How to interpret this __ctype_b based predicate?

Question

I am getting this simplified function from the decomplication results of Ghidra, and I am having a hard time interpreting what the predicate would evaluate to since I do not have access to __ctype_b structure, in other words, what is this predicate indicating (eg. no blank spaces, digits only, ect...) ?

int myFunc(char myChar) {

    if ((*(unsigned short*)((char)myChar * 2 + __ctype_b) >> 6 & 1) == 0) {
        return true;
    } else {
        return false;
    }

}

Answer 1

_ctype is a common name for an array with flags used for implementing the is... family of C runtime function-like macros from ctype.h (isupper, islower, isalpha, isdigit and so on). For example, see this file from early glibc:

CONST unsigned short int __ctype_b_C[] =
  {
    0,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl,
    _IScntrl|_ISspace|_ISblank,

If we assume that __ctype_b is an array of shorts, the function seems to be equivalent to:

return (__ctype_b[myChar]>>6 & 1)==0;

or

return (__ctype_b[myChar]& (1<<6))==0;

If the bits in the table use standard values, 1<<6 corresponds to the _ISpunct flag, so the function seems to return !ispunct(myChar);