, allowing the function to modify the original variable.

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