In the sqrt function of most languages (though here I'm mostly interested in C and Haskell), are there any guarantees that the square root of a perfect square will be returned exactly? For example, if I do sqrt(81.0) == 9.0, is that safe or is there a chance that sqrt will return 8.999999998 or 9.00000003?

If numerical precision is not guaranteed, what would be the preferred way to check that a number is a perfect square? Take the square root, get the floor and the ceiling and make sure they square back to the original number?

Thank you!

c haskell floating-point
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