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Re: SCM_POSFIXABLE(-1) fails with MSVC++ 6.0
From: |
Lars J. Aas |
Subject: |
Re: SCM_POSFIXABLE(-1) fails with MSVC++ 6.0 |
Date: |
Sun, 25 Aug 2002 15:27:27 +0200 |
User-agent: |
Mutt/1.3.24i |
Dirk Herrmann <address@hidden> wrote:
: On Fri, 23 Aug 2002, Lars J. Aas wrote:
:
: > Lars J. Aas <address@hidden> wrote:
: > : Changing the alternative definition of SCM_LONG_BITS to cast the value
: > : to long like this works...
: > :
: > : # define SCM_LONG_BIT ((long)(SCM_CHAR_BIT * sizeof (long) / sizeof
(char)))
: >
: > Come to think of it, the cast is better placed on the inside so it's more
: > evident what is being done (stopping the unsigned-ness of the sizeof()-
: > operations to propagate incorrectly out through the macro invokation) so
: > it's not removed later by someone not knowing about this compiler bug.
: >
: > # define SCM_LONG_BIT (SCM_CHAR_BIT * ((long) (sizeof (long) / sizeof
(char))))
: >
: > or s/long/int/ on the cast?
: >
: > BTW, is anyone reading these mails? ;)
: >
: > Lars J
:
: I am not sure I understand the problem here: is the failure of testing
: -1 <= sizeof(long) a compiler bug, or is the strange behaviour defined
: in the C language?
I haven't studied the C specification under a microscope, but it has got
to be a compiler bug. What I guess is happening is that sizeof() is defined
to return an unsigned value, and consequently -1 is interpreted as an unsigned
value. Exactly how data type qualifiers are supposed to propagate over unary
tests isn't something I know for sure, but gcc handles it as one would expect.
BTW, I don't think the define makes sense at all. I believe it's part of
the C standard that sizeof(char) should always return 1, whether the char type
is the size of a byte or not. Besides, if it had returned the number of bytes
a char is - why would one want to divide by it? Multiply perhaps, but that
would only be if sizeof(long) returned the number of _chars_ in a long, and
sizeof(char) returned the number of _bytes_ in a char, which wouldn't make
sense either. So there's some stuff I think is wrong there, but I guess that
part of the #if/else isn't that often used, and when it's used, it doesn't
affect the result since it's a divide-by-one thing and char is always a byte
long.
I propose that the define is changed to:
# define SCM_LONG_BIT (SCM_CHAR_BIT * ((int) sizeof(long)))
Lars J