Keywords in C Last Updated : 13 Jul, 2025 Comments Improve Suggest changes Like Article Like Report Keywords are predefined or reserved words that have special meanings to the compiler. These are part of the syntax and cannot be used as identifiers in the program. A list of keywords in C or reserved words in the C programming language are mentioned below:autobreakcasecharconstcontinuedefaultdodoubleelseenumexternfloatforgotoifintlongregisterreturnshortsignedsizeofstaticstructswitchtypedefunionunsignedvoidvolatilewhileWe cannot use these keywords as identifiers (such as variable names, function names, or struct names). The compiler will throw an error if we try to do so.Example: C #include <stdio.h> int main() { int return = 10; printf("%d\n", return); return 0; } Output./Solution.c: In function 'main':./Solution.c:4:9: error: expected identifier or '(' before 'return' int return = 10; ^./Solution.c:5:20: error: expected expression before 'return' printf("%d\n", return); ^Let's categorize all keywords based on context for a more clear understanding.CategoryKeywords Data Type Keywords char, int, float, double, void, short, long, signed, unsignedOperator & Utility Keywordssizeof, return, goto, typedefControl Flow Keywordsif, else, switch, case, default, for, while, do, break, continueStorage Class Keywordsauto, register, static, externType Qualifiersconst, volatileUser Defined Typesstruct, union, enumDifference Between Keywords and IdentifiersKeywordsIdentifiersReserved Words in C that have a specific meaning and use in the syntaxNames given to variables, functions, structs, etc.Cannot be used as variable names.Can be used as variable names (if not a keyword).Examples: int, return, if, whileExamples: x, total, countPart of the C language grammar.User-defined, meaningful names in the code. Cannot be redefined or repurposed.Can be defined redefined and reused as needed. Difference Between Variables and KeywordsVariablesKeywordsUsed to store data and manipulate dataReserved words used to define structure and control flowCreated by the user using valid identifiersDefined by C standard and recognized by the compiler Examples: age, total, marksExamples: for, while, int, returnCan hold values and be used in operationsUsed to define the structure of C code.Naming is flexible within identifier rulesNames are fixed and cannot be altered or used differently Comment More infoAdvertise with us Next Article C Variables K kamleshjoshi18 Follow Improve Article Tags : C Language Similar Reads C Programming Language Tutorial C is a general-purpose mid-level programming language developed by Dennis M. Ritchie at Bell Laboratories in 1972. It was initially used for the development of UNIX operating system, but it later became popular for a wide range of applications. Today, C remains one of the top three most widely used 5 min read C BasicsC Language IntroductionC is a general-purpose procedural programming language initially developed by Dennis Ritchie in 1972 at Bell Laboratories of AT&T Labs. It was mainly created as a system programming language to write the UNIX operating system.Main features of CWhy Learn C?C is considered mother of all programmin 6 min read Features of C Programming LanguageC is a procedural programming language. It was initially developed by Dennis Ritchie in the year 1972. It was mainly developed as a system programming language to write an operating system.The main features of C language include low-level access to memory, a simple set of keywords, and a clean style 3 min read C Programming Language StandardIntroduction:The C programming language has several standard versions, with the most commonly used ones being C89/C90, C99, C11, and C18.C89/C90 (ANSI C or ISO C) was the first standardized version of the language, released in 1989 and 1990, respectively. This standard introduced many of the feature 6 min read C Hello World ProgramThe “Hello World†program is the first step towards learning any programming language. It is also one of the simplest programs that is used to introduce aspiring programmers to the programming language. It typically outputs the text "Hello, World!" to the console screen.C Program to Print "Hello Wor 1 min read Compiling a C Program: Behind the ScenesThe compilation is the process of converting the source code of the C language into machine code. As C is a mid-level language, it needs a compiler to convert it into an executable code so that the program can be run on our machine.The C program goes through the following phases during compilation:C 4 min read C CommentsThe comments in C are human-readable notes in the source code of a C program used to make the program easier to read and understand. They are not a part of the executable program by the compiler or an interpreter.Example:C#include <stdio.h> int main() { // This is a comment, the below // state 3 min read Tokens in CIn C programming, tokens are the smallest units in a program that have meaningful representations. Tokens are the building blocks of a C program, and they are recognized by the C compiler to form valid expressions and statements. Tokens can be classified into various categories, each with specific r 4 min read Keywords in CKeywords are predefined or reserved words that have special meanings to the compiler. These are part of the syntax and cannot be used as identifiers in the program. A list of keywords in C or reserved words in the C programming language are mentioned below:autobreakcasecharconstcontinuedefaultdodoub 