C Identifiers

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Identifiers in C Language

Understanding C Identifiers: Rules, Types, and Examples


C is a powerful and widely used programming language, follows a set of rules for constructing identifiers. Identifiers are names used to identify variables, functions, labels, and other user-defined entities in a C program. In this blog post, we will delve into the rules for constructing C identifiers, explore the different types of identifiers, and provide programming examples to illustrate their usage.

What is Identifier in C?

In C programming, an identifier is a user-defined name used to identify variables, functions, labels, and other entities in a program. It consists of letters (both uppercase and lowercase), digits, and underscores (_). The first character must be a letter or an underscore, and identifiers are case-sensitive. Identifiers provide unique and meaningful names to program elements, facilitating effective referencing and manipulation.

Rules for Constructing C Identifiers:

  • Valid characters: Identifiers in C can consist of letters (both uppercase and lowercase), digits, and the underscore character (_). The first character of an identifier must be a letter or an underscore.
  • Length: C identifiers can have any length, but only the first 31 characters are significant. C is a case-sensitive language, so uppercase and lowercase letters are considered distinct.
  • Reserved words: Identifiers cannot be the same as reserved words, which are predefined by the C language (e.g., if, while, int). Using reserved words as identifiers will result in a compilation error.
  • Meaningful names: Choose meaningful and descriptive names for identifiers to enhance code readability and maintainability. For example, a variable storing a person’s age could be named personAge.

C Identifiers

Types of Identifiers:

1. Variable identifiers: These are used to represent memory locations that hold values during program execution. Variable identifiers follow the rules for constructing C identifiers and can be declared for various data types like int, float, char, etc. For instance:

int count;
float temperature;
char initial;

2. Function identifiers: Function identifiers are used to represent user-defined functions. They follow the same rules as variable identifiers. Here’s an example:

int calculateSum(int a, int b) {
    return a + b;

3. Constant identifiers: Constants are fixed values that cannot be modified during program execution. Constant identifiers are typically defined using the const keyword and follow the same rules as variable identifiers. An example:

const int MAX_VALUE = 100;

4. Label identifiers: Labels are used in control statements like goto and switch to specify a target location within the program. Label identifiers follow the same rules as variable identifiers. Example:

void printNumbers() {
    int i;
    for (i = 0; i < 10; i++) {
        if (i == 5) {
            goto end;
        printf("%d ", i);
    printf("End of program");

Difference Between Keywords and Identifier;



Keyword is a pre-defined word. The identifier is a user-defined word
It must be written in a lowercase letter. It can be written in both lowercase and uppercase letters.
Its meaning is pre-defined in the c compiler. Its meaning is not defined in the c compiler.
It is a combination of alphabetical characters. It is a combination of alphanumeric characters.
It does not contain the underscore character. It can contain the underscore character.

Programming Examples:

Let’s see a practical example of using identifiers in a C program:

#include <stdio.h>
int main() {
    int radius = 5;
    float area, circumference;

    area = 3.14 * radius * radius;
    circumference = 2 * 3.14 * radius;

    printf("Area: %f\n", area);
    printf("Circumference: %f\n", circumference);
    return 0;

In the above example, we define variable identifiers (radius, area, circumference) to calculate the area and circumference of a circle. We then use the printf function (identified by its name) to display the calculated values.


Understanding and following the rules for constructing C identifiers is crucial for writing clean and error-free code. By choosing meaningful names and using the appropriate types of identifiers, you can enhance the readability and maintainability of your C programs. Incorporating these principles into your coding practices will contribute to more efficient and professional programming.

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