C Programming Language Tutorial

Introduction to C Programming

C is a general-purpose, procedural programming language developed by Dennis Ritchie at Bell Labs in the early 1970s. It is one of the most widely used programming languages of all time and forms the basis for many other languages like C++, Java, and C#.

Creator of C · Co-creator of Unix

Dennis Ritchie — Creator of C

1941 – 2011 Bell Labs Turing Award, 1983

Dennis MacAlistair Ritchie was an American computer scientist who shaped modern computing. While working at Bell Telephone Laboratories, he designed and implemented the C programming language (around 1972–1973) and, with Ken Thompson, built the Unix operating system. C gave programmers close control over memory and hardware while staying more portable than assembly language—ideas that still underpin operating systems, embedded software, and language design today.

Ritchie and Thompson shared the Turing Award in 1983 and the U.S. National Medal of Technology in 1998. With Brian Kernighan, he co-authored The C Programming Language (1978), often called “K&R C,” which taught generations of programmers. C directly influenced C++, Objective-C, and countless other languages; Unix and its descendants power servers, phones, and the internet worldwide.

• Created C to rewrite Unix in a portable, efficient language
• Introduced types, structs, and a clear compilation model still used today
• Helped standardize C (ANSI C / C89) for consistent compilers everywhere

#include <stdio.h>

int main() {
    printf("Hello, World!\n");
    return 0;
}

Basic Syntax and Structure

C programs are composed of various tokens which can be keywords, identifiers, constants, string literals, and symbols. A typical program is organized into clear sections—from preprocessor directives and global declarations through main() and its statements.

Variables and Data Types

Variables are used to store data in memory. Each variable has a specific type, which determines the size and layout of the variable's memory.

Rules to Declare a Variable

Before you use a variable in C, you must declare it with a data type and a valid name. These rules apply to local variables inside functions and global variables outside functions.

1. Declaration syntax

2. Naming rules

3. Scope and placement rules

• Local variables — declared inside a function (usually at the start of a block); visible only in that block.
• Global variables — declared outside all functions; visible to the whole file (use sparingly).
• Declare before use — in older C (C89), all declarations must appear before executable statements in a block. In C99 and later, you may declare variables anywhere in the block.
• No redeclaration — you cannot declare the same name twice in the same scope (int x; int x; is an error).
• Reassignment is allowed — after declaration, change the value with = (e.g. age = 30;).

#include <stdio.h>

int main() {
    // Variable declaration and initialization
    int age = 25;
    float salary = 45000.50;
    double pi = 3.1415926535;
    char grade = 'A';
    _Bool isEmployed = 1;
    
    // Display values
    printf("Age: %d\n", age);
    printf("Salary: %.2f\n", salary);
    printf("Grade: %c\n", grade);
    printf("Is Employed: %d\n", isEmployed);
    
    return 0;
}

C Data Types Reference