Your First C Program, Line by Line

Alice types five lines, presses Run, and the screen says Hello, C. She is pleased for about four seconds. Then she deletes the line return 0;, presses Run again, and it still says Hello, C.

So what was that line for? A program that runs is not the same thing as a program you understand. This lesson takes the five lines apart, one word at a time, and puts Alice's line back with a reason.

C has 32 keywords. Your first program uses two of them, so the other 30 can wait.

The five lines, once

Here is the whole program. Do not try to understand it yet. Let its shape settle in your eyes first.

#include <stdio.h>

int main(void)
{
    printf("Hello, C.\n");
    return 0;
}

Hello, C.

Four things carry meaning here: a line starting with #, a function header, a pair of braces, and two statements inside them. Every C program you write for the rest of your life is a longer version of this shape.

Line 1: the include directive

The # at the start is a signal. It says: this line is not for the compiler. It is for the preprocessor, the tool that runs first and tidies up your file (Module 0, lesson 4).

include is its instruction, and it is blunt: copy the whole of that file in here, right now, before compiling starts.

stdio.h is short for standard input output header. It is a plain text file that came with your compiler. Inside it are the declarations of printf, scanf and about forty more functions.

A declaration is a promise, not the code. Think of a menu: it lists which dishes exist and what each one needs. The kitchen, the real machine code of printf, lives in the C standard library and joins at the linker.

The angle brackets < > send the preprocessor to the system's own folders. Double quotes, as in #include "myfile.h", send it to your project folder first.

So a directive is not a statement, and it takes no semicolon. Put one there and you have pasted a stray ; into your program.

Line 3: the function header

This is where your program starts running. When the operating system launches the program, it calls main. Not the first line in the file. Not the first function you wrote. main, always.

Three pieces, left to right:

• int is the return type. It promises that when main finishes, it hands one whole number back to the operating system.
• main is the name, and the C standard fixes it. Main, MAIN and main2 are three different names, and none of them is the one the system calls.
• (void) is the parameter list. Parameters are values a function accepts, and void here means "this one accepts nothing".

You will also meet int main(int argc, char *argv[]), which is how a program reads the words typed after its name. That is Module 15. Until then, (void) says what you mean.

So int main(void) is a promise and a name, and the machine keeps its half by calling it first.

Lines 4 and 7: the braces

Curly braces mark the body of the function: where it begins and where it ends. Everything between them is what main does.

Braces come in pairs, always. A brace with no partner is the most common beginner mistake in C. The message you get for it is strange, because the compiler only notices at the end of the file.

So if an error names the last line of a file you did not touch, count your braces.

Line 5: the statement, argument by argument

This line is a function call. You are asking printf to do its job, and handing it one thing to work with.

• A value you hand to a function is an argument. Here the argument is the text "Hello, C.\n".
• Double quotes make a string literal, which is C's word for a fixed piece of text.
• \n is an escape sequence. It means newline: move to the start of the next line.
• The semicolon ends the statement. In C a statement ends where you say it ends, not where the line ends.

Look at \n again, because it surprises everyone. You typed two characters, a backslash and an n. The output holds one character, the invisible one that ends a line. The backslash is not printed; it is an instruction to the compiler about the character that follows it.

So printf("A\nB\n") prints two lines from one call, and the string is 4 characters long, not 6.

Line 6: return 0, and what Alice deleted

main promised an int, so it returns one. By convention 0 means "finished, no problems", and any other number means something went wrong.

That number is the program's exit status, and it is not decoration. The operating system keeps it. On Linux or macOS, run a program and then type echo $? to see the number it returned.

$ ./hello
Hello, C.
$ echo $?
0

This is how a script decides what to do next. Every "build failed, stopping here" message in a CI pipeline is a program that returned something other than zero.

Now Alice's experiment. Since the C99 standard, if control reaches the closing brace of main, the effect is the same as return 0;. Her program was correct because the standard filled the line in for her.

Write it anyway. It is one line, it says what you mean, and main is the only function in C that gets this favour. Leave it out of a function that returns a value and you have a real bug.

So a missing return 0; in main changes nothing, and a missing return anywhere else changes everything.

#include <header>        directive: for the preprocessor, no semicolon

int main(void)            return type, name, parameter list
{                         body starts
    statement;            one instruction, ended by a semicolon
    return 0;             the exit status handed to the system
}                         body ends

#include <stdio.h>

int main(void)
{
    printf("Hello, C.\n");
    return 0;
}

Hello, C.

#include <stdio.h>

int main(void)
{
    printf("Welcome to Progsity\n");
    printf("Track: C, from zero\n");
    printf("Part Zero: before you write a single line\n");
    return 0;
}

Welcome to Progsity
Track: C, from zero
Part Zero: before you write a single line

#include <stdio.h>

int main(void)
{
    printf("+----------------+\n");
    printf("|   PROGSITY C   |\n");
    printf("|   Part Zero    |\n");
    printf("+----------------+\n");
    return 0;
}

+----------------+
|   PROGSITY C   |
|   Part Zero    |
+----------------+

int main(void)
{
    printf("Hello, C.\n");
    return 0;
}

#include <stdio.h>

int main(void)
{
    Printf("Hello, C.\n");
    return 0;
}

#include <stdio.h>;

int main(void)
{
    printf("Hello, C.\n");
    return 0;
}

printf("Hello, C.\n")
return 0;

#include <stdio.h>

int main(void)
{
    printf("Chittagong");
    printf("Sylhet\n");
    return 0;
}

=== PROGSITY ===
Stall n

#include <stdio.h>

int main(void)
{
    int n;
    scanf("%d", &n);

    /* Your two printf calls go here. */

    return 0;
}

Maths: a
Physics: b
Chemistry: c

#include <stdio.h>

int main(void)
{
    int a, b, c;
    scanf("%d %d %d", &a, &b, &c);

    /* Three printf calls, one per subject. */

    return 0;
}

Bob wrote:	"Hello, C."
Score:	s

#include <stdio.h>

int main(void)
{
    int s;
    scanf("%d", &s);

    /* Two printf calls. You need \t and \" here. */

    return 0;
}

Next you will meet the two kinds of wrong. One the compiler refuses, and one it builds happily while giving you the wrong answer.