Defect Report #265

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Submitter: UK C Panel
Submission Date: 2001-09-07
Source: Clive D.W. Feather <clive@demon.net>
Version: 1.2
Date: 2002-05-15
Subject: preprocessor arithmetic

Problem

Assume that both compile-time and run-time arithmetic have 2's complement, no trap representations, 8/16/32/48/64 bit integer types. Consider the code:

   #if -0xFFFFFFFF < 0

Is this expression true or false ? 6.10.1#3 reads, in part:

and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6, except that all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>.

Does the "except" wording apply to the conversion to a token, or only to the evaluation of the expression ? If the former, then 0xFFFFFFFF can be represented in an int (intmax_t), it has a signed type, and the expression is true. If the latter, 0xFFFFFFFF cannot be represented in an int but can be represented in an unsigned int, so it has unsigned type and the expression is false.

I believe that the former was intended, with the preprocessor only having to consider one pair of integer types.

Suggested Technical Corrigendum

Change the cited text to:

and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6. For the purposes of the conversion and evaluation all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>.

(bold type shows the changed words)

Add a footnote reference to the end of this text, and add the footnote:

140a Thus on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant 0x8000 is signed within a #if expression even though it is unsigned in translation phase 7.


Proposed Technical Corrigendum

Change 6.10.1#3 to read:

...

and then each preprocessing token is converted into a token. The resulting tokens compose the controlling constant expression which is evaluated according to the rules of 6.6. For the purposes of this token conversion and evaluation all signed integer types and all unsigned integer types act as if they have the same representation as, respectively, the types intmax_t and uintmax_t defined in the header <stdint.h>.

Add a footnote to the end of 6.10.1#3 to read:

Thus on an implementation where INT_MAX is 0x7FFF and UINT_MAX is 0xFFFF, the constant 0x8000 is signed and positive within a #if expression even though it is unsigned in translation phase 7.


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