Keywords in C Language – BKNMU Junagadh | BCA Sem 1 Notes

BCA Sem 1  |  Unit 1  |  Computer Concepts & C Programming

Keywords in C Language — BKNMU Junagadh

Complete Guide to All 32 ANSI Keywords, Categories, Rules, and Code Usage

BKNMU Junagadh NEP 2020 32 C Keywords Reserved Words C Syntax Rules

In simple words: Keywords in C are predefined, reserved words that carry fixed, special meanings to the C compiler. Because these 32 words serve as the fundamental grammar of the C language, they cannot be used as identifiers (variable names, function names, or array names).

📖 Master List of All 32 Keywords in ANSI C

Here are all 32 standard keywords recognized by ANSI C (C89/C90) compilers:

auto

break

case

char

const

continue

default

do

double

else

enum

extern

float

for

goto

if

int

long

register

return

short

signed

sizeof

static

struct

switch

typedef

union

unsigned

void

volatile

while

ℹ️ Lowercase Rule: Every single keyword in C is written entirely in lowercase letters. Words like INT, If, or RETURN are NOT recognized as keywords by the compiler!

🔍 Categorization of C Keywords by Purpose

Data Types & Modifiers

• int, char, float, double
• void, short, long
• signed, unsigned
• const, volatile

Control Flow & Loops

• if, else, switch, case
• default, goto
• for, while, do
• break, continue, return

Storage Classes & Structures

• auto, register, static, extern
• struct, union, enum
• typedef, sizeof

1️⃣ Frequently Used Keywords & Their Descriptions

Here is how the most essential keywords operate inside a C program:

Keyword	Category	What It Does	Example Usage
int	Data Type	Declares integer variables	int count = 5;
if ... else	Control Flow	Executes code based on a boolean condition	if (x > 0) { ... }
for	Loop Control	Executes a block of statements repeatedly	for (i=0; i<10; i++)
return	Function Control	Exits a function and optionally returns a value	return 0;
const	Type Qualifier	Makes a variable's value read-only (constant)	const float PI = 3.14;
sizeof	Compile-time Operator	Returns the memory size (in bytes) of a data type/variable	sizeof(int);
struct	User-Defined Type	Groups variables of different types under one name	struct Student { ... };
typedef	Type Definition	Creates an alias or nickname for an existing data type	typedef int number;

💻 Code Sample — Identifying Keywords in Source Code

Observe how keywords (highlighted in bold blue) form the structural backbone of C programs:

#include <stdio.h> int main() { const float pi = 3.14159; int radius = 5; if (radius > 0) { float area = pi * radius * radius; printf("Area: %f\n", area); } else { printf("Invalid radius!\n"); } return 0; }

💡 Exam Tip: In theory exams, if asked to list rules for keywords, emphasize that keywords cannot be used as variable names. Writing int double = 10; will cause a compile error!

🔄 Rules for Using Keywords in C

1

Strict Lowercase Requirement

Keywords must be written strictly in lowercase (e.g., while, not WHILE).

2

Cannot be Used as Variable Names

You cannot use keywords as identifiers (e.g., int float = 5; is invalid).

3

Predefined Meaning

The compiler already understands what each keyword does; you cannot redefine their purpose.

⚠️ Common Mistake: Newer standards like C99 added keywords like inline and _Bool, but for university exams (BKNMU), focus on the **32 standard ANSI C keywords**.

📝 Quick Revision — Key Exam Points

• Definition: Reserved words with fixed meanings built into the C compiler.
• Total Count: Exactly 32 keywords in ANSI C (C89).
• Case Sensitivity: All keywords MUST be written in lowercase letters.
• Identifier Rule: Keywords cannot be re-purposed as variable or function names.
• Data Type Keywords: int, char, float, double, void.
• Loop Keywords: for, while, do.
• Decision Keywords: if, else, switch, case, default.
• Storage Class Keywords: auto, register, static, extern.