🟢 Beginner · Lesson 05

Structure of a C Program

C Program की संरचना

Parts of a C Program

Every well-written C program follows a standard structure. Understanding this structure helps you write organized, readable, and professional code.

Standard C Program Structure

/* =====================================================
   1. DOCUMENTATION SECTION (Program description)
   ===================================================== */

// 2. PREPROCESSOR / HEADER FILE SECTION
#include <stdio.h>
#include <math.h>
#define PI 3.14159

// 3. GLOBAL DECLARATION SECTION
int counter = 0;       // Global variable

// 4. FUNCTION PROTOTYPE SECTION
void printMenu();
int  add(int, int);

// 5. MAIN FUNCTION
int main() {
    // Local variables
    int a = 5, b = 3;
    printMenu();
    printf("Sum = %d\n", add(a, b));
    return 0;
}

// 6. USER-DEFINED FUNCTIONS
void printMenu() {
    printf("=== Calculator ===\n");
}
int add(int x, int y) {
    return x + y;
}

Preprocessor Section

Lines starting with # are processed by the preprocessor — before the compiler sees the code. This section includes:

Header files — #include <stdio.h>, #include <math.h>, #include <string.h>
Macro definitions — #define PI 3.14159
Conditional compilation — #ifdef, #endif

Header File	Functions it provides
`<stdio.h>`	printf, scanf, getchar, putchar, fgets, puts
`<math.h>`	sqrt, pow, abs, sin, cos, log, ceil, floor
`<string.h>`	strlen, strcpy, strcat, strcmp
`<stdlib.h>`	malloc, free, rand, srand, atoi, exit
`<ctype.h>`	isdigit, isalpha, isupper, islower, toupper, tolower
`<time.h>`	time, clock, difftime

Global Declaration Section

Variables declared outside all functions are global variables. They are accessible from any function in the file and exist for the lifetime of the program.

C Language – Global vs Local

#include <stdio.h>

int globalCount = 0;  // Global: accessible everywhere

void increment() {
    globalCount++;     // Can access global variable
    int local = 10;   // Local: only inside increment()
}

int main() {
    increment();
    increment();
    printf("Count: %d\n", globalCount);  // 2
    // printf("%d", local); // ERROR: local not visible here
    return 0;
}

⚠️ Use Global Variables Sparingly

Overuse of global variables makes code hard to debug and maintain (any function can change them). Prefer local variables and pass values through function parameters.

main() Function

The main() function is the entry point of every C program. The operating system calls main() when you run the program. Key points:

Every C program must have exactly one main() function
Return type is always int (in modern C)
return 0 indicates successful execution; non-zero = error
Can accept command-line arguments: int main(int argc, char *argv[])

User-Defined Functions

Functions can be defined before or after main(). If after, use a prototype before main(). Good practice: one function per logical task.

Comments

C Language – Comments

// Single-line comment (C99 and later)

/* Multi-line comment
   Can span multiple lines
   Used for documentation */

/**
 * Function: calculateArea
 * Parameters: radius (float)
 * Returns: area of circle (float)
 */
float calculateArea(float r) { return 3.14f * r * r; }

Complete Well-Structured Program Example

C Language – Student Grade Calculator

/*
 * Program: Student Grade Calculator
 * Author:  CodeKaFunda
 * Date:    2025
 * Purpose: Calculate grade from marks input
 */

#include <stdio.h>    // printf, scanf
#include <string.h>  // strcmp

#define MAX_MARKS  100
#define MIN_PASS   40

/* Function prototypes */
char* getGrade(int marks);
void  printResult(char *name, int marks);

/* Main function */
int main() {
    char name[50];
    int  marks;
    printf("Enter student name: ");
    scanf("%s", name);
    printf("Enter marks (0-%d): ", MAX_MARKS);
    scanf("%d", &marks);
    printResult(name, marks);
    return 0;
}

char* getGrade(int marks) {
    if (marks >= 90) return "A+";
    if (marks >= 80) return "A";
    if (marks >= 70) return "B";
    if (marks >= 60) return "C";
    if (marks >= MIN_PASS) return "D";
    return "F";
}

void printResult(char *name, int marks) {
    printf("\n=== Result Card ===\n");
    printf("Student: %s\n", name);
    printf("Marks:   %d/%d\n", marks, MAX_MARKS);
    printf("Grade:   %s\n", getGrade(marks));
    printf("Result:  %s\n", marks >= MIN_PASS ? "PASS" : "FAIL");
    printf("===================\n");
}

