C Notes

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Identifiers in C Programming

Complete guide to identifiers in C — naming rules, conventions, valid and invalid examples, and the difference between identifiers and keywords.

When you write a C program, you create variables, functions, arrays, structures, and many other elements. Each one needs a name so you can refer to it later in your code. These names are called identifiers. Choosing good identifiers is one of the simplest yet most impactful things you can do to make your code readable and maintainable.

Rules for Naming Identifiers in C

C has strict rules about what constitutes a valid identifier. If you break any of these rules, the compiler will reject your code with an error.

Rule 1: Must Begin with a Letter or Underscore

The first character of an identifier must be:

An uppercase letter (A–Z)
A lowercase letter (a–z)
An underscore (_)

It cannot start with a digit (0–9).

c example

int name;     // ✓ starts with letter
int _count;   // ✓ starts with underscore
int Age;      // ✓ starts with uppercase letter
// int 2ndPlace;  // ✗ starts with digit — INVALID

Rule 2: Can Contain Letters, Digits, and Underscores

After the first character, identifiers can include:

Letters (A–Z, a–z)
Digits (0–9)
Underscores (_)

No other characters are allowed — no spaces, hyphens, periods, or special symbols.

c example

int student1;     // ✓ digits allowed after first character
int max_value;    // ✓ underscores allowed
int totalMarks2;  // ✓ mix of letters and digits
// int my-var;    // ✗ hyphen not allowed
// int total$;    // ✗ dollar sign not allowed

Rule 3: Cannot Be a C Keyword

Identifiers must not clash with any of C's 32 reserved keywords. The compiler uses these words for specific purposes and won't let you redefine them.

c example

int value;    // ✓ not a keyword
int number;   // ✓ not a keyword
// int int;   // ✗ 'int' is a keyword
// int return; // ✗ 'return' is a keyword
// int while; // ✗ 'while' is a keyword

Rule 4: Case-Sensitive

C treats uppercase and lowercase letters as different characters. Age, age, and AGE are three completely different identifiers.

c example

#include <stdio.h>

int main() {
    int age = 25;
    int Age = 30;
    int AGE = 35;

    // These are three DIFFERENT variables
    printf("age = %d\n", age);
    printf("Age = %d\n", Age);
    printf("AGE = %d\n", AGE);

    return 0;
}

Output

age = 25
Age = 30
AGE = 35

Rule 5: No Length Limit (But Significance Varies)

The C standard guarantees that at least the first 31 characters of an internal identifier are significant (63 characters in C99). For external identifiers (across files), at least the first 6 characters must be significant in C89 (31 in C99).

In practice, most modern compilers support much longer significant names, but keeping identifiers reasonably short is good practice anyway.

Rule 6: No Spaces Allowed

Identifiers cannot contain spaces. If you need a multi-word name, use underscores or camelCase.

c example

int first_name;    // ✓ use underscore
int firstName;     // ✓ use camelCase
// int first name; // ✗ spaces not allowed

Valid and Invalid Identifiers — Quick Reference

Identifier	Valid?	Reason
`sum`	✓	Starts with letter, all lowercase
`_temp`	✓	Starts with underscore
`MAX_VALUE`	✓	Uppercase with underscore
`student1`	✓	Digit after first character is fine
`calculateArea`	✓	camelCase, all valid characters
`my_var_2`	✓	Mix of letters, digits, underscore
`__internal`	✓*	Valid but reserved by convention
`2ndYear`	✗	Cannot start with a digit
`my-variable`	✗	Hyphens not allowed
`total marks`	✗	Spaces not allowed
`float`	✗	Keyword — reserved word
`price$`	✗	Special characters not allowed
`for`	✗	Keyword — reserved word
`hello@world`	✗	@ symbol not allowed

Note: Identifiers starting with double underscore (__) or underscore followed by an uppercase letter (_A) are reserved for the compiler and standard library. Avoid using them in your own code.

Naming Conventions and Best Practices

While C doesn't enforce naming conventions (only rules), following established conventions makes your code professional and readable.

Common Conventions

Convention	Style	Used For	Example
camelCase	`firstName`	Variables, functions	`totalAmount`, `getUserInput()`
snake_case	`first_name`	Variables, functions	`total_amount`, `get_user_input()`
UPPER_SNAKE_CASE	`MAX_SIZE`	Constants, macros	`BUFFER_SIZE`, `PI`
PascalCase	`StudentRecord`	Structs, typedefs	`LinkedList`, `TreeNode`
Prefix notation	`g_count`, `p_data`	Globals, pointers	`g_totalUsers`, `p_head`

Best Practices

Example

// ✓ GOOD: descriptive and clear int studentAge; float averageScore; char firstName[50]; int calculateTotal(int price, int quantity); // ✗ BAD: cryptic and unclear int x; float a; char n[50]; int calc(int p, int q);

