Keywords in C

01What are Keywords?

📝 Definition

Keywords (also called reserved words) are predefined words in C that have special meanings. They are part of C's syntax and cannot be used as identifiers (variable names, function names, etc.).

❌ Invalid

main.c

int int = 5;     // ERROR!
float return;    // ERROR!
char if = 'a';   // ERROR!

✅ Valid

main.c

int number = 5;     // OK
float result;       // OK
char initial = 'a'; // OK

💡 Key Points

• C has 32 keywords (C89/C90 standard)
• All keywords are in lowercase
• Keywords are case-sensitive (if ≠ IF)
• Cannot be used as variable/function names

02All 32 Keywords in C

📋 Complete List of C Keywords (32 Total)

auto

break

case

char

const

continue

default

double

else

enum

extern

float

for

goto

int

long

return

short

signed

sizeof

static

struct

switch

typedef

union

unsigned

void

volatile

while

📝 Note: C99 & C11 Added More

C99 added: inline, restrict, _Bool, _Complex, _Imaginary
C11 added: _Alignas, _Alignof, _Atomic, _Generic, etc.

03Data Type Keywords

These keywords define the type of data a variable can hold.

Keyword	Description	Example
int	Integer (whole numbers)	int age = 25;
float	Single-precision decimal	float pi = 3.14f;
double	Double-precision decimal	double pi = 3.14159;
char	Single character	char grade = 'A';
void	No type / empty	void printHello();
short	Short integer	short num = 100;
long	Long integer	long big = 100000L;
signed	Can be negative (default)	signed int x = -5;
unsigned	Only positive (0+)	unsigned int x = 5;

data_type_keywords.c

1#include <stdio.h>
2
3int main() {
4    // Data type keywords in action
5    int age = 25;
6    float salary = 50000.50f;
7    double pi = 3.14159265359;
8    char grade = 'A';
9    short small = 100;
10    long large = 1000000L;
11    unsigned int positive = 42;
12    
13    printf("Age: %d\n", age);
14    printf("Salary: %.2f\n", salary);
15    return 0;
16}

04Control Flow Keywords

These keywords control the flow of execution in your program.

🔀 Decision Making

if- Execute if true

else- Execute if false

switch- Multi-way branch

case- Switch case label

default- Default switch case

🔄 Loops

for- Count-controlled loop

while- Pre-test loop

do- Post-test loop (do-while)

break- Exit loop/switch

continue- Skip to next iteration

goto- Jump to label

control_flow_keywords.c

1#include <stdio.h>
2
3int main() {
4    int x = 10;
5    
6    // if-else
7    if (x > 5) {
8        printf("x is greater than 5\n");
9    } else {
10        printf("x is 5 or less\n");
11    }
12    
13    // for loop
14    for (int i = 0; i < 3; i++) {
15        printf("i = %d\n", i);
16    }
17    
18    // while loop
19    while (x > 5) {
20        printf("x = %d\n", x);
21        x--;
22    }
23    
24    // switch
25    switch (x) {
26        case 5:
27            printf("x is 5\n");
28            break;
29        default:
30            printf("x is not 5\n");
31    }
32    
33    return 0;
34}

05Storage Class Keywords

These keywords define where a variable is stored and how long it exists.

Keyword	Description	Lifetime
auto	Automatic (default for locals)	Block scope
static	Persists between calls	Program lifetime
extern	Defined in another file	Program lifetime
register	Hint: store in CPU register	Block scope

storage_class_keywords.c

1void counter() {
2    static int count = 0;  // Persists between calls
3    count++;
4    printf("Called %d times\n", count);
5}
6
7extern int globalVar;  // Defined elsewhere
8
9register int fast = 0;  // Hint for faster access

06User-Defined Type Keywords

Keyword	Description	Example
struct	Group of related variables	struct Person { ... }
union	Share memory location	union Data { ... }
enum	Named integer constants	enum Color { RED, GREEN }
typedef	Create type alias	typedef int Integer;

user_defined_types.c

1// struct - group related data
2struct Person {
3    char name[50];
4    int age;
5};
6
7// enum - named constants
8enum Color { RED, GREEN, BLUE };
9
10// typedef - create alias
11typedef unsigned int uint;
12
13// union - shared memory
14union Data {
15    int i;
16    float f;
17};

07Other Important Keywords

Keyword	Description	Example
return	Exit function, return value	return 0;
sizeof	Get size in bytes	sizeof(int) → 4
const	Make read-only	const int PI = 3;
volatile	Can change unexpectedly	volatile int flag;

other_keywords.c

1#include <stdio.h>
2
3int add(int a, int b) {
4    return a + b;  // return: exit with value
5}
6
7int main() {
8    const int MAX = 100;    // const: cannot change
9    
10    printf("int size: %zu bytes\n", sizeof(int));
11    printf("Sum: %d\n", add(5, 3));
12    
13    return 0;  // return success
14}

08Keywords by Category

📊Data Types (9)

intfloatdoublecharvoidshortlongsignedunsigned

🔀Control Flow (12)

ifelseswitchcasedefaultforwhiledobreakcontinuegotoreturn

📦Storage Classes (4)

autostaticexternregister

🏗️User-Defined Types (4)

structunionenumtypedef

🔧Other Keywords (3)

sizeofconstvolatile

09Common Mistakes

❌ Using Keywords as Variable Names

int for = 5; // ERROR: 'for' is a keyword

❌ Wrong Case

IF (x > 5) // ERROR: should be lowercase 'if'

❌ Misspelling Keywords

swicth (x) // ERROR: should be 'switch'

10Summary

🎯 Key Takeaways

•C has 32 reserved keywords
•All keywords are lowercase
•Keywords cannot be used as identifiers
•Categories: Data Types, Control Flow, Storage Classes, User Types

09Next Steps

Now learn about data types and variables in C:

Next: Data Types→Review: Identifiers

What You Will Learn