Enter student name: Rahul Enter marks (0-100): 87 === Result Card === Student: Rahul Marks: 87/100 Grade: A Result: PASS ===================

Summary

A C program has 6 parts: Documentation → Preprocessor → Global declarations → Prototypes → main() → User-defined functions
#include brings in library function definitions
Global variables are accessible everywhere; prefer local variables
Every program has exactly one main() — execution starts there
return 0 = success; non-zero = error
Comments (// and /* */) improve readability — use them!

C Program के Parts

हर well-written C program एक standard structure follow करता है। इस structure को समझने से organized, readable और professional code लिख सकते हैं।

Standard C Program Structure

/* 1. DOCUMENTATION — Program description */

// 2. PREPROCESSOR / HEADER FILES
#include <stdio.h>
#define PI 3.14159

// 3. GLOBAL DECLARATIONS (सभी functions के लिए accessible)
int counter = 0;

// 4. FUNCTION PROTOTYPES
void printMenu();

// 5. MAIN FUNCTION (program यहाँ से शुरू होता है)
int main() {
    printMenu();
    return 0;
}

// 6. USER-DEFINED FUNCTIONS
void printMenu() { printf("Menu\n"); }

Preprocessor Section

# से शुरू होने वाली lines preprocessor process करता है — compiler से पहले।

Header File	Functions
`<stdio.h>`	printf, scanf, fgets, puts
`<math.h>`	sqrt, pow, sin, cos, floor
`<string.h>`	strlen, strcpy, strcat, strcmp
`<stdlib.h>`	malloc, free, rand, exit, atoi
`<ctype.h>`	isdigit, isalpha, toupper, tolower

main() Function

main() function हर C program का entry point है। OS इसे तभी call करता है जब आप program run करते हैं।

हर C program में exactly एक main() होनी चाहिए
Return type हमेशा int होता है
return 0 = successful; non-zero = error

Comments

C Language

// Single-line comment
/* Multi-line comment
   Multiple lines ho sakta hai */

💡 Comments ज़रूरी क्यों?

Comments code को readable बनाते हैं। 6 महीने बाद आप खुद का code भूल जाते हैं! अच्छे comments बताते हैं कि function क्या करता है, parameters क्या हैं, और edge cases कौन से हैं।

Complete Example Program

C Language – Student Grade Calculator

/* Program: Student Grade Calculator | CodeKaFunda */
#include <stdio.h>
#define MAX 100
#define PASS 40

char* grade(int m);
void  result(char *n, int m);

int main() {
    char name[50]; int marks;
    printf("Naam daalen: "); scanf("%s", name);
    printf("Marks (0-%d): ", MAX); scanf("%d", &marks);
    result(name, marks);
    return 0;
}

char* grade(int m) {
    if(m>=90) return "A+"; if(m>=80) return "A";
    if(m>=70) return "B";  if(m>=PASS) return "C";
    return "F";
}

void result(char *n, int m) {
    printf("\n=== Result ===\nNaam: %s\nMarks: %d/%d\nGrade: %s\nResult: %s\n",
           n, m, MAX, grade(m), m>=PASS ? "PASS":"FAIL");
}

Naam daalen: Priya Marks (0-100): 87 === Result === Naam: Priya Marks: 87/100 Grade: A Result: PASS

सारांश

C program के 6 parts: Documentation → Preprocessor → Global → Prototypes → main() → User-defined functions
#include library function definitions लाता है
Global variables सब जगह accessible; prefer local variables
हर program में exactly एक main() — execution यहाँ से शुरू
return 0 = success; non-zero = error
Comments (// और /* */) readability improve करते हैं — ज़रूर use करें!

← Back to C Tutorial