In C programming, functions are blocks of code that perform a specific task. They are a fundamental building block for structuring programs. Functions in C can be classified into two main categories:

1. Built-in Functions: These are functions that are predefined in C libraries. You don’t need to write them; instead, you can directly use them by including the corresponding header files.
2. User-defined Functions: These are functions that you, as a programmer, define to perform specific tasks based on your program’s needs.

1. Built-in Functions

These functions are provided by the C standard library, and they simplify common tasks like input/output, mathematical operations, string manipulations, memory management, etc. Here are some examples:

• Input/Output Functions:
  • printf(): Prints formatted output to the screen.
  • scanf(): Takes input from the user.
• Mathematical Functions:
  • sqrt(): Returns the square root of a number.
  • pow(): Returns the value of a number raised to a power.
  • abs(): Returns the absolute value of a number.
• String Functions:
  • strlen(): Returns the length of a string.
  • strcpy(): Copies one string to another.
  • strcmp(): Compares two strings.
• Memory Management Functions:
  • malloc(): Allocates memory dynamically.
  • free(): Frees the dynamically allocated memory.
  • calloc(): Allocates memory for an array of objects and initializes them to zero.
• Other Utility Functions:
  • exit(): Terminates the program.
  • time(): Returns the current system time

To use these functions, you generally need to include specific header files like:

#include <stdio.h> // For input/output functions
#include <stdlib.h> // For memory management functions and exit
#include <math.h> // For mathematical functions
#include <string.h> // For string functions
#include <time.h> // For time functions

2. User-defined Functions

In C, you can create your own functions to perform custom tasks. A user-defined function typically consists of:

• Function Declaration (Prototype): Specifies the function’s name, return type, and parameters.
• Function Definition: Contains the actual code to perform the task.
• Function Call: Used to invoke the function.

Syntax of a Function in C

1. Function Declaration (Prototype):

return_type function_name(parameters);

Example:

int add(int, int); // Function prototype

Function Definition: The function’s implementation.

return_type function_name(parameters)
{
// Function body
}

Example:

int add(int a, int b) // Function definition
{
return a + b;
}

 Function Call: Calling the function from the main() function or any other function.

int result = add(5, 3); // Function call

          Example of a User-Defined Function

#include <stdio.h>

// Function declaration (prototype)
int add(int, int);

int main() {
int a = 5, b = 3;
int sum = add(a, b); // Function call
printf(“Sum: %d\n”, sum);
return 0;
}

// Function definition
int add(int a, int b) {
return a + b;
}

Key Points About Functions in C:

• Return Type: Specifies what type of value the function will return (e.g., int, float, char, void).
• Function Name: The name you give to the function, which you use to call it.
• Parameters (Arguments): These are the values that you pass to the function. Functions can have zero or more parameters.
• Return Statement: If the function has a return type other than void, you must use a return statement to return a value.

Types of User-Defined Functions

Functions with No Arguments and No Return Value: These functions don’t accept parameters and don’t return any value.

void greet() {
printf(“Hello, World!\n”);
}

Functions with Arguments but No Return Value: These functions take parameters but don’t return anything.

void printSum(int a, int b) {
printf(“Sum: %d\n”, a + b);
}

Functions with Arguments and a Return Value: These functions take parameters and return a value.

int multiply(int a, int b) {
return a * b;
}

Functions with No Arguments but a Return Value: These functions don’t take parameters but return a value.

int getRandomNumber() {
return rand() % 100;
}

Conclusion

In C, functions help to modularize the program, make the code more readable, reusable, and maintainable. Built-in functions are predefined in the C standard library, while user-defined functions are created by the programmer to perform specific tasks.