C语言 为什么 Visual Studio 2010 中的 ssize_t 被定义为无符号？
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Why ssize_t in Visual Studio 2010 is defined as unsigned?
提问by Dror Harari
I have a portable program which uses ssize_t under the assumption it is a signed integer. Conceptually it does something like:
我有一个便携式程序，它使用 ssize_t 假设它是一个有符号整数。从概念上讲，它执行以下操作：
#include <stdint.h>
#include <stdio.h>
int main(int argc, char *argv[])
{
size_t size_10 = 10;
size_t size_20 = 20;
ssize_t len_diff;
len_diff = (ssize_t)size_10 - (ssize_t)size_20;
if (len_diff < 0)
printf("negative\n");
else if (len_diff > 0)
printf("positive\n");
else
printf("zero\n");
}
One would expect the program to print 'negative' but instead it prints 'positive'. The reason is easy to see from how ssize_t is defined (in sourceannotations.h):
人们会期望程序打印“负”，但它会打印“正”。从 ssize_t 是如何定义的（在 sourceannotations.h 中）很容易看出原因：
#ifndef _SSIZE_T_DEFINED
#ifdef _WIN64
typedef unsigned __int64 ssize_t;
#else
typedef _W64 unsigned int ssize_t;
#endif
#define _SSIZE_T_DEFINED
#endif
And so, subtracting two unsigned values results in an unsigned value and hence the result.
因此，将两个无符号值相减会产生一个无符号值，从而得到结果。
In older versions of the Windows SDK (e.g. V7.0A) the ssize_t was correctly defined as:
在旧版本的 Windows SDK（例如 V7.0A）中，ssize_t 被正确定义为：
//
// SIZE_T used for counts or ranges which need to span the range of
// of a pointer. SSIZE_T is the signed variation.
//
typedef ULONG_PTR SIZE_T, *PSIZE_T;
typedef LONG_PTR SSIZE_T, *PSSIZE_T;
Can anyone explain this change? Are we supposed to stop using ssize_t on Windows?
谁能解释一下这个变化？我们应该停止在 Windows 上使用 ssize_t 吗？
Update:Based on all the answers, it appears to be a bug in Visual Studio 2010 which includes ssize_t but defined incorrectly. This is a sneaky and nasty bug.
更新：根据所有答案，它似乎是 Visual Studio 2010 中的一个错误，其中包括 ssize_t 但定义不正确。这是一个狡猾和讨厌的错误。
Last Update:This bug has been fixed in VS2012 and VS2016. Also from the comment discussion it appears that this way of calculating len_diff is problematic when the values compared are of different signs when casted to SSIZE_T
最后更新：此错误已在 VS2012 和 VS2016 中修复。同样从评论讨论中可以看出，当比较的值在转换为 SSIZE_T 时具有不同的符号时，这种计算 len_diff 的方法是有问题的
采纳答案by Hans Passant
ssize_tis notstandard C, it is a typedef from Posix. That you found it in a code analysis header for VS2010 probably has something to do with the origin, most code analysis tooling started on Unix. It is removed again in VS2012 and up.
ssize_t是不标准的C，它是从Posix的一个typedef。你在 VS2010 的代码分析头文件中发现它可能与起源有关，大多数代码分析工具都是在 Unix 上开始的。它在 VS2012 及更高版本中再次被删除。
That it is present in the BaseTsd.h SDK file in all capscertainly is not a mistake, Windows supported a Posix subsystem. These typedefs insulate the operating system from compiler implementation details, the basic reason that Windows managed to survive architecture changes, moving from 16 to 32 to 64-bit.
它以全大写形式出现在 BaseTsd.h SDK 文件中当然不是错误，Windows 支持 Posix 子系统。这些 typedef 将操作系统与编译器实现细节隔离开来，这是 Windows 设法在架构变化中幸存下来的基本原因，从 16 位到 32 位再到 64 位。
So the realproblem is that you are trying to compile a Posix program on Windows but without using Posix headers. Trivial to solve, just add your own typedef before the #includes.
所以真正的问题是你试图在 Windows 上编译 Posix 程序，但没有使用 Posix 头文件。解决起来很简单，只需在#includes 之前添加您自己的typedef。
回答by Scott Wardlaw
Would this be a good solution?
这会是一个很好的解决方案吗？
#if defined(_MSC_VER)
#include <BaseTsd.h>
typedef SSIZE_T ssize_t;
#endif
回答by alk
Although it's definitely not conforming the POSIX standard to have ssize_tbeing an unsigned integer, the OP's code risks to also break on systems conforming the POSIX standard.
尽管ssize_t作为无符号整数绝对不符合 POSIX 标准，但 OP 的代码也有在符合 POSIX 标准的系统上中断的风险。
As POSIX defines ssize_tto (at least) just cover the -1and nothing else negativ:
正如POSIX 定义的那样ssize_t（至少）只覆盖 the-1而没有其他否定：
ssize_t
Used for a count of bytes or an error indication.
[...]
The type ssize_tshall be capable of storing values at least in the range [-1, {SSIZE_MAX}].
ssize_t
用于字节计数或错误指示。
[...]
类型ssize_t应能够存储至少在 [-1, {SSIZE_MAX}] 范围内的值。
