为什么在WinSock2.h(c ++)中将无效套接字定义为〜0? [英] Why is invalid socket defined as ~0 in WinSock2.h (c++)?

问题描述

在WinSock2.h中，无效的套接字和套接字错误定义为这些吗?有什么意义吗?

#define INVALID_SOCKET  (SOCKET)(~0)
#define SOCKET_ERROR            (-1)

解决方案

在二进制补码系统(Windows始终是二进制补码)上，~0等于-1，因此对编译器没有意义. /p>

对读者可能有重要意义:~0强调这是一个所有位都已设置的值，而-1强调这是一个小于0的值.

在旁边:

在不是二进制补码的系统上，并且假设SOCKET是无符号类型，通常写(SOCKET)(~0)是错误.原因是在这样的系统上，~0不代表值-1，它是INT_MIN，负零或陷阱表示之一.因此，它不一定会转换为类型SOCKET作为所有位为零的值，而是会转换为INT_MAX+2，0或善意-什么(也许是将所有位都设置为零的值).

因此，通常应使用-1初始化无符号类型，以获取设置了所有位的值.如果您知道要处理的是无符号类型，则可以 使用UINT_MAX或~0UL或类似名称.但这不值得，因为-1适用于所有无符号类型.

In WinSock2.h, the invalid socket and socket error are defined as these? Is there any significance to this?

#define INVALID_SOCKET  (SOCKET)(~0)
#define SOCKET_ERROR            (-1)

解决方案

On a two's complement system (and Windows is always two's complement), ~0 is equal to -1, so there's no significance to the compiler.

There may be a significance to the reader: ~0 emphasizes that it's a value with all bits set, whereas -1 emphasizes that it's a value 1 less than 0.

Aside:

On a system which is not two's complement, and assuming that SOCKET is an unsigned type, it is generally wrong to write (SOCKET)(~0). The reason is that on such systems, ~0 does not represent the value -1, it's one of INT_MIN, negative zero, or a trap representation. Hence it will not necessarily convert to type SOCKET as the value with all bits zero, rather it will convert as INT_MAX+2, 0, or goodness-knows-what (perhaps the value with all bits set).

So generally you should initialize unsigned types with -1 to get the value with all bits set. You could use UINT_MAX, or ~0UL, or similar, if you know which unsigned type you're dealing with. But it's not worth it, because -1 works for all unsigned types.

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