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ctype

I wonder how this code works. ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])


#ifndef _CTYPE_H_
#define _CTYPE_H_

#include "_ansi.h"

_BEGIN_STD_C

int _EXFUN(isalnum, (int __c));
int _EXFUN(isalpha, (int __c));
int _EXFUN(iscntrl, (int __c));
int _EXFUN(isdigit, (int __c));
int _EXFUN(isgraph, (int __c));
int _EXFUN(islower, (int __c));
int _EXFUN(isprint, (int __c));
int _EXFUN(ispunct, (int __c));
int _EXFUN(isspace, (int __c));
int _EXFUN(isupper, (int __c));
int _EXFUN(isxdigit,(int __c));
int _EXFUN(tolower, (int __c));
int _EXFUN(toupper, (int __c));

#if !defined(__STRICT_ANSI__) || defined(__cplusplus) || __STDC_VERSION__ >= 199901L
int _EXFUN(isblank, (int __c));
#endif

#ifndef __STRICT_ANSI__
int _EXFUN(isascii, (int __c));
int _EXFUN(toascii, (int __c));
#define _tolower(__c) ((unsigned char)(__c) - 'A' + 'a')
#define _toupper(__c) ((unsigned char)(__c) - 'a' + 'A')
#endif

#define _U  01
#define _L  02
#define _N  04
#define _S  010
#define _P  020
#define _C  040
#define _X  0100
#define _B  0200

#ifndef _MB_CAPABLE
_CONST
#endif
extern  __IMPORT char   *__ctype_ptr__;

#ifndef __cplusplus
/* These macros are intentionally written in a manner that will trigger
   a gcc -Wall warning if the user mistakenly passes a 'char' instead
   of an int containing an 'unsigned char'.  Note that the sizeof will
   always be 1, which is what we want for mapping EOF to __ctype_ptr__[0];
   the use of a raw index inside the sizeof triggers the gcc warning if
   __c was of type char, and sizeof masks side effects of the extra __c.
   Meanwhile, the real index to __ctype_ptr__+1 must be cast to int,
   since isalpha(0x100000001LL) must equal isalpha(1), rather than being
   an out-of-bounds reference on a 64-bit machine.  */
#define __ctype_lookup(__c) ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])

#define isalpha(__c)    (__ctype_lookup(__c)&(_U|_L))
#define isupper(__c)    ((__ctype_lookup(__c)&(_U|_L))==_U)
#define islower(__c)    ((__ctype_lookup(__c)&(_U|_L))==_L)
#define isdigit(__c)    (__ctype_lookup(__c)&_N)
#define isxdigit(__c)   (__ctype_lookup(__c)&(_X|_N))
#define isspace(__c)    (__ctype_lookup(__c)&_S)
#define ispunct(__c)    (__ctype_lookup(__c)&_P)
#define isalnum(__c)    (__ctype_lookup(__c)&(_U|_L|_N))
#define isprint(__c)    (__ctype_lookup(__c)&(_P|_U|_L|_N|_B))
#define isgraph(__c)    (__ctype_lookup(__c)&(_P|_U|_L|_N))
#define iscntrl(__c)    (__ctype_lookup(__c)&_C)

#if defined(__GNUC__) && \
    (!defined(__STRICT_ANSI__) || __STDC_VERSION__ >= 199901L)
#define isblank(__c) \
  __extension__ ({ __typeof__ (__c) __x = (__c);        \
        (__ctype_lookup(__x)&_B) || (int) (__x) == '\t';})
#endif


/* Non-gcc versions will get the library versions, and will be
   slightly slower.  These macros are not NLS-aware so they are
   disabled if the system supports the extended character sets. */
# if defined(__GNUC__)
#  if !defined (_MB_EXTENDED_CHARSETS_ISO) && !defined (_MB_EXTENDED_CHARSETS_WINDOWS)
#   define toupper(__c) \
  __extension__ ({ __typeof__ (__c) __x = (__c);    \
      islower (__x) ? (int) __x - 'a' + 'A' : (int) __x;})
#   define tolower(__c) \
  __extension__ ({ __typeof__ (__c) __x = (__c);    \
      isupper (__x) ? (int) __x - 'A' + 'a' : (int) __x;})
#  else /* _MB_EXTENDED_CHARSETS* */
/* Allow a gcc warning if the user passed 'char', but defer to the
   function.  */
#   define toupper(__c) \
  __extension__ ({ __typeof__ (__c) __x = (__c);    \
      (void) __ctype_ptr__[__x]; (toupper) (__x);})
#   define tolower(__c) \
  __extension__ ({ __typeof__ (__c) __x = (__c);    \
      (void) __ctype_ptr__[__x]; (tolower) (__x);})
#  endif /* _MB_EXTENDED_CHARSETS* */
# endif /* __GNUC__ */
#endif /* !__cplusplus */

