C Syntax Basics

Every language has grammar. In English, sentences end with a period and names start with a capital letter. C syntax is the grammar of C: the rules that decide whether the compiler understands your code or rejects it. The good news is that C's rules are few and consistent. In this lesson, you'll learn about statements, semicolons, blocks, whitespace, case sensitivity, keywords and the rules for naming things.

What is C syntax?

C syntax is the set of rules for how C code must be written: where semicolons go, how braces pair up, which words are reserved and how names are formed. Code that breaks these rules causes a syntax error, and the compiler refuses to build the program until you fix it.

Here's a small program that uses every rule in this lesson:

#include <stdio.h>

int main(void) {
    printf("Thermostat on\n");
    printf("Target: 21 degrees\n");
    return 0;
}

Thermostat on
Target: 21 degrees

Statements and semicolons

A statement is one complete instruction, like "print this" or "return 0". In C, every statement ends with a semicolon ;. The semicolon is how the compiler knows where one instruction stops and the next begins; it doesn't look at line breaks at all.

Not every line needs a semicolon. Lines that start with # (like #include) are instructions for the preprocessor, not statements, and lines that open or close a block with { or } don't take one either.

Blocks and curly braces

A block is a group of statements wrapped in curly braces { }. The body of main is a block. Later, you'll use blocks to group the statements inside if conditions and loops. A block can even contain another block:

#include <stdio.h>

int main(void) {
    printf("Outside the inner block\n");
    {
        printf("Inside the inner block\n");
    }
    printf("Outside again\n");
    return 0;
}

Outside the inner block
Inside the inner block
Outside again

The inner block doesn't change what is printed here, but blocks matter a lot once you learn about if-else and variable scope. The key rule: every { must have a matching }.

Does whitespace matter in C?

Mostly, no. Whitespace means spaces, tabs and line breaks, and the compiler ignores the extra ones. Indentation in C is purely for humans (unlike Python, where it's part of the grammar). This program is ugly, but it's valid C:

#include <stdio.h>
int main(void){printf("Cramped but valid\n");return 0;}

Cramped but valid

And so is this very spread-out version:

#include <stdio.h>

int    main  (  void  )
{
    printf(  "Spacious but valid\n"  )  ;

    return    0 ;
}

Spacious but valid

There are three exceptions. Spaces inside quotes are printed exactly as typed. You can't put a space inside a name or keyword (ret urn is not return). And each #include must be on its own line.

Is C case-sensitive?

Yes. C treats uppercase and lowercase letters as completely different. printf works, but Printf and PRINTF are unknown names. All of C's keywords are lowercase, and so is main.

The same applies to your own names. In the next program, score and Score are two separate variables. (You'll learn about variables properly in C Variables; for now, int score = 90; stores the whole number 90 under the name score, and %d prints it.)

#include <stdio.h>

int main(void) {
    int score = 90;
    int Score = 75;
    printf("score = %d\n", score);
    printf("Score = %d\n", Score);
    return 0;
}

score = 90
Score = 75

It works, but don't do this in real code: names that differ only by capital letters are very easy to mix up.

What are keywords in C?

Keywords are words that have a fixed meaning in C, so you can't use them as names. C17 has 44 keywords (C23 adds a few more, such as true, false and bool). Here are the ones you'll use most, grouped by job:

You'll meet each of these in later chapters. You don't need to memorize the list; your editor colors keywords so you can spot them.

Rules for identifiers (names) in C

An identifier is a name you choose for something, such as a variable or a function. C identifiers must follow these rules:

• Use only letters (a–z, A–Z), digits (0–9) and underscores (_).
• Don't start with a digit.
• No spaces, hyphens or other symbols.
• Don't use a keyword such as int or return.
• Avoid names that start with an underscore followed by a capital letter (like _Total) or with two underscores; C reserves those for itself.

Most C programmers write names in snake_case: lowercase words joined by underscores, like seats_left.

The structure of a C program

Put everything together and almost every C program you write in this tutorial has the same skeleton:

#include <stdio.h>      /* 1. preprocessor lines */

int main(void) {        /* 2. main function starts the block */
    /* 3. your statements go here, each ending in ; */
    return 0;           /* 4. report success */
}                       /* 5. close the block */

The text between /* and */ is a comment, ignored by the compiler; you'll learn more in the next lesson. Start every new program from this skeleton and fill in step 3.

Common C syntax mistakes

1. A missing semicolon

printf("Ready\n")
printf("Go\n");

GCC says something like error: expected ';' before 'printf'. The error points at the next line, because that's where the compiler realized the statement never ended.

2. A missing closing brace

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

GCC reports error: expected declaration or statement at end of input: it reached the end of the file still waiting for }. Consistent indentation makes this easy to see.

3. A semicolon after main(void)

int main(void); {
    return 0;
}

Semicolons end statements, not the line that opens a block. This gives an error such as expected identifier or '(' before '{' token. Remove the semicolon before {.

4. Capital letters in keywords

Return 0; or Int main(void) fail, because C keywords are all lowercase.

Practice exercise

#include <stdio.h>

Int main(void) {
    Printf("Parking lot B\n");
    printf("12 spaces free\n")
    return 0;

#include <stdio.h>

int main(void) {
    printf("Parking lot B\n");
    printf("12 spaces free\n");
    return 0;
}

Parking lot B
12 spaces free

What's next?

You know the grammar of C. Next, learn how to leave notes for yourself and other programmers in Comments in C.