Why Do Leading Zeros in C Integer Literals Sometimes Produce Unexpected Results?

Clarifying Numerical Anomalies in C : The Significance of Leading Zeros

In the realm of C programming, the behavior of integer literals containing leading zeros can be puzzling at times. Let's delve into this curious matter and illuminate its underlying mechanics.

The C standard defines integer literals as sequences of digits without decimal points or exponents. Prefixing the literal with 0 signifies an octal (base-8) number, while the absence of a prefix indicates a decimal (base-10) number.

However, consider the following code snippets:

int i = 07; // i == 7
int i = 16; // i == 16
int i = 00016; // i == 14, why?
int i = 05016; // i == 2574, wow )

In the first example, i is set to the value 7 because 07 is interpreted as an octal value (8 * 0 7 = 7). This is consistent with C 's convention of interpreting leading zeros without a prefix as octal numbers.

However, in the subsequent examples, leading zeros appear before decimal digits, which raises questions. According to the standard, these literals should not be accepted as octal numbers. So, what is happening here?

The answer lies in a compiler optimization known as "octal suppression". When the compiler encounters a leading zero followed by a decimal digit, it suppresses the octal interpretation and treats the literal as decimal instead. This explains why 00016 is evaluated as 14 and 05016 as 2574.

On the other hand, 08 triggers a compile error because it violates the standard's rule that octal literals cannot contain the digits 8 or 9. The compiler expects a valid octal number, but it finds an invalid character and raises an error.

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