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🔑 C Keywords – Reserved Words in C Programming

🧲 Introduction – What Are Keywords in C?
Keywords in C are reserved words predefined by the language that have special meaning to the compiler. These words are part of the C syntax and cannot be used for naming variables, functions, or identifiers. They are essential for writing syntactically valid C programs and form the backbone of C language constructs like data types, control statements, and loops.

🎯 In this guide, you’ll learn:

The list of standard C keywords and their purpose
How keywords are used in program structure
Why you cannot redefine or use them as identifiers
The difference between keywords and identifiers

📋 What Are C Keywords?

Keywords are built-in, reserved terms that are used to perform specific operations or define language constructs. The C compiler interprets them as commands rather than user-defined names.

📚 List of ANSI C (C89/C90) Keywords

Keyword	Purpose
`auto`	Declares automatic storage class
`break`	Exits from a loop or switch
`case`	Defines a case in a switch block
`char`	Declares a character variable
`const`	Declares a constant value
`continue`	Skips to the next loop iteration
`default`	Default case in a switch block
`do`	Loop that executes at least once
`double`	Declares a double precision float
`else`	Executes if `if` is false
`enum`	Declares an enumeration
`extern`	Declares an external variable
`float`	Declares a floating-point variable
`for`	Looping construct
`goto`	Jumps to a labeled statement
`if`	Conditional execution
`int`	Declares an integer variable
`long`	Declares a long integer
`register`	Declares a register variable
`return`	Returns from a function
`short`	Declares a short integer
`signed`	Declares a signed variable
`sizeof`	Returns the size of a variable
`static`	Declares a static variable
`struct`	Declares a structure
`switch`	Multi-way branching
`typedef`	Defines a new name for a type
`union`	Declares a union type
`unsigned`	Declares an unsigned variable
`void`	Declares functions with no return
`volatile`	Declares a volatile variable
`while`	Looping construct

🧪 Keywords Introduced in C99

Keyword	Purpose
`inline`	Suggests that a function be inlined
`_Bool`	Boolean data type
`_Complex`	Complex number type
`_Imaginary`	Imaginary number type
`restrict`	Optimizes pointer usage in functions

🧪 Keywords Introduced in C11

Keyword	Purpose
`_Alignas`	Aligns variables in memory
`_Alignof`	Returns alignment requirement of a type
`_Atomic`	Atomic access for variables
`_Generic`	Enables generic programming
`_Noreturn`	Indicates functions that do not return
`_Static_assert`	Compile-time assertion
`_Thread_local`	Declares thread-local storage

⚠️ Rules for Using Keywords

❌ Cannot be used as variable, function, or identifier names
✅ Must be written in lowercase (e.g., int, not Int)
✅ Cannot be redefined or overridden
✅ Are recognized by the compiler as special symbols

✅ Examples of Keyword Usage

int main() {
    int age = 30;      // 'int' is a keyword
    if (age > 18) {    // 'if' and 'return' are keywords
        return 1;
    }
    return 0;
}

📌 Summary – Recap & Next Steps

C keywords are reserved identifiers with fixed meanings that define how your program works. From declaring variables and types to controlling program flow, keywords play a vital role in every C application.

🔍 Key Takeaways:

C has 32 keywords in ANSI C, more in later versions like C99 and C11.
Keywords are case-sensitive and cannot be used as identifiers.
They are fundamental to all C syntax and logic structures.
Knowing how and when to use keywords is essential for proper code flow.

⚙️ Real-World Relevance:

A strong grasp of C keywords enables efficient and error-free coding. They are foundational in systems programming, embedded development, and any C-based application.

❓ Frequently Asked Questions (FAQ)

❓ How many keywords are there in C?

✅ The original ANSI C (C89/C90) standard includes 32 keywords. C99 and C11 standards added additional keywords like _Bool, inline, _Atomic, and others.

❓ Can I use a keyword as a variable name?

✅ No. Keywords are reserved and cannot be used for variable, function, or macro names. Doing so will cause a compilation error.

❓ Are C keywords case-sensitive?

✅ Yes. C is a case-sensitive language. Int, INT, and int are treated as different. Only int is recognized as a valid keyword.

❓ Can keywords be redefined or overridden?

✅ No. Keywords have predefined meaning in the C language and cannot be redefined, overridden, or changed in any way.

❓ What is the difference between a keyword and an identifier?

✅ A keyword is a reserved word used by the C language (e.g., return, int), while an identifier is a name defined by the programmer (e.g., count, age).

❓ Are keywords the same across all C compilers?

✅ All standard-compliant C compilers support the official list of keywords for each C version (C89, C99, C11). Some may include non-standard extensions, but these are not portable.

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