C First Program

Your first C program is like saying hello for the first time. You write a main function that shows a message on the screen and then finishes.

In this page:

The Main Function
Printing Output
Returning Zero
Header Files
Executing the Program

Syntax

#include <stdio.h>

int main() {
  printf("text");
  return 0;
}

The Main Function

Execution of every C program begins inside the function named main -- the operating system looks specifically for this function and calls it first, regardless of how many other functions your file defines.

Example: The Main Function

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("Execution starts here, inside main.");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Printing Output

printf writes formatted text to the screen and is how a C program produces any visible output; without it (or a similar output call), your program could run perfectly and you'd never know, since nothing would appear on screen.

Example: Printing Output

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("Hello, World!");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Returning Zero

Returning 0 from main is a signal to the operating system (and any script or tool that launched your program) that execution completed without errors -- returning a non-zero value is the conventional way to report failure.

Example: Returning Zero

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("Program completed successfully.");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Header Files

#include <stdio.h> pulls in the declarations for standard input/output functions like printf and scanf from the C standard library; without it, the compiler wouldn't know these functions exist and would refuse to compile your call to them.

Example: Header Files

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Print formatted text to the screen
	printf("stdio.h provides printf and scanf declarations.");
	// Return 0 to signal that the program finished successfully
	return 0;
}

Executing the Program

Compiling and actually running your program -- not just getting it to compile cleanly -- is the only way to confirm it behaves the way you expect; a program with zero compiler errors can still produce completely wrong output.

Example: Executing the Program

// Include standard input/output (printf, scanf)
#include <stdio.h>
// Program execution starts in main()
int main() {
	// Declare result and set it to 5 / 2
	int result = 5 / 2;
	// Print formatted text to the screen
	printf("Result: %d", result);
	// Return 0 to signal that the program finished successfully
	return 0;
}

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