Your First C Program: Hello World Explained Line by Line

Programmers traditionally start a new language by printing "Hello, World!". Let us write it and then understand every line.

The program#

#include <stdio.h>

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

Hello, World!

Compile and run it as shown in Installing a C Compiler:

gcc -std=c17 -Wall -Wextra hello.c -o hello
./hello

Line by line#

Escape sequences#

\n inside a string is a new line. Without it, the next output continues on the same line. A backslash starts an escape sequence:

printf("Line one\nLine two\n");
printf("Name:\tAsha\n");
printf("She said \"hi\"\n");
printf("Path: C:\\temp\n");

Line one
Line two
Name:	Asha
She said "hi"
Path: C:\temp

Several statements#

Statements run one after another, from top to bottom:

printf("Step 1: boil water\n");
printf("Step 2: add tea leaves\n");
printf("Step 3: add milk and sugar\n");

Step 1: boil water
Step 2: add tea leaves
Step 3: add milk and sugar

Printing numbers#

printf uses format specifiers such as %d to insert values into the text:

printf("%d apples cost %d rupees\n", 3, 3 * 30);
printf("Total items: %d\n", 3 + 4);

3 apples cost 90 rupees
Total items: 7

%d prints an int. You will learn the other specifiers in Input and Output.

Your first errors#

Mistakes are normal. The compiler reports them with the file name, line and column.

Forgetting a semicolon:

#include <stdio.h>

int main(void) {
    printf("Hello")
    return 0;
}

GCC reports something like error: expected ';' before 'return', pointing at the line after the missing semicolon.

Forgetting #include <stdio.h> gives a warning or error about an implicit declaration of printf. Read the first message carefully; later messages are often caused by the first one.

Practice#

Item	Qty	Total
Pen	12	48

#include <stdio.h>

int main(void) {
    printf("Item\tQty\tTotal\nPen\t%d\t%d\n", 12, 12 * 4);
    return 0;
}

Item	Qty	Total
Pen	12	48

Key takeaway#