NULL 宏何时不为 0? [英] When was the NULL macro not 0?

问题描述

我依稀记得几年前读过这篇文章，但我在网上找不到任何参考资料.

I vaguely remember reading about this a couple of years ago, but I can't find any reference on the net.

你能举个例子说明 NULL 宏没有扩展为 0 吗?

Can you give me an example where the NULL macro didn't expand to 0?

为清晰起见进行今天它扩展为 ((void *)0)、(0) 或 (0L).然而，有些架构早已被遗忘，这不是真的，NULL 扩展到不同的地址.类似的东西

Edit for clarity: Today it expands to either ((void *)0), (0), or (0L). However, there were architectures long forgotten where this wasn't true, and NULL expanded to a different address. Something like

#ifdef UNIVAC
     #define NULL (0xffff)
#endif

我正在寻找此类机器的示例.

I'm looking for an example of such a machine.

更新以解决问题:

我不是在当前标准的上下文中提出这个问题，也不是因为我的错误术语而使人们感到不安.但是，我的假设得到了公认的答案的证实:

I didn't mean this question in the context of current standards, or to upset people with my incorrect terminology. However, my assumptions were confirmed by the accepted answer:

后来的模型使用了 [blah]，显然是为了弥补所有现存的编写不当的 C 代码，这些代码做出了不正确的假设.

Later models used [blah], evidently as a sop to all the extant poorly-written C code which made incorrect assumptions.

有关当前标准中空指针的讨论，请参阅这个问题.

For a discussion about null pointers in the current standard, see this question.

推荐答案

C 常见问题解答提供了一些具有非 0 NULL 表示的历史机器示例.

The C FAQ has some examples of historical machines with non-0 NULL representations.

来自 C 常见问题列表，问题 5.17:

问:说真的，有没有真正使用过非零空值的实际机器指针，或指向不同的指针的不同表示类型?

Q: Seriously, have any actual machines really used nonzero null pointers, or different representations for pointers to different types?

A:Prime 50 系列使用段 07777，偏移量为 0 表示空指针，至少对于 PL/I.后来的模型使用段 0，偏移量 0 用于C 中的空指针，需要新指令，例如 TCNP(测试C Null Pointer)，显然是对 [footnote] 所有现存内容的补充写得不好的 C 代码做出了错误的假设.年长的，字寻址的 Prime 机器也因需要更大的内存而臭名昭著字节指针(char * 的)比字指针(int * 的).

A: The Prime 50 series used segment 07777, offset 0 for the null pointer, at least for PL/I. Later models used segment 0, offset 0 for null pointers in C, necessitating new instructions such as TCNP (Test C Null Pointer), evidently as a sop to [footnote] all the extant poorly-written C code which made incorrect assumptions. Older, word-addressed Prime machines were also notorious for requiring larger byte pointers (char *'s) than word pointers (int *'s).

Data General 的 Eclipse MV 系列在架构上具有三个支持的指针格式(字、字节和位指针)，其中两种被 C 编译器使用:char * 和 void * 的字节指针，以及 word其他一切的指针.期间由于历史原因32 位 MV 线从 16 位 Nova 线演变而来，字指针和字节指针具有偏移、间接和环字中不同位置的保护位.通过不匹配指向函数的指针格式导致保护错误.最终，MVC 编译器添加了许多兼容性选项来尝试处理具有指针类型不匹配错误的代码.

The Eclipse MV series from Data General has three architecturally supported pointer formats (word, byte, and bit pointers), two of which are used by C compilers: byte pointers for char * and void *, and word pointers for everything else. For historical reasons during the evolution of the 32-bit MV line from the 16-bit Nova line, word pointers and byte pointers had the offset, indirection, and ring protection bits in different places in the word. Passing a mismatched pointer format to a function resulted in protection faults. Eventually, the MV C compiler added many compatibility options to try to deal with code that had pointer type mismatch errors.

某些 Honeywell-Bull 大型机使用位模式 06000(内部)空指针.

Some Honeywell-Bull mainframes use the bit pattern 06000 for (internal) null pointers.

CDC Cyber​​ 180 系列具有由环组成的 48 位指针，段和偏移量.大多数用户(在环 11 中)的空指针为0xB00000000000.在旧的 CDC 补码机器上很常见使用全一位字作为各种数据的特殊标志，包括无效地址.

The CDC Cyber 180 Series has 48-bit pointers consisting of a ring, segment, and offset. Most users (in ring 11) have null pointers of 0xB00000000000. It was common on old CDC ones-complement machines to use an all-one-bits word as a special flag for all kinds of data, including invalid addresses.

旧的 HP 3000 系列使用不同的字节寻址方案地址而不是字地址；像上面的几台机器因此它对 char * 和 void * 使用不同的表示指针而不是其他指针.

The old HP 3000 series uses a different addressing scheme for byte addresses than for word addresses; like several of the machines above it therefore uses different representations for char * and void * pointers than for other pointers.

Symbolics Lisp Machine，一个标记的架构，甚至没有传统的数字指针；它使用 对(基本上是一个不存在的 句柄)作为 C 空指针.

The Symbolics Lisp Machine, a tagged architecture, does not even have conventional numeric pointers; it uses the pair <NIL, 0> (basically a nonexistent <object, offset> handle) as a C null pointer.

根据使用的内存模型"，8086 系列处理器(PC兼容)可以使用 16 位数据指针和 32 位函数指针，反之亦然.

Depending on the "memory model" in use, 8086-family processors (PC compatibles) may use 16-bit data pointers and 32-bit function pointers, or vice versa.

一些 64 位 Cray 机器在低 48 位表示 int *单词;char * 另外使用一些高 16 位来表示字内的字节地址.

Some 64-bit Cray machines represent int * in the lower 48 bits of a word; char * additionally uses some of the upper 16 bits to indicate a byte address within a word.

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