关于整型不会小于0警告 怎么处理？

babysnail

RT

  u8 i;

  for (i=31;i>=0;i--)

一般来讲写 i>=0 比较放心，但如果 这样写就会Warning:

pointless comparison of unsigned integer with zero

laujc

编译器是对的,如果你这样写,程序会一直执行,不会结束的

babysnail

哈，是 i-- 哦

babysnail

网上搜到一文：

原地址： http://mtlin.org/archives/3434

拿 unsigned integer 跟 0 做大小比較沒意義嗎？Linus Torvalds 不這麼認為…
Posted on November 24, 2004 by mtlin

有人要動 file system 底下的程式 fcntl.c，原因是要修掉一個 compiler 的 warning: pointless comparison of unsigned integer with zero。但有人說他以前就提出過，卻被 Linus 給回絕。於是這次提出的傢伙就不信邪地再 CC 去問 Linus … XD

但是 Linus Torvalds 卻還是認為有留下檢查的必要，底下是他的回覆：

    No, please don’t.

    The warning is sometimes useful, but when it comes to a construct like

    　　　if (a < 0 || a > X)

    the fact that “a” is unsigned does not make the construct silly. First off, it’s (a) very readable and (b) the type of “a” may not be immediately obvious if it’s a user typedef, for example.

    In fact, the type of “a” might depend on the architecture, or even compiler flags. Think about “char” – which may or may not be signed depending on ABI and things like -funsigned-char.

    In other places, it’s not “unsigned” that is the problem, but the fact that the range of a type is smaller on one architecture than another. So you might have

    　　　inf fn(pid_t a)
    　　　{
    　　　　　　if (a > 0xffff)
    　　　　　　…
    　　　}

    which might warn on an architecture where “pid_t” is just sixteen bits wide. Does that make the code wrong? Hell no.

    IOW, a lot of the gcc warnings are just not valid, and trying to shut gcc

    It’s not even that I will drop the patches, it’s literally that “fixing” the code so that gcc doesn’t complain can be a BUG. We’ve gone through that.

    Linus

原始討論串在此：http://kerneltrap.org/mailarchiv ... 11/21/307540/thread 後頭還有一些其他人的意見，可以參考。

Goselff

这样做，肯定会一直执行，因为是U8类型，所以0-1等于255，还是大于0，程序会死在这里的。
可以这样写：for(i=31;i<=31;i--);这样当从0减到255时，就停止执行了。

babysnail

Goselff 发表于 2012-4-29 23:59
这样做，肯定会一直执行，因为是U8类型，所以0-1等于255，还是大于0，程序会死在这里的。
可以这样写：for( ...

嗯，有道理。

orange-208

for(i=31;i<=31;i--); 有道理！

learner123

语法糖

linus扯了半天，根本原因就是编译器优化问题

while（1）也是一个道理的。