Tips for Choosing Good Names

Be descriptive — numberOfStudents is better than n
Be concise — maxLen is fine; maximumLengthOfTheStringBuffer is too long
Use consistent style — pick camelCase or snake_case and stick with it
Avoid single-letter names — except for loop counters (i, j, k)
Make names pronounceable — stuCnt is harder to discuss than studentCount
Avoid abbreviations unless universally understood (max, min, len are fine)

Identifiers vs Keywords — Key Differences

Feature	Identifiers	Keywords
Definition	User-defined names	Pre-defined reserved words
Purpose	Name program entities	Define language syntax/structure
Flexibility	Can be anything (following rules)	Fixed — cannot be changed
Case	Case-sensitive (user decides)	Always lowercase
Examples	`sum`, `age`, `printResult`	`int`, `if`, `return`, `while`
Total count	Unlimited	32 (in C89)

Scope and Identifier Visibility

The same identifier name can be used in different scopes without conflict:

c example

#include <stdio.h>

int value = 100;  // global 'value'

void display() {
    int value = 50;  // local 'value' — shadows global
    printf("Inside display: %d\n", value);
}

int main() {
    int value = 25;  // local to main
    printf("Inside main: %d\n", value);
    display();
    return 0;
}

Output

Inside main: 25
Inside display: 50

Each value is a separate variable despite sharing the same identifier name — they exist in different scopes.

Identifiers in Different Contexts

c example

#include <stdio.h>

#define MAX 100          // MAX is a macro identifier

typedef int Integer;     // Integer is a typedef identifier

enum Color { RED, GREEN, BLUE };  // Color, RED, GREEN, BLUE are identifiers

struct Point {           // Point is a struct tag identifier
    int x;              // x is a member identifier
    int y;              // y is a member identifier
};

int globalVar = 10;     // globalVar is a variable identifier

int add(int a, int b) { // add, a, b are identifiers
    return a + b;
}

int main() {            // main is an identifier
    struct Point p1;    // p1 is a variable identifier
    p1.x = 5;
    p1.y = 10;

    Integer sum = add(p1.x, p1.y);
    printf("Sum: %d\n", sum);
    printf("Max allowed: %d\n", MAX);

    return 0;
}

Output

Sum: 15
Max allowed: 100

Common Mistakes with Identifiers

Mistake 1: Starting with a Digit

c example

// int 1st_student;  // ERROR
int first_student;   // CORRECT
int student_1;       // CORRECT

Mistake 2: Using Special Characters

c example

// int total-marks;   // ERROR (hyphen interpreted as minus)
// int email@id;      // ERROR
int total_marks;      // CORRECT
int email_id;         // CORRECT

Mistake 3: Using Keywords

c example

// int case;    // ERROR
// float break; // ERROR
int testCase;   // CORRECT
float breakPoint; // CORRECT

Mistake 4: Confusing Case

c example

int count = 10;
// printf("%d", Count);  // ERROR: 'Count' is not declared
printf("%d", count);     // CORRECT: case must match exactly

Interview Questions on Identifiers in C

Q1: What is an identifier in C? Give examples.

An identifier is a user-defined name given to program elements such as variables, functions, arrays, and structures. Examples: age, calculateSum, MAX_SIZE, struct Student. Identifiers must follow naming rules — start with a letter/underscore, contain only alphanumeric characters and underscores, and not be a keyword.

Q2: What is the maximum length of an identifier in C?

The C89 standard guarantees that the first 31 characters of an internal identifier are significant. C99 extended this to 63 characters. In practice, most compilers support even longer names, but only the standard-defined number of characters are guaranteed to be distinguishing.

Q3: Are identifiers case-sensitive in C? Explain with an example.

Yes, C identifiers are case-sensitive. total, Total, and TOTAL are three different identifiers. If you declare int count; and later try to use Count, the compiler will report an "undeclared identifier" error.

Q4: Why can't identifiers start with a digit?

This is a parser design decision. If identifiers could start with digits, the compiler would have difficulty distinguishing between numeric literals (like 123) and identifiers (like 123abc). By requiring the first character to be a letter or underscore, the compiler can immediately determine whether a token is a number or a name.

Q5: What identifiers are reserved by convention in C?

Identifiers starting with a double underscore (__) or an underscore followed by a capital letter (_X) are reserved for the compiler implementation and standard library. Additionally, identifiers starting with a single underscore at file scope are reserved. Using them in user code may cause conflicts.

