The C standard specifies the following for isdigit() and the other character classification functions:

  The functions in this subclause return nonzero (true) if and only if the value of the argument c conforms to that in the description of the function.

Although Glibc strictly conforms to this requirement, it returns a different non-zero value from the function than from the corresponding macro.  I think returning the same value from both would improve the quality of the implementation.

In addition, returning 1 from both instead of a seemingly arbitrary non-zero value would prevent bugs like those discussed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82272.  (See WG14 paper N2229 for a proposal to help detect these bugs.)

#include <assert.h>
#include <ctype.h>
#include <stdio.h>

int main (void)
{
  int i = isdigit ('1');
  int j = (isdigit)('1');
  printf ("%i %i\n", i, j);

  assert (isdigit ('1') == (isdigit)('1'));
  
  return 0;
}
2048 1
tst-isdigit.c:12: main: Assertion `isdigit ('1') == (isdigit)('1')' failed.