Identifier in C

An identifier is the name used to identify variables, functions, arrays, structures, and other user-defined entities in a C program. Think of a contact list on your phone — the contact name is the identifier, and the phone number is the value stored at that name. Similarly, an identifier gives a name to a memory location.

Rules for Identifiers

Can contain letters (A–Z, a–z), digits (0–9), and underscore (_)
Must begin with a letter or underscore — not a digit
Cannot be a C keyword (e.g. int, for, while)
Are case-sensitive — total, Total, and TOTAL are three different identifiers
No special characters allowed (@, $, %, #, etc.)
No spaces allowed in an identifier name

Valid vs Invalid Identifiers

Identifier	Valid?	Reason
`age`	✅ Valid	Letters only
`_salary`	✅ Valid	Starts with `_`
`num1`	✅ Valid	Letter then digit
`1num`	❌ Invalid	Starts with a digit
`int`	❌ Invalid	Reserved keyword
`my var`	❌ Invalid	Contains a space
`@value`	❌ Invalid	Special character

Example

int age = 25;
float salary = 50000.50;
char grade = 'A';

Here, age, salary, and grade are identifiers — each names a memory location that stores its respective value.

Key Point

Identifiers are case-sensitive in C. This means:

int total = 10;
int Total = 20;
int TOTAL = 30;
// All three are different variables!