在第一列需要 # 的编译器？

Question

是否有广泛使用的 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 # 之前是否允许空格。摘自原文 The C Programming Language，附录 A 中 "C Reference Manual" 的§12¶1：

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，未指定是否允许在 # 之前使用空格。这意味着如果指令没有从第一列开始，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使用空格作为前缀。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，但进一步阐明：

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

简答：是。

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 编译器就不会抱怨 "unrecognized preprocessor directive '#error'"。这本来是 Ritchie 的原始 cc 用于 pdp11 或 pcc（"portable C compiler"，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——绝对要求 # 位于第一列。（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，在那些日子里它总是一个单独的程序。）

在第一列需要 # 的编译器？

Compilers that required # on the first column?

c

c-preprocessor

kernighan-and-ritchie