Defect Report #170

Submission Date: 16 Oct 95
Submittor: BSI
Source: Clive D.W. Feather
Question
Submitted to BSI by Clive D.W. Feather [email protected] .
In this Defect Report, identifiers lexically identical to those declared in standard headers refer to the identifiers declared in those standard headers, whether or not the header is explicitly mentioned.
This Defect Report has been prepared with considerable help from Mark Brader, Jutta Degener, Ronald Guilmette, and a person whose employment conditions require anonymity. However, except where stated, opinions expressed or implied should not be assumed to be those of any person other than myself.
Defect Report UK 018: Operators and punctuators
The description of operators and punctuators is confusing, and the constraints are contradictory.
Subclause 6.1.5 Constraints reads:
The operators [ ], ( ), and ? : shall occur in pairs, possibly separated by expressions. The operators # and ## shall occur in macro-defining preprocessing directives only.
Subclause 6.1.6 Constraints reads:
The punctuators [ ], ( ), and { } shall occur (after translation phase 4) in pairs, possibly separated by expressions, declarations, or statements. The punctuator # shall occur in preprocessing directives only.
Consider the code:
#define STR(x) #x STR ({) /* Line A */ STR (:) /* Line B */ STR ([) /* Line C */ STR (#) /* Line D */
Line A appears to be strictly conforming, since the first sentence of the constraint of subclause 6.1.6 does not apply during translation phase 4. Line B violates the constraint of subclause 6.1.5. The interpretation of line C depends on whether the [ is an operator or a punctuator!
Line D violates both constraints, but again which one depends on whether it is an operator or a punctuator, something which is not made clear in the C Standard.
Assuming that the intent was for line B to be strictly conforming, and that (after translation phase 4) was inadvertently omitted from subclause 6.1.5, the first sentence of each of these Constraints is nugatory, as any program that violates these constraints also violates a syntax rule elsewhere in clause 6. The remaining sentences would be better expressed as part of subclause 6.8. It is also arguable that the concepts of operator and punctuator are better merged at the syntactic level, and separated out only at the semantic level.
Suggested Technical Corrigendum
Delete the Constraints of subclauses 6.1.5 and 6.1.6. Add the following constraint to 6.8:
A # preprocessing token shall only occur within a replacement-list or when permitted by the syntax rules of this subclause. A ## preprocessing token shall only occur within a replacement-list.
Add to the end of the Constraints of subclause 6.1, just before the full stop:
, and shall not be # or ##
Alternative Suggested Technical Corrigendum
In subclause 6.1 syntax, delete both occurences of operator and replace the second occurence of punctuator by pp-punctuator.
Delete subclauses 6.1.5 and 6.1.6, and replace them by the following:
6.1.5 Punctuators
Syntax:
pp-punctuator: punctuator pp-only-punctuator pp-only-punctuator: one of # ## defined punctuator: [ ] ( ) { } . - ++ -- & * + - ~ ! sizeof / % = = == != ^ | && || ? : , : ; ... = *= /= %= += -= = = &= ^= |=
Semantics:
A punctuator is a symbol that has independent syntactic and semantic significance. Depending on context, some punctuators may specify an operation to be performed (an evaluation) that yields a value, or yields a designator, or produces a side-effect, or a combination thereof; in that context, the punctuator is known as an operator. An operand is an entity on which an operator acts.
Add the following constraint to 6.8:
A # preprocessing token shall only occur within a replacement-list or when permitted by the syntax rules of this subclause. A ## preprocessing token shall only occur within a replacement-list.
Response
This is a work in progress item.
General feeling is that this should be cleaned up for C9X along the lines of C++ pp-punctuator grammar.
Suggested response is to add words to subclause 6.1.5 along the lines, shall occur in pairs within expressions...
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