What Happens When You Use the Modulo Operator with Zero?

Addressing the Undefined Behavior of Modulo by Zero

In programming, the modulo operator (%) is employed to determine the remainder when dividing one number by another. However, when used with a divisor of 0, the expression X % 0 becomes invalid and invokes undefined behavior.

This behavior stems from the mathematical definition of modulo, where division by zero is undefined. Logically, some might expect X % 0 to equal X, as the remainder of dividing X by zero should be X. However, the C Standard explicitly states that modulo by zero results in undefined behavior (§5.6/4):

[...] If the second operand of / or % is zero the behavior is undefined; [...]

Thus, the expressions:

X / 0; // Undefined behavior
X % 0; // Undefined behavior

are considered invalid in C . It's crucial to avoid such expressions in your code to prevent unpredictable and erroneous behavior.

Additionally, it's important to note that the sign of the remainder when one operand is negative is implementation-defined (§5.6/4):

[...] If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined.

Therefore, expressions like -5 % 2 may not always return the expected result, and the behavior may vary depending on the implementation.

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