The C89 standard divides translation into eight ``phases''. C99 keeps the same phases and just adds Unicode and the like. I will quote an old C99 draft below (with some small edits for text representation). Read footnote 5 carefully, and then consider the fact that ``#if'' happens in phase 4, but pp-tokens are not converted to regular tokens until phase 7. Since the ``sizeof'' keyword is a regular token rather than a pp-token, that makes it impossible for ``#if'' to recognize it. In fact, the line:
# if ((sizeof aszColors / sizeof *aszColors) != COLOR_NB)
must (in the absence of various #defines) behave as if it read:
# if ((0 0 / 0 *0) != 0)
so a diagnostic is required (because ``0 0'' is a syntax error).
(Once the required diagnostic has been emitted, of course, a compiler is free to do anything it likes, including go back and discover that the ``0''s came from ``sizeof'', work out the sizes, examine the enumeration constant, and so on. For a compiler to do this, however, its phase-4 code must be able to peek at phase-7 data, such as enumeration constant values and array sizes. That in turns requires an integrated compiler -- preprocessing cannot occur in a separate program the way it does in, e.g., gcc. That in turn makes using multiple CPUs, e.g., via ``cpp | cc1'', much harder -- so it may be a good idea to retain this sequencing, and footnote 5.)
[excerpt from c9x.n2620.txt below]
Working Draft, 1997-11-21, WG14/N794 J11/97-158
22.214.171.124 Translation phases
[#1] The precedence among the syntax rules of translation is specified by the following phases.[footnote 5]
- Physical source file multibyte characters are mapped to the source character set (introducing new-line characters for end-of-line indicators) if necessary. Any multibyte source file character not in the basic source character set is replaced by the universal-character-name that designates that multibyte character. Then, trigraph sequences are replaced by corresponding single-character internal representations.
- Each instance of a backslash character immediately followed by a newline character is deleted, splicing physical source lines to form logical source lines. Only the last backslash on any physical source line shall be eligible for being part of such a splice. A source file that is not empty shall end in a new-line character, which shall not be immediately preceded by a backslash character before any such splicing takes place.
- The source file is decomposed into preprocessing tokens and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or comment. Each comment is replaced by one space character. New-line characters are retained. Whether each nonempty sequence of white-space characters other than new-line is retained or replaced by one space character is implementation-defined.
- Preprocessing directives are executed, macro invocations are expanded, and pragma unary operator expressions are executed. If a character sequence that matches the syntax of a universal-character-name is produced by token concatenation (126.96.36.199), the behavior is undefined. A #include preprocessing directive causes the named header or source file to be processed from phase 1 through phase 4, recursively. All preprocessing directives are then deleted.
- Each source character set member, escape sequence, and universal-character-name in character constants and string literals is converted to a member of the execution character set.
- Adjacent character string literal tokens are concatenated and adjacent wide string literal tokens are concatenated.
- White-space characters separating tokens are no longer significant. Each preprocessing token is converted into a token. The resulting tokens are syntactically and semantically analyzed and translated as a translation unit.
- All external object and function references are resolved. Library components are linked to satisfy external references to functions and objects not defined in the current translation. All such translator output is collected into a program image which contains information needed for execution in its execution environment.
__________[footnotes 5 through 7]
- Implementations must behave as if these separate phases occur, even though many are typically folded together in practice.
- The process of handling extended characters is specified in terms of mapping to an encoding that uses only the basic source character set, and, in the case of character literals and strings, further mapping to the execution character set. In practical terms, however, any internal encoding may be used, so long as an actual extended character encountered in the input, and the same extended character expressed in the input as a universal-character-name (i.e., using the \U or \u notation), are handled equivalently.
- As described in 6.1, the process of dividing a source file's characters into preprocessing tokens is context-dependent. For example, see the handling of < within a #include preprocessing directive.
In-Real-Life: Chris Torek, Berkeley Software Design Inc
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