编译器需要在第一列添加 # 吗？

Question

是否存在广泛使用的 ANSI C 之前的编译器^†，要求 # 位于第一列？

_{† I would accept any compiler on this list. If I can find mention of it in the comp.lang.c Usenet newsgroup in a post dated before 1995, I would accept it.}

K&R C 没有指定# 之前是否允许有空格。摘自《C 编程语言》原文，附录 A 中“C 引用手册”的第 12¶1 节:

The C compiler contains a preprocessor capable of macro substitution, conditional compilation, and inclusion of named files. Lines beginning with # communicate with this preprocessor.

因此，未指定 # 之前是否允许有空格。这意味着如果指令不是从第一列开始，则 ANSI 之前的编译器可能无法编译程序。

在 ISO C(以及之前的 ANSI C)中，C 预处理指令被明确允许以空格作为前缀。在 ANSI C (C-89) 中:

A preprocessing directive consists of a sequence of preprocessing tokens that begins with a # preprocessing token that is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character, and is ended by the next new-line character.

ISO C.2011 有类似的语言，但进一步澄清:

A preprocessing directive consists of a sequence of preprocessing tokens that satisfies the following constraints: The first token in the sequence is a # preprocessing token that (at the start of translation phase 4) is either the first character in the source file (optionally after white space containing no new-line characters) or that follows white space containing at least one new-line character. The last token in the sequence is the first newline character that follows the first token in the sequence.¹⁶⁵⁾ A new-line character ends the preprocessing directive even if it occurs within what would otherwise be an invocation of a function-like macro.
_{165) Thus, preprocessing directives are commonly called ‘‘lines’’. These ‘‘lines’’ have no other syntactic significance, as all white space is equivalent except in certain situations during preprocessing (see the # character string literal creation operator in 6.10.3.2, for example).}

Answer 1

简短回答:是的。

我记得写过这样的东西

#if foo
    /* ... */
#else
#if bar
    /* ... */
#else
 #error "neither foo nor bar specified"
#endif
#endif

这样我曾经使用过的各种 pre-ANSI 编译器就不会提示“无法识别的预处理器指令‘#error’”。这可能是 Ritchie 最初的 pdp11 cc 或 pcc(“可移植 C 编译器”，IIRC，是 80 年代左右 Vax cc 的基础)。这两个编译器(更准确地说，是与这两个编译器一起使用的预处理器)肯定要求 # 位于第一列中。 (实际上，尽管这些编译器非常不同，但它们可能都使用了基本相同的预处理器的不同变体，而在当时预处理器始终是一个单独的程序。)