C qsort compare function issue

Dr. Klahn
Dr. Klahn used Ask the Experts™
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I'm attempting to sort what would normally be a multi-dimensional array (CIDR block in 0, CIDR mask in 1) as a single-dimensional array of structures.  Using qsort, telling it the data size is the size of the structure, and a hopefully-clever compare function to compare only the CIDR blocks using structure offsets.

The compilation fails in the compare function with the error:

sorttest.c: In function 'ib_cmpfunc':
sorttest.c:22:27: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
                           ^
sorttest.c:22:62: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
                                                              ^
sorttest.c:23:27: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;
                           ^
sorttest.c:23:62: error: request for member 'ib_blackaddr' in something not a structure or union
   if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;

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Here's the code used to generate the test program.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

#define BLACKLIST_SIZE 512

typedef struct {
    /* IPV4 address as 32-bit integer */
  unsigned long ib_blackaddr;
    /* CIDR mask as 32-bit integer */
  unsigned long ib_blackmask;
}  ib_arrayelement;

  /* Number of entries in the blacklist table */
int ib_blacklist_size;

  /* IPv4 address - CIDR mask blacklist */
ib_arrayelement ib_blackarray[BLACKLIST_SIZE];

static int ib_cmpfunc (const void *a, const void *b) {
  if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) > 0) return 1;
  if ((*(ib_arrayelement)a.ib_blackaddr - *(ib_arrayelement)b.ib_blackaddr) < 0) return -1;
  return 0;
}

void main (int argc, char **argv) {

  qsort(ib_blackarray, ib_blacklist_size,
        sizeof(ib_arrayelement), ib_cmpfunc);

  return;
}

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The qsort compare function is too deep in pointers for me to follow what the problem is; I don't normally do anything this obscure.  Can someone point me at a solution?

Side note:  This is c on a Debian linux host.  Not C++, nor Windows.
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See if this works for you.
static int ib_cmpfunc (const void *a, const void *b) {
  const ib_arrayelement* aa = reinterpret_cast<const ib_arrayelement*>(a);
  const ib_arrayelement* bb = reinterpret_cast<const ib_arrayelement*>(b);
  if ( aa->ib_blackaddr - bb->ib_blackaddr > 0) return 1;
  if ( aa->ib_blackaddr - bb->ib_blackaddr < 0) return -1;
  return 0;
}

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Dr. KlahnPrincipal Software Engineer

Author

Commented:
Doesn't seem to be available on my system, at least not with the includes I'm using.  Perhaps it's only available in C++ ?

root@www: cc test.c
test.c: In function 'ib_cmpfunc':
test.c:22:31: error: 'reinterpret_cast' undeclared (first use in this function)
   const ib_arrayelement* aa = reinterpret_cast<const ib_arrayelement*>(a);
                               ^
test.c:22:31: note: each undeclared identifier is reported only once for each function it appears in
test.c:22:48: error: expected expression before 'const'
   const ib_arrayelement* aa = reinterpret_cast<const ib_arrayelement*>(a);
                                                ^
test.c:23:48: error: expected expression before 'const'
   const ib_arrayelement* bb = reinterpret_cast<const ib_arrayelement*>(b);

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Right, C, not C++.

static int ib_cmpfunc (const void *a, const void *b) {
  const ib_arrayelement* aa = (const ib_arrayelement*)(a);
  const ib_arrayelement* bb = (const ib_arrayelement*)(b);
  if ( aa->ib_blackaddr - bb->ib_blackaddr > 0) return 1;
  if ( aa->ib_blackaddr - bb->ib_blackaddr < 0) return -1;
  return 0;
}

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Dr. KlahnPrincipal Software Engineer

Author

Commented:
Thanks very much, phoffric.  The second version of the compare function reversed the array, but there was no compile error.  Shortly after reviewing (again, but at least for the last time) some qsort compare example functions I ended up with this:

static int ib_cmpfunc (const void *a, const void *b) {
  const ib_arrayelement* aa = (const ib_arrayelement*)(a);
  const ib_arrayelement* bb = (const ib_arrayelement*)(b);
  if (aa->ib_blackaddr > bb->ib_blackaddr) return 1;
  if (bb->ib_blackaddr > aa->ib_blackaddr) return -1;
  return 0;
}

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And it works as hoped-for, the masks are still "attached" to their CIDR blocks as the array is sorted.

mod_ipblock:  Blacklist dump
  Entry 0 38.38.38.0 mask 255.255.255.0
  Entry 1 38.38.0.0 mask 255.255.0.0
  Entry 2 38.0.0.0 mask 255.0.0.0
  Entry 3 255.255.255.255 mask 255.255.255.255
  Entry 4 87.87.232.0 mask 255.255.255.0
  Entry 5 16.255.0.0 mask 255.255.252.0
mod_ipblock:  Blacklist dump
  Entry 0 16.255.0.0 mask 255.255.252.0
  Entry 1 38.0.0.0 mask 255.0.0.0
  Entry 2 38.38.0.0 mask 255.255.0.0
  Entry 3 38.38.38.0 mask 255.255.255.0
  Entry 4 87.87.232.0 mask 255.255.255.0
  Entry 5 255.255.255.255 mask 255.255.255.255

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Dr. KlahnPrincipal Software Engineer

Author

Commented:
Glad to have this one out of the way.  Excellent support from phoffric.
Glad to have helped.
Keep in mind for future reference that the . operator has higher precedence that the (type) operator.
http://en.cppreference.com/w/c/language/operator_precedence

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