Octal literals in C

In C, we can use octal literals by typing a zero before the actual number. For example, if an octal number is 25, then we have to write 025. Octal literals use base-8 numbering system and contain only digits 0-7.

Syntax

0octal_digits

Where octal_digits are valid octal digits (0-7).

Example

The following example demonstrates how to use octal literals in C −

#include <stdio.h>

int main() {
    int a = 025;  /* Octal 25 = Decimal 21 */
    int b = 063;  /* Octal 63 = Decimal 51 */
    int c = 0777; /* Octal 777 = Decimal 511 */
    
    printf("Decimal of 25(Octal) is %d<br>", a);
    printf("Decimal of 63(Octal) is %d<br>", b);
    printf("Decimal of 777(Octal) is %d<br>", c);
    
    return 0;
}

Decimal of 25(Octal) is 21
Decimal of 63(Octal) is 51
Decimal of 777(Octal) is 511

How It Works

Octal literals are converted to decimal using base-8 system:

• 025 = 2×8¹ + 5×8? = 16 + 5 = 21
• 063 = 6×8¹ + 3×8? = 48 + 3 = 51
• 0777 = 7×8² + 7×8¹ + 7×8? = 448 + 56 + 7 = 511

Key Points

• Octal literals must start with 0 (zero).
• Only digits 0-7 are valid in octal literals.
• Using digits 8 or 9 will cause a compilation error.
• Octal literals are commonly used for file permissions in Unix systems.

Conclusion

Octal literals in C provide a convenient way to represent base-8 numbers by prefixing with zero. They are automatically converted to decimal values during compilation and are particularly useful in system programming.