Summary

Identifiers are user-defined names for variables, functions, arrays, and other program elements.
They must start with a letter or underscore, followed by letters, digits, or underscores.
Identifiers cannot be C keywords and are case-sensitive.
At least 31 characters are significant in C89 (63 in C99).
Following consistent naming conventions (camelCase, snake_case, UPPER_CASE) improves code readability.
Avoid starting identifiers with double underscores or underscore + capital letter (reserved).
Good identifier names are descriptive, concise, consistent, and pronounceable.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Identifiers in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

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c-programming, c programming, programming, basics, identifiers, identifiers in c programming

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BasicsC Syntax Rules — Complete GuideLearn the fundamental syntax rules of C programming — statements, expressions, semicolons, code blocks, whitespace handling, and common syntax errors explained with examples.BasicsComments in C ProgrammingLearn about comments in C — single-line comments, multi-line comments, best practices, nested comments, and when to use comments effectively in your code.BasicsConstants in C ProgrammingLearn about constants in C programming — const keyword, #define macro, enum constants, and literal constants with practical examples and best practices.BasicsData Types in CComplete guide to C data types — primary types (int, char, float, double), derived types (arrays, pointers, structures), type modifiers (short, long, signed, unsigned), sizeof operator, type conversion, and memory representation with examples.BasicsKeywords in C ProgrammingComplete guide to all 32 keywords in C programming — categorized by purpose with explanations, usage examples, and comparison tables.BasicsStructure of a C ProgramComplete explanation of C program structure — documentation section, preprocessor directives, global declarations, main function, user-defined functions, and how each section works together with practical examples.

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C Notes

Identifiers in C Programming

Complete guide to identifiers in C — naming rules, conventions, valid and invalid examples, and the difference between identifiers and keywords.

Rules for Naming Identifiers in C

C has strict rules about what constitutes a valid identifier. If you break any of these rules, the compiler will reject your code with an error.

Rule 1: Must Begin with a Letter or Underscore

The first character of an identifier must be:

An uppercase letter (A–Z)
A lowercase letter (a–z)
An underscore (_)

It cannot start with a digit (0–9).

c example

int name;     // ✓ starts with letter
int _count;   // ✓ starts with underscore
int Age;      // ✓ starts with uppercase letter
// int 2ndPlace;  // ✗ starts with digit — INVALID

Rule 2: Can Contain Letters, Digits, and Underscores

After the first character, identifiers can include:

Letters (A–Z, a–z)
Digits (0–9)
Underscores (_)

No other characters are allowed — no spaces, hyphens, periods, or special symbols.

c example

int student1;     // ✓ digits allowed after first character
int max_value;    // ✓ underscores allowed
int totalMarks2;  // ✓ mix of letters and digits
// int my-var;    // ✗ hyphen not allowed
// int total$;    // ✗ dollar sign not allowed

Rule 3: Cannot Be a C Keyword

Identifiers must not clash with any of C's 32 reserved keywords. The compiler uses these words for specific purposes and won't let you redefine them.

c example

int value;    // ✓ not a keyword
int number;   // ✓ not a keyword
// int int;   // ✗ 'int' is a keyword
// int return; // ✗ 'return' is a keyword
// int while; // ✗ 'while' is a keyword

Rule 4: Case-Sensitive

C treats uppercase and lowercase letters as different characters. Age, age, and AGE are three completely different identifiers.

c example

#include <stdio.h>

int main() {
    int age = 25;
    int Age = 30;
    int AGE = 35;

    // These are three DIFFERENT variables
    printf("age = %d\n", age);
    printf("Age = %d\n", Age);
    printf("AGE = %d\n", AGE);

    return 0;
}

Output

age = 25
Age = 30
AGE = 35

Rule 5: No Length Limit (But Significance Varies)

In practice, most modern compilers support much longer significant names, but keeping identifiers reasonably short is good practice anyway.

Rule 6: No Spaces Allowed

Identifiers cannot contain spaces. If you need a multi-word name, use underscores or camelCase.

c example

int first_name;    // ✓ use underscore
int firstName;     // ✓ use camelCase
// int first name; // ✗ spaces not allowed

Valid and Invalid Identifiers — Quick Reference

Identifier	Valid?	Reason
`sum`	✓	Starts with letter, all lowercase
`_temp`	✓	Starts with underscore
`MAX_VALUE`	✓	Uppercase with underscore
`student1`	✓	Digit after first character is fine
`calculateArea`	✓	camelCase, all valid characters
`my_var_2`	✓	Mix of letters, digits, underscore
`__internal`	✓*	Valid but reserved by convention
`2ndYear`	✗	Cannot start with a digit
`my-variable`	✗	Hyphens not allowed
`total marks`	✗	Spaces not allowed
`float`	✗	Keyword — reserved word
`price$`	✗	Special characters not allowed
`for`	✗	Keyword — reserved word
`hello@world`	✗	@ symbol not allowed

Note: Identifiers starting with double underscore (__) or underscore followed by an uppercase letter (_A) are reserved for the compiler and standard library. Avoid using them in your own code.

