Submitter: Ivan A. Kosarev (Unicals Group, RU)
Submission Date: 2005-04-19
Reference Document: N/A
Subject: Tokenization obscurities
188.8.131.52 #2 ("Translation phases") says:
[#2] ...2. Each instance of a backslash character (\) immediately followed by a new-line 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.
3. The source file is decomposed into preprocessing tokens6) and sequences of white-space characters (including comments). A source file shall not end in a partial preprocessing token or in a partial 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.
Assuming there is a non-empty source file legally ending with a new-line character, what are examples of such partial preprocessing tokens that could end the file? And, generally, what the partial preprocessing tokens are?
184.108.40.206 ("Character constants") says:
[#3] The single-quote ', the double-quote ", the question-mark ?, the backslash \, and arbitrary integer values are representable according to the following table of escape sequences...
[#5] The octal digits that follow the backslash in an octal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant...
[#6] The hexadecimal digits that follow the backslash and the letter x in a hexadecimal escape sequence are taken to be part of the construction of a single character for an integer character constant or of a single wide character for a wide character constant...
[#8] In addition, characters not in the basic character set are representable by universal character names and certain nongraphic characters are representable by escape sequences consisting of the backslash \ followed by a lowercase letter: \a, \b, \f, \n, \r, \t, and \v.64)
64) The semantics of these characters were discussed in 5.2.2. If any other character follows a backslash, the result is not a token and a diagnostic is required. See ''future language directions'' (6.11.4).
6.4 #3 ("Lexical elements") says:
[#3] ...The categories of preprocessing tokens are: header names, identifiers, preprocessing numbers, character constants, string literals, punctuators, and single non-white-space characters that do not lexically match the other preprocessing token categories.58) If a ' or a " character matches the last category, the behavior is undefined.
What in the formal content of the standard says that if any other character follows a backslash, there should be a diagnostic? Does such a case causes undefined behaviour? Furthermore, if a character sequence that is coming just after a double quote " (that is not terminating a string literal) begins with, say, \l and the result of the tokenization is not a token, then what the result is?
6.10.9 #2 ("Pragma operator") gives the following example:
[#2] EXAMPLE A directive of the form:#pragma listing on "..\listing.dir"
can also be expressed as:_Pragma ( "listing on \"..\\listing.dir\"" )
The previous summary says that if there an unknown escape sequence encountered during tokenization of a character constant or string literal, then the result is not token. The question is whether the example above is well-defined.
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