Re: Declaration of isspace in C/C++ not consistent?

[OndÅej BÃlka <neleai at seznam dot cz>]

On Sun, Jun 02, 2013 at 11:42:11PM +0800, Hongxu Chen wrote:
> Mike Frysinger <vapier@gentoo.org> writes:
> 
> > On Sunday 02 June 2013 10:45:21 Hongxu Chen wrote:
> >> Hi list,
> >> 
> >> Maybe this question is a bit silly, but I just cannot understand why
> >> `isspace` seems not consistent for C and C++(I have put this question in
> >> stackoverflow but no satisfactory answer has been given yet).
> >> 
> >> I am using *clang* analyzer to get the definition information and I know
> >> quite little about the mechanism behind it, so the declaration result
> >> might not be accurate; but I am just confused.
> >> 
> >> For c code like this:
> >> 
> >>     // test.c
> >>     #include <ctype.h>
> >>     int main(int argc, char *argv[]) {
> >>       isspace('a');
> >>       return 0;
> >>     }
> >> 
> >> clang reports below as the declaration of isspace:
> >> 
> >>     # define isspace(c)	__isctype((c), _ISspace)  // LINE 207 in
> >> /usr/include/ctype.h
> >> 
> >> and when for this snippet:
> >> 
> >>     // test.cpp
> >>     #include <cctype>
> >>     int main() {
> >>       std::isspace('t');
> >>       return 0;
> >>     }
> >> 
> >> clang reports the declaration here:
> >> 
> >>     __exctype (isspace);  // LINE 120 in /usr/include/ctype.h
> >>     // #define	__exctype(name)	extern int name (int) __THROW
> >> 
> >> So why should there be such a difference?
> >
> > glibc provides ctype.h which follows POSIX:
> > http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/ctype.h.html
> > we provide both real funcs and macros for each because the standard allows it, 
> > and the macro ends up producing better code at runtime.
> 
> You mean that the c code is a macro implementation and it generates
> better runtime binary while C++ code uses the function one? By saying
> macro you mean `__ctype_b_loc'?(Actually I don't know what this symbol
> does)
> 
> # define __isctype(c, type) \
>   ((*__ctype_b_loc ())[(int) (c)] & (unsigned short int) type)
> 
> Also there is another implementation called `__isctype_f', which is
> defined as:
> 
> # define __isctype_f(type) \
>   __extern_inline int							      \
>   is##type (int __c) __THROW						      \
>   {									      \
>     return (*__ctype_b_loc ())[(int) (__c)] & (unsigned short int) _IS##type; \
>   }
> 
> Then what's this supposed to be doing?
> 
Replace function call by simple table lookup where table is 1 for character
inside class and 0 otherwise.