qsort const指针问题

Question

char  **av;
SIZE_T  ac;

char WordCopy[256] = {0};
av[ac] = strdup(p);

qsort(av, ac, sizeof (char *), compare);

STATIC int compare(const void *p1, const void *p2)
{
    const char *pp1 = *(const char **)p1;
    const char *pp2 = *(const char **)p2;
    return strcmp(pp1, pp2);
}

But, it gives me the following error:

complete.c: In function ‘compare’:
complete.c:26:24: error: cast discards ‘__attribute__((const))’ qualifier from pointer target type [-Werror=cast-qual]
     const char *pp1 = *(const char **)p1;
                        ^
complete.c:27:24: error: cast discards ‘__attribute__((const))’ qualifier from pointer target type [-Werror=cast-qual]
     const char *pp2 = *(const char **)p2;
                        ^

The old way it was written, works:

STATIC int
compare(p1, p2)
    CONST char  **p1;
    CONST char  **p2;
{
    return strcmp(*p1, *p2);
}

I don't understand what the difference is... why am I getting the casting error? I found a few other similar-lokking issues here, which seemed to suggest that the syntax I am using should work... but it clearly does not.

I'd rather do it "properly" and not just disable the warning (disabling warnings will not pass code review).

EDIT : User chux asked what happened if I set a new var.. here is the result:

const char **s1 = (const char **)p1;

Produces:

complete.c: In function ‘compare’:
complete.c:26:23: error: cast discards ‘__attribute__((const))’ qualifier from pointer target type [-Werror=cast-qual]
     const char **s1 = (const char **)p1;
                       ^

Answer 1

You need to make what the pointer points to const as well:

const char *pp1 = *(const char * const *)p1;
const char *pp2 = *(const char * const *)p2;

Answer 2

A solution should not need any cast.

Examples of assigning a const void *

  const void *p1 = NULL;
  char *q1 = p1;        // warning: initialization discards 'const' qualifier from pointer target type
  const char *q2 = p1;
  const char **q3 = p1; // warning: initialization discards 'const' qualifier from pointer target type
  const char * const *q4 = p1;
  char * const *q5 = p1;

const void *p1 points to const data. The assignment target type needs to also be to a type that points to const data. q1 and q3 above do not point to const data. Either q4,q5 will work for OP's compare.

int compare5(const void *p1, const void *p2) {
  char * const *pp1 = p1;
  char * const *pp2 = p2;
  return strcmp(*pp1, *pp2);
}