what does x =!x means?

Question

While readinf MuPDF comes across the source code line

x = !x;

Want to know what does the above code do?

While compilation is also gives error as

assuming signed overflow does not occur when simplifying 
condition to constant [-Werror=strict-overflow ]

Answer 1

!x will evaluate to either 0 or 1, depending on the value of x.

• If x is 0, it evaluates to 1.
• Otherwise, it evaluates to 0.