It may sound obvious, but just out of curiosity: is program translation direction well-defined (i.e. top-to-bottom, left-to-right)? Is it explicitly defined in the standard?
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A source file, and the translation unit that results from including headers via the #include directive, is implicitly a sequence of characters with one dimension. I do not see this explicitly stated in the standard, but there are numerous references to this dimension in the standard, referring to characters “followed by” (C 2018 5.1.1.2 1 1) or “before” (6.10.2 5) other characters, scopes begin “after” the appearance of a tag or declarator (6.2.1 7), and so on.
A compiler is free to read and compute with the parts of translation units in any order it wants, but the meaning of the translation unit is defined in terms of this start-to-finish order.
There is no “up” or “down” between lines. Lines are meaningful in certain parts of C translation, such as the fact that a preprocessing directive ends with a new-line character. However, there is no relationship defined between the same columns on different lines, so the standard does not define anything meaningful for going up or down by lines beyond the fact this means going back or forth in the stream of characters by some amount.
The standard does allow that source files might be composed of lines of text, perhaps with Hollerith cards (punched cards) in which each individual card is a line or with fixed-length-record files in which each record is a fixed number of bytes (such as 80) and there are no new-line characters physically recorded in the file. (Implicitly, each line of text ends after 80 characters.) The standard treats these files as if a new-line character were inserted at the end of the line (5.2.1 3), thus effectively converting the file to a single stream of characters.
于 2020-11-10T19:56:23.863 回答