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

[Hongxu Chen <leftcopy dot chx at gmail dot com>]

Mike Frysinger <vapier@gentoo.org> writes:

> On Sunday 02 June 2013 20:40:35 Hongxu Chen wrote:
>> OndÅej BÃlka <neleai@seznam.cz> writes:
>> > 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.
>> 
>> Would you please tell some more details? You mean __isctype_f
>> implementation would actually lookup a _ISspacetype table and find
>> whether `c' is 1 in the table? Or you are talking about __isctype?
>
> __ctype_b_loc is a function call that returns the address of the data table.  
> then the char is used an index into that table.

Got it, Many thanks!

> -mike

-- 
Regards,
Hongxu Chen