！是否定。因此， !! 是否定之否定。最重要的是一个事实，即结果将是一个 INT 。

• ！X 如果 X == 0 是 0！，即！1 ，即 0 。


• ！X 如果 X！= 0 是 !!（！0） ，即 !! 1 ，即！0 ，即 1 。

!! 是常用的，如果你想任何非零值转换为1而被肯定，0仍是0。

事实上， !! NULL == NULL ，因为！== NULL 0！和 0！= =！1 最后！1 == 0 。

因此​​，在一小段code你提到的数组下标将是要么 0 如果括号中的前pression的值 NULL 和 1 其他。

I have encountered the following snippet:

pt->aa[!!(ts->flags & MASK)] = -val;

1. What does !! stand for in c ?
2. Isn't (!!NULL) == NULL ?

! is negation. So !! is negation of negation. What is important is the fact that the result will be an int.

• !!x if x == 0 is !!0, that is !1, that is 0.
• !!x if x != 0 is !!(!0), that is !!1, that is !0, that is 1.

!! is used commonly if you want to convert any non-zero value to 1 while being certain that 0 remains a 0.

And indeed, !!NULL == NULL, since !!NULL == !!0 and !!0 == !1 and finally !1 == 0.

Consequently, in the short piece of code you cited the array subscript will be either 0 if the value of the expression in parenthesis is NULL, and 1 otherwise.