C - Hello World and Program Structure

Hello World and Program Structure

Code Properties

• Language: C

Overview

Every C program follows a fundamental structure consisting of preprocessor directives, function declarations, and a main() entry point.

This note covers the essential building blocks of a C program.

Code

Minimal Hello World

The simplest valid C program:

#include <stdio.h>
 
int main(void) {
    printf("Hello, World!\n");
    return 0;
}

Program Structure Breakdown

A complete C program with common structural elements:

// preprocessor directives
#include <stdio.h>   // standard input/output
#include <stdlib.h>  // standard library (malloc, exit, etc.)
#include <string.h>  // string manipulation
 
// macro definitions
#define MAX_SIZE 100
#define VERSION "1.0.0"
 
// type definitions
typedef unsigned int uint;
 
// function prototypes (forward declarations)
void greet(const char *name);
int add(int a, int b);
 
// global variables (use sparingly)
static int counter = 0;
 
// main entry point
int main(int argc, char *argv[]) {
    // argc: argument count
    // argv: argument vector (array of strings)
    
    printf("Program: %s\n", argv[0]);
    printf("Version: %s\n", VERSION);
    
    greet("World");
    
    int result = add(5, 3);
    printf("5 + 3 = %d\n", result);
    
    return EXIT_SUCCESS;  // 0 indicates success
}
 
// function definitions
void greet(const char *name) {
    printf("Hello, %s!\n", name);
    counter++;
}
 
int add(int a, int b) {
    return a + b;
}

Command Line Arguments

Handling command line arguments:

#include <stdio.h>
 
int main(int argc, char *argv[]) {
    printf("Number of arguments: %d\n", argc);
    
    for (int i = 0; i < argc; i++) {
        printf("argv[%d]: %s\n", i, argv[i]);
    }
    
    // alternative main signature (equivalent)
    // int main(int argc, char **argv)
    
    return 0;
}

Details

Compilation

Compile a C program using GCC:

# basic compilation
gcc hello.c -o hello
 
# with warnings and debugging symbols
gcc -Wall -Wextra -g hello.c -o hello
 
# optimized release build
gcc -O2 -Wall hello.c -o hello

Using Clang:

clang hello.c -o hello

On Windows with MSVC:

cl hello.c

Return Values

Return Value	Meaning
0 or EXIT_SUCCESS	Program completed successfully
1 or EXIT_FAILURE	Program encountered an error
Other non-zero	Application-specific error codes

Common Preprocessor Directives

Directive	Purpose
#include <file>	Include system header
#include "file"	Include local header
#define NAME value	Define a macro constant
#ifdef / #ifndef	Conditional compilation
#pragma once	Include guard (non-standard but widely supported)

Header Guards

Standard pattern to prevent multiple inclusion:

// myheader.h
#ifndef MYHEADER_H
#define MYHEADER_H
 
// declarations here
 
#endif // MYHEADER_H

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Appendix

Note created on 2025-12-31 and last modified on 2025-12-31.

See Also

• C Code Index
• MOC - Computer Science
• MOC - Development

Backlinks

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