2 min read C Variables and ConstantsC VariablesA variable in C is a named piece of memory which is used to store data and access it whenever required. It allows us to use the memory without having to memorize the exact memory address.Syntax for Creating VariablesTo create a variable in C, we have to specify a name and the type of data it is goin 4 min read Constants in CIn C programming, const is a keyword used to declare a variable as constant, meaning its value cannot be changed after it is initialized. It is mainly used to protect variables from being accidentally modified, making the program safer and easier to understand. These constants can be of various type 4 min read Const Qualifier in CThe qualifier const can be applied to the declaration of any variable to specify that its value will not be changed (which depends upon where const variables are stored, we may change the value of the const variable by using a pointer). The result is implementation-defined if an attempt is made to c 6 min read Different ways to declare variable as constant in CThere are many different ways to make the variable as constant in C. Some of the popular ones are: Using const KeywordUsing MacrosUsing enum Keyword1. Using const KeywordThe const keyword specifies that a variable or object value is constant and can't be modified at the compilation time. Syntaxconst 2 min read Scope rules in CThe scope of a variable in C is the block or the region in the program where a variable is declared, defined, and used. Outside this region, we cannot access the variable, and it is treated as an undeclared identifier.The scope is the area under which a variable is visible.The scope of an identifier 5 min read Internal Linkage and External Linkage in CIn C, linkage is a concept that describes how names/identifiers can or cannot refer to the same entity throughout the whole program or a single translation unit. The above sounds similar to scope, but it is not so. To understand what the above means, let us dig deeper into the compilation process.Be 4 min read Global Variables in CPrerequisite: Variables in C In a programming language, each variable has a particular scope attached to them. The scope is either local or global. This article will go through global variables, their advantages, and their properties. The Declaration of a global variable is very similar to that of a 3 min read C Data TypesData Types in CEach variable in C has an associated data type. It specifies the type of data that the variable can store like integer, character, floating, double, etc.Example:C++int number;The above statement declares a variable with name number that can store integer values.C is a statically type language where 5 min read Literals in CIn C, Literals are the constant values that are assigned to the variables. Literals represent fixed values that cannot be modified. Literals contain memory but they do not have references as variables. Generally, both terms, constants, and literals are used interchangeably. For example, “const int = 4 min read Escape Sequence in CThe escape sequence in C is the characters or the sequence of characters that can be used inside the string literal. The purpose of the escape sequence is to represent the characters that cannot be used normally using the keyboard. Some escape sequence characters are the part of ASCII charset but so 5 min read bool in CThe bool in C is a fundamental data type in most that can hold one of two values: true or false. It is used to represent logical values and is commonly used in programming to control the flow of execution in decision-making statements such as if-else statements, while loops, and for loops. In this a 5 min read Integer Promotions in CSome data types like char , short int take less number of bytes than int, these data types are automatically promoted to int or unsigned int when an operation is performed on them. This is called integer promotion. For example no arithmetic calculation happens on smaller types like char, short and e 2 min read Character Arithmetic in CAs already known character range is between -128 to 127 or 0 to 255. This point has to be kept in mind while doing character arithmetic. What is Character Arithmetic?Character arithmetic is used to implement arithmetic operations like addition, subtraction, multiplication, and division on characters 2 min read Type Conversion in CIn C, type conversion refers to the process of converting one data type to another. It can be done automatically by the compiler or manually by the programmer. The type conversion is only performed to those data types where conversion is possible. Let's take a look at an example:C#include <stdio. 