投放ssize_t或size_t

问题描述

在我在项目中使用的源文件中， ssize_t 和 size_t 变量之间有一个比较：

  ssize_t sst; 
 size_t st; 
 
 if（sst == st）{...} 
  

I想要摆脱警告：

 警告：有符号和无符号整数表达式之间的比较
  
 
 
 
但我不知道，应该将哪个变量投给另一个？
  if（（size_t）sst == st）{...} 
  
 
 $ b b 
或
  if（sst ==（ssize_t）st）{...} 
  
什么是更安全，更好，更清洁？感谢
解决方案
这个问题没有正确的答案。有几种可能的答案，取决于您对这些变量可能采用的值的先验。
 
 
 
 
 
 
如果您知道 sst 是非负数，那么您可以安全地将 sst  c $ c> size_t ，因为这不会更改值（顺便说一句，如果你没有任何转换会发生什么）。
 
 
 
 
如果 sst 可能为负值，但您知道 st 永远不会大于 SSIZE_MAX ，那么您可以安全地将 st 转换为 ssize_t  
 
 
 
 
如果 sst 可能为负值， st 可能大于 SSIZE_MAX ，则两个强制转换都不正确;任何一个可以更改值，导致不正确的比较。相反，您将执行以下 if（sst> = 0&&（size_t）sst == st）。
 
 
 
 
 
 
如果您不是绝对确定，前两种情况之一适用，请选择第三个选项，因为它是正确的
 
In source files which I am using in my project, there is a comparison between ssize_t and size_t variables:
ssize_t sst;
size_t st;

if(sst == st){...}
I would like to get rid of the warning:
warning: comparison between signed and unsigned integer expressions
But I am not sure, which variable should I cast to the other?
if((size_t)sst == st){...}
or
if(sst == (ssize_t)st){...}
What is safer, better, cleaner? Thanks
 解决方案 
There is no one right answer to this question.  There are several possible answers, depending on what you know a priori about the values that those variables may take on.
If you know that sst is non-negative, then you can safely cast sst to size_t, as this will not change the value (incidentally, this is what happens if you have no cast at all).
If sst might be negative but you know that st will never be larger than SSIZE_MAX, then you can safely cast st to ssize_t, as this will not change the value.
If sst might be negative, and st might be larger than SSIZE_MAX, then neither cast is correct; either one could change the value, resulting in an incorrect comparison.  Instead, you would do the following if (sst >= 0 && (size_t)sst == st).
If you’re not absolutely certain that one of the first two situations applies, choose the  third option as it is correct in all cases.
                        
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