Your First Program

Every C program follows the same basic shape: include the headers you need, define a main function where execution starts, and return a status code when you're done. This lesson takes the classic first program apart line by line.

Hello, world

#include <stdio.h>

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

Output

Hello, world!

Line by line

Code	Meaning
`#include <stdio.h>`	Pulls in the declarations of the standard input/output functions, including `printf`. Lines starting with `#` are handled by the preprocessor before compiling.
`int main(void)`	Defines the function `main`. Every program has exactly one; it's where the program starts. `int` means it returns a whole number; `void` means it takes no arguments.
`{ … }`	Braces enclose the function's body.
`printf("Hello, world!\n");`	Calls `printf` to print text. `\n` is a newline. Each statement ends with a semicolon.
`return 0;`	Ends `main` and reports success to the operating system. Any non-zero value signals failure.

Printing several lines

#include <stdio.h>

int main(void) {
    printf("Line one\n");
    printf("Line two, ");
    printf("still line two\n");
    puts("puts adds the newline for you");
    return 0;
}

Output

Line one
Line two, still line two
puts adds the newline for you

printf prints exactly what you give it — without \n, the next output continues on the same line. puts prints a string followed by a newline.

Escape sequences

Some characters can't be typed directly inside a string, so they're written with a backslash:

Sequence	Meaning
`\n`	Newline
`\t`	Tab
`\"`	Double quote
`\\`	Backslash
`%%`	A percent sign (inside `printf`, where `%` starts a format)

#include <stdio.h>

int main(void) {
    printf("Name:\tAda\n");
    printf("She said \"hello\".\n");
    printf("Path: C:\\temp\\notes.txt\n");
    printf("Progress: 100%%\n");
    return 0;
}

Output

Name:	Ada
She said "hello".
Path: C:\temp\notes.txt
Progress: 100%

Printing numbers

printf fills in values using format specifiers such as %d for whole numbers. Lesson 11 covers them all.

#include <stdio.h>

int main(void) {
    printf("%d + %d = %d\n", 19, 23, 19 + 23);
    return 0;
}

Output

19 + 23 = 42

The exit status

The value returned from main goes back to whoever ran the program — the terminal, a script or a build tool. In a terminal you can see it with echo $? straight after running a program. 0 means success. (If main reaches its closing brace without a return, it returns 0 automatically, but writing it out is clearer.)

A compile error

Forget a semicolon and the compiler stops, pointing at the problem:

#include <stdio.h>

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

Compiler error

main.c:4:21: error: expected ';' after expression

The message gives the file, line and column (main.c:4:21). Read errors from the top: the first one is usually the real problem, and later ones are often knock-on effects. GCC's wording differs slightly from Clang's, but it points to the same place.

Try it yourself

Print a small receipt using tabs to line up the columns.

Output

Item		Price
Coffee		3
Croissant	2
Total		5

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