C Programming Language: Complete Guide with Commands, Examples, and Applications

Introduction to C Language

C is one of the most powerful and widely used programming languages in computer science. It is known as the “mother of all programming languages” because many modern languages such as C++, Java, Python, and JavaScript are either directly or indirectly influenced by C. Developed in the early 1970s, C remains popular due to its efficiency, portability, and flexibility.

History and Evolution of C

C was developed in 1972 by Dennis Ritchie at Bell Laboratories. It was created as an evolution of the B programming language to support system software development, especially for the UNIX operating system.
Key milestones:

• 1960s: Assembly language and machine code dominated programming.
• 1969: Ken Thompson developed B programming language.
• 1972: Dennis Ritchie developed C.
• 1989: ANSI C standardization (ANSI X3.159-1989).
• 1999: C99 standard introduced new features.
• 2011 and beyond: C11 and C18 standards refined the language further.

Features of C

C has several features that make it a foundational programming language:

• Simple and efficient.
• Mid-level language (supports both low-level memory management and high-level constructs).
• Structured programming approach.
• Rich library functions.
• Portable across different hardware platforms.
• Supports dynamic memory allocation.
• Powerful pointer manipulation.

Structure of a C Program

A C program follows a specific structure.

#include <stdio.h>  // Preprocessor directive

int main() {        // Main function
    printf("Hello, World!"); // Output statement
    return 0;       // Exit status
}

Basic structure includes:

1. Preprocessor directives (#include).
2. Main function (int main()).
3. Declarations and statements.
4. Return statement.

Basic Syntax and Commands

Some basic commands and syntax in C:

• Printing output:

printf("Hello, C Language\n");

• Reading input:

int x;
scanf("%d", &x);

• Comments:

// Single-line comment
/* Multi-line 
   comment */

• Compile and run (command line):

gcc program.c -o program
./program

Data Types & Variables

C provides multiple data types to store different values.

• Basic data types:
  • int → integers
  • float → decimal numbers
  • double → large decimal numbers
  • char → characters

Example:

int age = 20;
float price = 99.5;
char grade = 'A';

• Variables: Named storage locations used in programs.

int num = 100;

Operators in C

C has various operators:

• Arithmetic operators: + - * / %
• Relational operators: == != < > <= >=
• Logical operators: && || !
• Assignment operators: = += -= *= /=
• Increment/Decrement: ++ --

Example:

int a = 5, b = 3;
printf("%d", a + b);  // Output: 8

Control Statements (if, switch, loops)

Control statements manage the flow of execution.

• if-else statement:

if (x > 0) {
    printf("Positive");
} else {
    printf("Negative or Zero");
}

• switch statement:

switch (day) {
    case 1: printf("Monday"); break;
    case 2: printf("Tuesday"); break;
    default: printf("Invalid");
}

• Loops:

// for loop
for(int i=1; i<=5; i++) {
    printf("%d ", i);
}

// while loop
int j=1;
while(j<=5) {
    printf("%d ", j);
    j++;
}

// do-while loop
int k=1;
do {
    printf("%d ", k);
    k++;
} while(k<=5);

Functions in C

Functions allow code reusability.

#include <stdio.h>

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

int main() {
    int result = add(5, 3);
    printf("Sum = %d", result);
    return 0;
}

Arrays, Strings, and Pointers

• Arrays: Collection of similar data types.

int arr[5] = {1, 2, 3, 4, 5};

• Strings: Character arrays ending with \0.

char name[] = "C Language";
printf("%s", name);

• Pointers: Variables storing memory addresses.

int x = 10;
int *ptr = &x;
printf("%d", *ptr); // Prints 10

Structures & Unions

• Structure:

struct Student {
    int id;
    char name[20];
};

struct Student s1 = {1, "John"};

• Union: Shares memory for all members.

union Data {
    int i;
    float f;
    char c;
};

File Handling in C

C allows file operations such as read, write, and append.

FILE *fptr;
fptr = fopen("data.txt", "w");
fprintf(fptr, "Hello File Handling");
fclose(fptr);

Memory Management (malloc, free, etc.)

Dynamic memory is managed with functions from <stdlib.h>.

int *ptr;
ptr = (int*) malloc(5 * sizeof(int));
if(ptr != NULL) {
    for(int i=0; i<5; i++) {
        ptr[i] = i+1;
    }
}
free(ptr);

Advantages & Limitations

• Advantages:
  • Fast execution.
  • Portable across systems.
  • Supports low-level programming.
  • Large community support.
• Limitations:
  • No built-in object-oriented features.
  • Manual memory management required.
  • Limited error handling compared to modern languages.

Real-Life Applications

C is widely used in:

• Operating systems (UNIX, Linux).
• Embedded systems.
• Game development.
• Compilers and interpreters.
• Database systems.
• System drivers and hardware control.

Sample Programs (with commands & output)

• Factorial Program:

#include <stdio.h>

int main() {
    int n, fact = 1;
    scanf("%d", &n);
    for(int i=1; i<=n; i++) {
        fact *= i;
    }
    printf("Factorial = %d", fact);
    return 0;
}

• Reverse a string:

#include <stdio.h>
#include <string.h>

int main() {
    char str[50];
    scanf("%s", str);
    for(int i=strlen(str)-1; i>=0; i--) {
        printf("%c", str[i]);
    }
    return 0;
}

Conclusion

C language remains a cornerstone of computer programming. It is not only fast and efficient but also lays the foundation for learning advanced languages like C++, Java, and Python. With its structured approach, direct hardware manipulation, and portability, C continues to be a relevant and powerful language in modern computing.