The Standard C Library 的一个问题

baozhaomail

p79的问题, #if (-1 + 0x0) >> 1 > 0x7fff 不知是如何确定大于16位的,我用VC测试#if (-1 + 0x0) >> 1 > 0x7fff
结果为假.


The presence of <limits .h> is also designed to discourage an old programming
trick that is extremely nonportable. Some programs attempted
to test the properties of the execution environment by writing #if directives:

#if (-1 + 0x0) >> 1 > 0x7fff
/* must have ints greater than 16 bits */
...
#endif

This code assumes that whatever arithmetic the preprocessor performs
is the same as what occurs in the execution environment. Those who deal
heavily with cross compilers know well that the translation environment
can differ markedly from the execution environment. For tricks like this
one to work, theC Standard would have to require that the translator mimic
the execution environment very closely. And translator families with a
common front end would have to adapt translation-time arithmetic to suit
each environment.

starwing83

这是有符号数。右移是否是算术右移是符号实现定义的。

应该这样：

#if -1u >>1 == 0x7fff

但是问题是，cpp不一定支持u后缀（我也不知道），而且cpp的integer-literal也不一定会和C的int类型相同。最简单的方法是

#if INT_MAX > 32770

稍微大一点点，免得出意外……

人家原文就指出这个方法了，即limits.h这个标准头文件，现在这个时代没有这个头文件的编译器已经找不到，所以放心地用吧。

baozhaomail

我知道应该使用limits.h这个标准头文件.

我的具体问题在于原来的背景下这个表达式怎么起作用的,我在VC 6.0 不起作用 .

  -1u >>1 == 0x7fff  对我来说不是问题, 但是原来加0x0的动机是什么.  (-1 + 0x0) 为什么成了unsigned类型

starwing83

The presence of <limits .h> is also designed to discourage an old programming
trick that is extremely nonportable. Some programs attempted
to test the properties of the execution environment by writing #if directives:

说了这种方法极端不可移植，所以现在不成立也正常。

很有可能是integer promotion之类的，有可能古代某些编译器的16进制常量是unsigned，这很自然吧？根据而且也许他们规定了整型提升（当然和现在定义的不一样），int+unsigned == unsigned，最终就成了unsigned的-1了。当时可能是没有数字后缀才这样，毕竟这玩意儿是标准提出来的，出现年代较晚。

但是这个规则现在肯定没有了。所以你试不出来。

baozhaomail

似乎有道理,谢谢,我再查证一下

starwing83

回复 6# pmerofc


    如果int是16位，那么最大就应该是2^15-1即32767，为了安全起见加了3，就是32770，不过也许加2就行了，Lua的头文件里面用的是：


/* avoid overflows in comparison */
#if INT_MAX-20 < 32760                /* { */
#define LUAI_BITSINT        16
#elif INT_MAX > 2147483640L        /* }{ */
/* int has at least 32 bits */
#define LUAI_BITSINT        32
#else                                /* }{ */
#error "you must define LUA_BITSINT with number of bits in an integer"
#endif                                /* } */


额……这个更保险一点，防止32770超过了int的长度导致不测……恩，Lua的质量是没的说的~

cdtits

呵呵....