BUILD_BUG_ON(condition) ((void)sizeof(char[1 - (2 * (!(condition)))]))

This is a macro definition in C that is used to ensure that a particular condition is false at compile-time. If the condition is true, the macro will cause a compiler error and prevent the program from being compiled.

The macro works by creating an array of characters with a size that depends on the value of the condition. If the condition is true, the size of the array will be negative, which is not allowed in C. This will generate a compiler error.

On the other hand, if the condition is false, the size of the array will be positive, but the expression inside the sizeof operator will evaluate to 0. The macro then casts this 0 to void to avoid any warnings about unused values.

Overall, the purpose of this macro is to catch errors at compile-time rather than run-time, which can help to improve the reliability and robustness of the code.