Naming Conventions and Best Practices

While C doesn't enforce naming conventions (only rules), following established conventions makes your code professional and readable.

Common Conventions

Convention	Style	Used For	Example
camelCase	`firstName`	Variables, functions	`totalAmount`, `getUserInput()`
snake_case	`first_name`	Variables, functions	`total_amount`, `get_user_input()`
UPPER_SNAKE_CASE	`MAX_SIZE`	Constants, macros	`BUFFER_SIZE`, `PI`
PascalCase	`StudentRecord`	Structs, typedefs	`LinkedList`, `TreeNode`
Prefix notation	`g_count`, `p_data`	Globals, pointers	`g_totalUsers`, `p_head`

Best Practices

Example

Tips for Choosing Good Names

Be descriptive — numberOfStudents is better than n
Be concise — maxLen is fine; maximumLengthOfTheStringBuffer is too long
Use consistent style — pick camelCase or snake_case and stick with it
Avoid single-letter names — except for loop counters (i, j, k)
Make names pronounceable — stuCnt is harder to discuss than studentCount
Avoid abbreviations unless universally understood (max, min, len are fine)

Identifiers vs Keywords — Key Differences

Feature	Identifiers	Keywords
Definition	User-defined names	Pre-defined reserved words
Purpose	Name program entities	Define language syntax/structure
Flexibility	Can be anything (following rules)	Fixed — cannot be changed
Case	Case-sensitive (user decides)	Always lowercase
Examples	`sum`, `age`, `printResult`	`int`, `if`, `return`, `while`
Total count	Unlimited	32 (in C89)

Scope and Identifier Visibility

The same identifier name can be used in different scopes without conflict:

c example

#include <stdio.h>

int value = 100;  // global 'value'

void display() {
    int value = 50;  // local 'value' — shadows global
    printf("Inside display: %d\n", value);
}

int main() {
    int value = 25;  // local to main
    printf("Inside main: %d\n", value);
    display();
    return 0;
}

Output

Inside main: 25
Inside display: 50

Each value is a separate variable despite sharing the same identifier name — they exist in different scopes.

Identifiers in Different Contexts

c example

#include <stdio.h>

#define MAX 100          // MAX is a macro identifier

typedef int Integer;     // Integer is a typedef identifier

enum Color { RED, GREEN, BLUE };  // Color, RED, GREEN, BLUE are identifiers

struct Point {           // Point is a struct tag identifier
    int x;              // x is a member identifier
    int y;              // y is a member identifier
};

int globalVar = 10;     // globalVar is a variable identifier

int add(int a, int b) { // add, a, b are identifiers
    return a + b;
}

int main() {            // main is an identifier
    struct Point p1;    // p1 is a variable identifier
    p1.x = 5;
    p1.y = 10;

    Integer sum = add(p1.x, p1.y);
    printf("Sum: %d\n", sum);
    printf("Max allowed: %d\n", MAX);

    return 0;
}

Output

Sum: 15
Max allowed: 100

Common Mistakes with Identifiers

Mistake 1: Starting with a Digit

c example

// int 1st_student;  // ERROR
int first_student;   // CORRECT
int student_1;       // CORRECT

Mistake 2: Using Special Characters

c example

// int total-marks;   // ERROR (hyphen interpreted as minus)
// int email@id;      // ERROR
int total_marks;      // CORRECT
int email_id;         // CORRECT

Mistake 3: Using Keywords

c example

// int case;    // ERROR
// float break; // ERROR
int testCase;   // CORRECT
float breakPoint; // CORRECT

Mistake 4: Confusing Case

c example

int count = 10;
// printf("%d", Count);  // ERROR: 'Count' is not declared
printf("%d", count);     // CORRECT: case must match exactly

Interview Questions on Identifiers in C

Q1: What is an identifier in C? Give examples.

Q2: What is the maximum length of an identifier in C?

Q3: Are identifiers case-sensitive in C? Explain with an example.

Q4: Why can't identifiers start with a digit?

Q5: What identifiers are reserved by convention in C?

Summary

Identifiers are user-defined names for variables, functions, arrays, and other program elements.
They must start with a letter or underscore, followed by letters, digits, or underscores.
Identifiers cannot be C keywords and are case-sensitive.
At least 31 characters are significant in C89 (63 in C99).
Following consistent naming conventions (camelCase, snake_case, UPPER_CASE) improves code readability.
Avoid starting identifiers with double underscores or underscore + capital letter (reserved).
Good identifier names are descriptive, concise, consistent, and pronounceable.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Identifiers in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, basics, identifiers, identifiers in c programming

Continue learning this concept

PreviousData Types in C NextKeywords in C Programming