4 min read C Input/OutputBasic Input and Output in CIn C, there are many input and output for different situations, but the most commonly used functions for Input/Output are scanf() and printf() respectively. These functions are part of the standard input/output library <stdio.h>. scanf() takes user inputs (typed using keyboard) and printf() di 4 min read Format Specifiers in CThe format specifier in C is used to tell the compiler about the type of data to be printed or scanned in input and output operations. They always start with a % symbol and are used in the formatted string in functions like printf(), scanf, sprintf(), etc.The C language provides a number of format s 5 min read printf in CIn C language, printf() function is used to print formatted output in many ways to the standard output stdout (which is generally the console screen).Example:C#include <stdio.h> int main() { // Using printf to print the text "Hi!" printf("Hi!"); return 0; }OutputHi!Explanation: In this program 5 min read scanf in CIn C, scanf() is a function is used to read data from stdin (standard input stream i.e. usually keyboard) and stores the result into the given arguments. It is defined in the <stdio.h> header file.Example:C#include <stdio.h> int main() { int n; // Reading an integer input scanf("%d", 3 min read Scansets in Cscanf family functions support scanset specifiers which are represented by %[]. Inside scanset, we can specify single character or range of characters. While processing scanset, scanf will process only those characters which are part of scanset. We can define scanset by putting characters inside squ 2 min read Formatted and Unformatted Input/Output functions in CIn C language, the Input/Output (I/O) functions are part of the standard library, and these functions are used for interacting with the user or other systems, to perform operations such as reading input and printing output. These functions provide ways to read data from files and other input devices 7 min read C OperatorsOperators in COperators are the basic components of C programming. They are symbols that represent some kind of operation, such as mathematical, relational, bitwise, conditional, or logical computations, which are to be performed on values or variables. The values and variables used with operators are called oper 11 min read Arithmetic Operators in CArithmetic operators are the type of operators used to perform basic math operations like addition, subtraction, and multiplication. Let's take a look at an example:C#include <stdio.h> int main() { // Calculate the area of the triangle int sum = 10 + 20; printf("%d", sum); return 0; }Output30E 5 min read Unary Operators in CIn C programming, unary operators are operators that operate on a single operand. These operators are used to perform operations such as negation, incrementing or decrementing a variable, or checking the size of a variable. They provide a way to modify or manipulate the value of a single variable in 5 min read Relational Operators in CIn C, relational operators are the symbols that are used for comparison between two values to understand the type of relationship a pair of numbers shares. The result that we get after the relational operation is a boolean value, that tells whether the comparison is true or false. Relational operato 4 min read Bitwise Operators in CIn C, bitwise operators are used to perform operations directly on the binary representations of numbers. These operators work by manipulating individual bits (0s and 1s) in a number.The following 6 operators are bitwise operators (also known as bit operators as they work at the bit-level). They are 6 min read C Logical OperatorsLogical operators in C are used to combine multiple conditions/constraints. Logical Operators returns either 0 or 1, it depends on whether the expression result is true or false. In C programming for decision-making, we use logical operators.We have 3 logical operators in the C language:Logical AND 4 min read Assignment Operators in CIn C, assignment operators are used to assign values to variables. The left operand is the variable and the right operand is the value being assigned. The value on the right must match the data type of the variable otherwise, the compiler will raise an error.Let's take a look at an example:C#include 4 min read Increment and Decrement Operators in CThe increment ( ++ ) and decrement ( -- ) operators in C are unary operators for incrementing and decrementing the numeric values by 1, respectively. They are one of the most frequently used operators in programming for looping, array traversal, pointer arithmetic, and many more.Increment Operator i 4 min read Conditional or Ternary Operator (?:) in CThe conditional operator in C is kind of similar to the if-else statement as it follows the same algorithm as of if-else statement but the conditional operator takes less space and helps to write the if-else statements in the shortest way possible. It is also known as the ternary operator in C as it 3 min read sizeof operator in CSizeof is a much-used operator in the C. It is a compile-time unary operator which can be used to compute the size of its operand. The result of sizeof is of the unsigned integral type which is usually denoted by size_t. sizeof can be applied to any data type, including primitive types such as integ 3 min read Operator Precedence and Associativity in COperator precedence and associativity are rules that decide the order in which parts of an expression are calculated. Precedence tells us which operators should be evaluated first, while associativity determines the direction (left to right or right to left) in which operators with the same preceden 7 min read C Control Statements Decision-MakingDecision Making in C (if , if..else, Nested if, if-else-if )In C, programs can choose which part of the code to execute based on some condition. This ability is called decision making and the statements used for it are called conditional statements. These statements evaluate one or more conditions and make the decision whether to execute a block of code or n 7 min read C - if StatementThe if in C is the simplest decision-making statement. It consists of the test condition and a block of code that is executed if