What does it mean when a numeric constant in C/C++ is prefixed with a 0?

When a numeric constant in C is prefixed with a 0, it indicates that the number is an octal number (base 8). Octal literals must start with 0 to distinguish them from decimal numbers. For example, the octal number 25 is written as 025 in C.

Syntax

0octal_digits

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

Example: Octal Number Conversion

The following program demonstrates how octal literals are converted to decimal values −

#include <stdio.h>

int main() {
    int a = 025;    /* Octal 25 = 2*8^1 + 5*8^0 = 16 + 5 = 21 */
    int b = 063;    /* Octal 63 = 6*8^1 + 3*8^0 = 48 + 3 = 51 */
    int c = 0100;   /* Octal 100 = 1*8^2 + 0*8^1 + 0*8^0 = 64 */
    
    printf("Decimal of 025(Octal) is %d\n", a);
    printf("Decimal of 063(Octal) is %d\n", b);
    printf("Decimal of 0100(Octal) is %d\n", c);
    
    return 0;
}

Decimal of 025(Octal) is 21
Decimal of 063(Octal) is 51
Decimal of 0100(Octal) is 64

Key Points

• Octal numbers use only digits 0-7. Using 8 or 9 will cause a compilation error.
• The leading zero is mandatory for octal literals in C.
• Octal is base 8, so each digit represents a power of 8.
• Common use cases include file permissions in Unix systems (e.g., 0755).

Conclusion

The 0 prefix in C indicates octal notation, where numbers are represented in base 8. This allows programmers to work with octal values directly in their code, which is particularly useful in system programming contexts.