Submitter: Clive Feather (UK)
Submission Date: 2000-04-04
Source:
Reference Document: N/A
Version: 1.4
Date: 2000-11-02 13:50:26
Subject: Source character encodings
Summary
The Standard is clear that the basic source character set need
not have the same encoding as the basic execution character
set, and that while the latter must be all positive, there is
no such requirement on the former:
6.2.5:
[#3] [...] If a member of the basic execution character set is stored in a char object, its value is guaranteed to be positive.
6.10.1:
[#3] [...] Whether the numeric value for these character constants matches the value obtained when an identical character constant occurs in an expression (other than within a #if or #elif directive) is implementation-defined.141) Also, whether a single-character character constant may have a negative value is implementation-defined.
However, there are two problems with this. Firstly, the cited wording in 6.2.5 conflicts with the definition of the basic execution character set:
5.2.1:
[#2] [...] A byte with all bits set to 0, called the null character, shall exist in the basic execution character set; it is used to terminate a character string.in that zero is not positive. Secondly, it is not clear whether a source character constant can have the value zero; in other words, can:
#if !'A'
#error Character A is zero
#endif
reach the #error directive ?
Suggested Technical Corrigendum
Change the cited wording in 6.2.5 to:
#3] [...] If a member of the basic execution character set (other than the null character) is stored in a char object, its value is guaranteed to be positive.and the last part of the cited wording in 6.10.1 to:
[#3] [...] Also, whether a single-character character constant may have a negative value is implementation-defined (nevertheless, it may not be zero).
Committee Response
Regarding the #error directive, 6.10.1
paragraph 3 states:
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>. This includes interpreting character constants, which may involve converting escape sequences into execution character set members. Whether the numeric value for these character constants matches the value obtained when an identical character constant occurs in an expression (other than within a #if or #elif directive) is implementation-defined.The evaluation of the controlling constant expression according to the rules of 6.6 implies that character constants are converted into an execution character set (translation phase 5) just as it also implies that preprocessor tokens representing integer constants are translated into integer constants (translation phase 7).
Thus, all character constants operated upon by #if have been translated to some execution character set. The liberty given by 6.10.1 paragraph 3 that allows the value of character constants to differ in preprocessor versus non-preprocessor expressions exists to allow cross-compilers to use a standalone "native" preprocessor that is unaware of the cross-compiled target and its execution character set.
Thus, in your example, the #error directive can never be reached on a conforming implementation.
Technical Corrigendum
Change the cited wording in 6.2.5 to:
[#3] [...] If a member of the basic execution character set is stored in a char object, its value is guaranteed to be non-negative.