and only if the given condition is true. Otherwise, it is skipped from execution.Let's take a look at an example:C#include <stdio.h> int main() { int n = 9; // if st 4 min read C if else StatementThe if else in C is an extension of the if statement which not only allows the program to execute one block of code if a condition is true, but also a different block if the condition is false. This enables making decisions with two possible outcomes.Example:C#include <stdio.h> int main() { in 3 min read C if , else if ladderIn C, if else if ladder is an extension of if else statement used to test a series of conditions sequentially, executing the code for the first true condition. A condition is checked only if all previous ones are false. Once a condition is true, its code block executes, and the ladder ends.Example:C 4 min read Switch Statement in CC switch statement is a conditional statement that allows you to execute different code blocks based on the value of a variable or an expression. It is often used in place of if-else ladder when there are multiple conditions.Example:C#include <stdio.h> int main() { // Switch variable int var = 5 min read Using Range in switch Case in CYou all are familiar with switch case in C, but did you know you can use a range of numbers instead of a single number or character in the case statement? Range in switch case can be useful when we want to run the same set of statements for a range of numbers so that we do not have to write cases se 2 min read C - LoopsIn C programming, there is often a need for repeating the same part of the code multiple times. For example, to print a text three times, we have to use printf() three times as shown in the code:C#include <stdio.h> int main() { printf( "Hello GfG\n"); printf( "Hello GfG\n"); printf( "Hello GfG 6 min read C for LoopIn C programming, the 'for' loop is a control flow statement that is used to repeatedly execute a block of code as many times as instructed. It uses a variable (loop variable) whose value is used to decide the number of repetitions. It is commonly used to iterate over a sequence such as an array or 4 min read while Loop in CThe while loop in C allows a block of code to be executed repeatedly as long as a given condition remains true. It is often used when we want to repeat a block of code till some condition is satisfied.Example: C#include <stdio.h> int main() { int i = 1; // Condition for the loop while (i <= 5 min read do...while Loop in CC do...while loop is a type of loop that executes a code block until the given condition is satisfied. Unlike the while loop, which checks the condition before executing the loop, the do...while loop checks the condition after executing the code block, ensuring that the code inside the loop is execu 4 min read For vs. WhileIn C, loops are the fundamental part of language that are used to repeat a block of code multiple times. The two most commonly used loops are the for loop and the while loop. Although they achieve the same result, their structure, use cases, and flexibility differ.The below table highlights some pri 4 min read Continue Statement in CThe continue statement in C is a jump statement used to skip the current iteration of a loop and continue with the next iteration. It is used inside loops (for, while, or do-while) along with the conditional statements to bypass the remaining statements in the current iteration and move on to the ne 4 min read Break Statement in CThe break statement in C is a loop control statement that breaks out of the loop when encountered. It can be used inside loops or switch statements to bring the control out of the block. The break statement can only break out of a single loop at a time.Example:C#include <stdio.h> int main() { 5 min read goto Statement in CThe goto statement in C allows the program to jump to some part of the code, giving you more control over its execution. While it can be useful in certain situations, like error handling or exiting complex loops, it's generally not recommended because it can make the code harder to read and maintain 4 min read C FunctionsC FunctionsA function in C is a set of statements that, when called, perform some specific tasks. It is the basic building block of a C program that provides modularity and code reusability. They are also called subroutines or procedures in other languages.Function DefinitionA function definition informs the c 6 min read User-Defined Function in CA user-defined function is a type of function in C language that is defined by the user himself to perform some specific task. It provides code reusability and modularity to our program. User-defined functions are different from built-in functions as their working is specified by the user and no hea 6 min read Parameter Passing Techniques in CIn C, passing values to a function means providing data to the function when it is called so that the function can use or manipulate that data. Here:Formal Parameters: Variables used in parameter list in a function declaration/definition as placeholders. Also called only parameters.Actual Parameters 3 min read Function Prototype in CIn C, a function prototype is a statement that tells the compiler about the function’s name, its return type, numbers, and data types of its parameters. Function prototype provides a means for the compiler to cross-check function parameters and their data type with the function definition and the fu 4 min read How can I return multiple values from a