Broken down into practical pointers and step-by-step instructions, the IT Service Excellence Tool Kit delivers expert advice for technology solution providers. Get your free copy now.
C qsort compare function issue
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:
Here's the code used to generate the test program.
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.
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;
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;
}
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.
Asked:
-
3
3
1 Solution
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);
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;
}
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:
And it works as hoped-for, the masks are still "attached" to their CIDR blocks as the array is sorted.
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;
}
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
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
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
Question has a verified solution.
Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.
Have a better answer? Share it in a comment.
Join & Write a Comment Already a member? Login.
Featured Post
Broken down into practical pointers and step-by-step instructions, the IT Service Excellence Tool Kit delivers expert advice for technology solution providers. Get your free copy now.
Tackle projects and never again get stuck behind a technical roadblock.
Join Now
See if this works for you.
Open in new window