#ifndef __STRICT_ANSI__
#define isascii(__c)    ((unsigned)(__c)<=0177)
#define toascii(__c)    ((__c)&0177)
#endif

/* For C++ backward-compatibility only.  */
extern  __IMPORT _CONST char    _ctype_[];

_END_STD_C

#endif /* _CTYPE_H_ */

This code is from the standard library ctype.h.

I was impressed by the following code:

#define isalpha(__c)    (__ctype_lookup(__c)&(_U|_L))

But how? How is the alphabet checked by just that operation? I think I should know what __ctype_lookup is.

However, __ctype_lookup was more odd.

#define __ctype_lookup(__c) ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])

Fortunately, there was a comment right above the code, but unfortunately I could not understand it.

/* These macros are intentionally written in a manner that will trigger a gcc -Wall warning if the user mistakenly passes a 'char' instead of an int containing an 'unsigned char'. Note that the sizeof will always be 1, which is what we want for mapping EOF to __ctype_ptr__[0]; the use of a raw index inside the sizeof triggers the gcc warning if __c was of type char, and sizeof masks side effects of the extra __c. Meanwhile, the real index to __ctype_ptr__+1 must be cast to int, since isalpha(0x100000001LL) must equal isalpha(1), rather than being an out-of-bounds reference on a 64-bit machine. */

Somebody help me!

And the __IMPORT in the following code was just defined somewhere. If it is important, I will find it.

extern  __IMPORT char   *__ctype_ptr__;

Solution

  • With reference to the chromium implementation of ctype.c (as hopefully it is representative):

    __ctype_ptr__ is a pointer to the start of the _ctype_ array:

    char *__ctype_ptr__ = (char *) _ctype_;
    

    The _ctype_ array is an array of const chars:

    _CONST char _ctype_[1 + 256] = {
        0,
        _CTYPE_DATA_0_127,
        _CTYPE_DATA_128_255
    };
    

    And finally the _CTYPE_DATA_0_127 and _CTYPE_DATA_128_255 entries are actually defines containing several chars, each of which is a bitfield:

    #define _CTYPE_DATA_0_127 \
    _C, _C, _C, _C, _C, _C, _C, _C, \
    _C, _C|_S, _C|_S, _C|_S,    _C|_S,  _C|_S,  _C, _C, \
    _C, _C, _C, _C, _C, _C, _C, _C, \
    _C, _C, _C, _C, _C, _C, _C, _C, \
    _S|_B,  _P, _P, _P, _P, _P, _P, _P, \
    _P, _P, _P, _P, _P, _P, _P, _P, \
    _N, _N, _N, _N, _N, _N, _N, _N, \
    _N, _N, _P, _P, _P, _P, _P, _P, \
    _P, _U|_X,  _U|_X,  _U|_X,  _U|_X,  _U|_X,  _U|_X,  _U, \
    _U, _U, _U, _U, _U, _U, _U, _U, \
    _U, _U, _U, _U, _U, _U, _U, _U, \
    _U, _U, _U, _P, _P, _P, _P, _P, \
    _P, _L|_X,  _L|_X,  _L|_X,  _L|_X,  _L|_X,  _L|_X,  _L, \
    _L, _L, _L, _L, _L, _L, _L, _L, \
    _L, _L, _L, _L, _L, _L, _L, _L, \
    _L, _L, _L, _P, _P, _P, _P, _C
    

    where each _U, _P etc is a different bit:

    #define _U  01
    #define _L  02
    #define _N  04
    #define _S  010
    #define _P  020
    #define _C  040
    #define _X  0100
    #define _B  0200
    

    So, the line:

    #define __ctype_lookup(__c) ((__ctype_ptr__+sizeof(""[__c]))[(int)(__c)])1

    Is just a lookup into that _ctype_ array, and then

    #define isalpha(__c) (__ctype_lookup(__c)&(_U|_L))

    is a bitwise mask of that lookup with the appropriate bitmask (in this case, and OR of uppercase and lowercase).


    1: Honestly not sure what sizeof(""[__c]) achieves, I'm guessing it's something to do with support for different compilers/architectures or something. It skips over the first '0' in the _ctype_ array