function?In C programming, a function can return only one value directly. However, C also provides several indirect methods in to return multiple values from a function. In this article, we will learn the different ways to return multiple values from a function in C.The most straightforward method to return 3 min read main Function in CThe main function is the entry point of a C program. It is a user-defined function where the execution of a program starts. Every C program must contain, and its return value typically indicates the success or failure of the program. In this article, we will learn more about the main function in C.E 5 min read Implicit Return Type int in CIn C, every function has a return type that indicates the type of value it will return, and it is defined at the time of function declaration or definition. But in C language, it is possible to define functions without mentioning the return type and by default, int is implicitly assumed that the ret 2 min read Callbacks in CIn C language, a callback is a function that is passed as an argument to another code, which is expected to call back (execute) the argument at a given time. In simple terms, a callback is the process of passing a function (executable code) to another function as an argument, which is then called by 4 min read Nested Functions in CNesting of functions refers to placing the definition of the function inside another functions. In C programming, nested functions are not allowed. We can only define a function globally.Example:C#include <stdio.h> int main() { void fun(){ printf("GeeksForGeeks"); } fun(); return 0; }Outputmai 4 min read Variadic Functions in CIn C, variadic functions are functions that can take a variable number of arguments. This feature is useful when the number of arguments for a function is unknown. It takes one fixed argument and then any number of arguments can be passed.Let's take a look at an example:C#include <stdio.h> #in 5 min read _Noreturn function specifier in CIn C, the _Noreturn specifier is used to indicate that a function does not return a value. It tells the compiler that the function will either exit the program or enter an infinite loop, so it will never return control to the calling function. This helps the compiler to optimize code and issue warni 2 min read Predefined Identifier __func__ in CBefore we start discussing __func__, let us write some code snippets and anticipate the output: C // C program to demonstrate working of a // Predefined Identifier __func__ #include <stdio.h> int main() { // %s indicates that the program will read strings printf("%s", __func__); return 0; } Ou 2 min read C Library math.h FunctionsThe math.h header defines various C mathematical functions and one macro. All the functions available in this library take double as an argument and return double as the result. Let us discuss some important C math functions one by one. C Math Functions1. double ceil (double x) The C library functio 6 min read C Arrays & StringsC ArraysAn array in C is a fixed-size collection of similar data items stored in contiguous memory locations. It can be used to store the collection of primitive data types such as int, char, float, etc., as well as derived and user-defined data types such as pointers, structures, etc. Creating an Array in 7 min read Properties of Array in CAn array in C is a fixed-size homogeneous collection of elements stored at a contiguous memory location. It is a derived data type in C that can store elements of different data types such as int, char, struct, etc. It is one of the most popular data types widely used by programmers to solve differe 8 min read Multidimensional Arrays in C - 2D and 3D ArraysA multi-dimensional array in C can be defined as an array that has more than one dimension. Having more than one dimension means that it can grow in multiple directions. Some popular multidimensional arrays include 2D arrays which grows in two dimensions, and 3D arrays which grows in three dimension 8 min read Initialization of Multidimensional Array in CIn C, multidimensional arrays are the arrays that contain more than one dimensions. These arrays are useful when we need to store data in a table or matrix-like structure. In this article, we will learn the different methods to initialize a multidimensional array in C. The easiest method for initial 4 min read Pass Array to Functions in CPassing an array to a function allows the function to directly access and modify the original array. In this article, we will learn how to pass arrays to functions in C.In C, arrays are always passed to function as pointers. They cannot be passed by value because of the array decay due to which, whe 3 min read How to pass a 2D array as a parameter in C?A 2D array is essentially an array of arrays, where each element of the main array holds another array. In this article, we will see how to pass a 2D array to a function.The simplest and most common method to pass 2D array to a function is by specifying the parameter as 2D array with row size and co 3 min read What are the data types for which it is not possible to create an array?In C, an array is a collection of variables of the same data type, stored in contiguous memory locations. Arrays can store data of primitive types like integers, characters, and floats, as well as user-defined types like structures.However, there are certain data types for which arrays cannot be dir 2 min read How to pass an array by value in C ?In C