Constants in C

As the name suggests the name constants is given to such variables or values in C programming language which cannot be modified once they are defined. They are fixed values in a program. There can be any types of constants like integer, float, octal, hexadecimal, character constants etc. Every constant has some range. The integers that is too big to fit into an int will be taken as a long. Now there are various ranges that differ from unsigned to signed bits. Under signed bit the range of an int, varies from -128 to +127 and under unsigned bit int varies from 0 to 255.

How to define constants?
In C program we can define constants in two ways as shown below:

  1. using #define preprocessor directive
  2. using a const keyword.

Let us now learn about above two ways in details:

  • using #define preprocessor directive : This directive used to declare an alias name for existing variable or any value. We can use this to declare a constant as shown below:

    #define identifierName value

    where identifierName is the name given to constant and value is any value assigned to it.

    Below is the C program to explain how to use #define to declare constants:

    #define val  10
    #define floatVal  4.5
    #define charVal 'G'
    int main()
        printf("Integer Constant: %d\n",val);
        printf("Floating point Constant: %f\n",floatVal);
        printf("Character Constant: %c\n",charVal);
        return 0;


    Integer Constant: 10
    Floating point Constant: 4.500000
    Character Constant: G

    Refer Macros and Preprocessors in C for details.

  • using a const keyword: Using const keyword to define constants is as simple as defining variables, the difference is you will have to precede the definition with a const keyword. Below program shows how to use const to declare costants of different data types:

    #include <stdio.h>
    int main()
        const int intVal = 10; // int constant
        const float floatVal = 4.14; // Real constant
        const char charVal = 'A'; // char constant
        const char stringVal[10] = "ABC"; // string constant
        printf("Integer constant :%d \n", intVal );
        printf("Floating point constant : %f \n", floatVal );
        printf("Character constant : %c \n", charVal );
        printf("String constant : %s \n", stringVal);
        return 0;


    Integer constant :10 
    Floating point constant : 4.140000 
    Character constant : A 
    String constant : ABC 

    Refer Const Qualifier in C for details.

  • String Literals : When string value is directly assigned to a pointer, in most of the compilers, it’s stored in a read only block (generally in data segment) that is shared among functions.
      char *str  =  "GfG";  

    In the above line “GfG” is stored in a read only location, but pointer str is stored in a read-write memory. You can change str to point something else but cannot change value at present str. So this kind of string should only be used when we don’t want to modify string at a later stage in program.

    The below program may crash (gives segmentation fault error) because the line *(str+1) = ‘n’ tries to write a read only memory.

    int main()
       char *str; 
       /* Stored in read only part of data segment */
       str = "GfG";     
       /* Problem:  trying to modify read only memory */
       *(str+1) = 'n'; 
       return 0;

    Refer Storage for Strings in C for details

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