Date: Fri, 11 Oct 2002 18:44:53 +1000 (EST) From: Bruce Evans <bde@zeta.org.au> To: Terry Lambert <tlambert2@mindspring.com> Cc: Craig Rodrigues <rodrigc@attbi.com>, <freebsd-standards@FreeBSD.ORG>, <freebsd-hackers@FreeBSD.ORG> Subject: Re: Problem detecting POSIX symbolic constants Message-ID: <20021011183032.D12170-100000@gamplex.bde.org> In-Reply-To: <3DA5D5BD.1DAFF031@mindspring.com>
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On Thu, 10 Oct 2002, Terry Lambert wrote: > Bruce Evans wrote: > > In Standard C, this is equivalent to the non-verbose version: > > > > #if _POSIX_REALTIME_SIGNALS != -1 > > ... > > #endif > > > > since if _POSIX_REALTIME_SIGNALS is not defined then it is equivalent to > > 0 in cpp expressions. The problem cases are if _POSIX_REALTIME_SIGNALS > > is defined to <empty> or <garbage>, but these are not permitted in > > POSIX.1-2001. These cases were permitted for many feature test macros > > in POSIX.1-1990. > > I know it's not fashionable to write code that's portable to > compilers other than GCC, but even if FreeBSD is going to ignore > portability for it's own source code, it's probably unreasonable > to expect ACE to ignore portability for theirs. Undefined symbols being 0 in cpp in expressions was in early C compilers if not the original one. Consult your archives for a posting 10-15 years or so ago by dmr in comp.std.c about him checking this in his archives. > > > Sigh. Why did the POSIX guys do this? :( > > > > Perhaps because they wanted you to use sysconf() instead of these mistakes. > > Actually, they didn't do this. _POSIX_REALTIME_SIGNALS is specified to > > have value 0, -1 or 200xxxL (draft 7 says xxx; I think the final standard > > says 112). > > This can't be the case; specifically, the sysconf() test will > only work at runtime, which means that the symbols had to be > there and resolvable at link time. Symbols can be resolved at runtime using dlopen(), etc. They would only actually be available on systems where sysconf() says that they are. > > > BTW, I think that: > > > #if defined(_POSIX_REALTIME_SIGNALS) && (_POSIX_REALTIME_SIGNALS != -1 ) > > > > > > should suffice, but I'll double-check with one of my portability gurus > > > to see if that is OK for ACE. > > > > This is variant of the above verbose version. It requires slightly more > > modern compilers, so it might fail for some 20-year old pre-Standard C > > compilers instead of only for some 25-year old ones. > > Uh, the 1990 standard, which allowed "#if" is only 12 years old. #if is in K&R1 (1978). Bruce To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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