programming, arrays are always passed as pointers to the function. There are no direct ways to pass the array by value. However, there is trick that allows you to simulate the passing of array by value by enclosing it inside a structure and then passing that structure by value. This will also p 2 min read Strings in CA String in C programming is a sequence of characters terminated with a null character '\0'. The C String is work as an array of characters. The difference between a character array and a C string is that the string in C is terminated with a unique character '\0'.DeclarationDeclaring a string in C i 5 min read Array of Strings in CIn C, an array of strings is a 2D array where each row contains a sequence of characters terminated by a '\0' NULL character (strings). It is used to store multiple strings in a single array.Let's take a look at an example:C#include <stdio.h> int main() { // Creating array of strings for 3 str 3 min read What is the difference between single quoted and double quoted declaration of char array?In C programming, the way we declare and initialize a char array can differ based on whether we want to use a sequence of characters and strings. They are basically same with difference only of a '\0' NULL character.Double quotes automatically include the null terminator, making the array a string l 2 min read C String FunctionsC language provides various built-in functions that can be used for various operations and manipulations on strings. These string functions make it easier to perform tasks such as string copy, concatenation, comparison, length, etc. The <string.h> header file contains these string functions.Th 6 min read C PointersC PointersA pointer is a variable that stores the memory address of another variable. Instead of holding a direct value, it holds the address where the value is stored in memory. It is the backbone of low-level memory manipulation in C. Accessing the pointer directly will just give us the address that is stor 9 min read Pointer Arithmetics in C with ExamplesPointer Arithmetic is the set of valid arithmetic operations that can be performed on pointers. The pointer variables store the memory address of another variable. It doesn't store any value. Hence, there are only a few operations that are allowed to perform on Pointers in C language. The C pointer 10 min read C - Pointer to Pointer (Double Pointer)In C, double pointers are those pointers which stores the address of another pointer. The first pointer is used to store the address of the variable, and the second pointer is used to store the address of the first pointer. That is why they are also known as a pointer to pointer.Let's take a look at 5 min read Function Pointer in CIn C, a function pointer is a type of pointer that stores the address of a function, allowing functions to be passed as arguments and invoked dynamically. It is useful in techniques such as callback functions, event-driven programs, and polymorphism (a concept where a function or operator behaves di 6 min read How to Declare a Pointer to a Function?A pointer to a function is similar to a pointer to a variable. However, instead of pointing to a variable, it points to the address of a function. This allows the function to be called indirectly, which is useful in situations like callback functions or event-driven programming.In this article, we w 2 min read Pointer to an Array | Array PointerA pointer to an array is a pointer that points to the whole array instead of the first element of the array. It considers the whole array as a single unit instead of it being a collection of given elements.Example:C #include<stdio.h> int main() { int arr[5] = { 1, 2, 3, 4, 5 }; int *ptr = arr; 5 min read Difference between constant pointer, pointers to constant, and constant pointers to constantsIn this article, we will discuss the differences between constant pointer, pointers to constant & constant pointers to constants. Pointers are the variables that hold the address of some other variables, constants, or functions. There are several ways to qualify pointers using const. Pointers to 3 min read Pointer vs Array in CMost of the time, pointer and array accesses can be treated as acting the same, the major exceptions being:  1. the sizeof operator sizeof(array) returns the amount of memory used by all elements in the array sizeof(pointer) only returns the amount of memory used by the pointer variable itself 2. 1 min read Dangling, Void , Null and Wild Pointers in CIn C programming pointers are used to manipulate memory addresses, to store the address of some variable or memory location. But certain situations and characteristics related to pointers become challenging in terms of memory safety and program behavior these include Dangling (when pointing to deall 6 min read Near, Far and Huge Pointers in CIn older times, the intel processors had 16-bit registers, but the address bus was 20-bits wide. Due to this, CPU registers were not able to hold the entire address at once. As a solution, the memory was divided into segments of 64 kB size, and the near pointers, far pointers, and huge pointers were 4 min read restrict Keyword in CThe restrict keyword is a type qualifier that was introduced in the C99 standard. It is used to tell the compiler that a pointer is the only reference or access point to the memory it points to, allowing the compiler to make optimizations based on that information.Let's take a look at an example:C#i 3 min read Like