We value your feedback.
Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!
COURSE of the MONTH
15 days, 4 hours left to enrollBecome a Premium Member and unlock a new, free course in leading technologies each month.
Solved
C qsort compare function issue
Posted on 2017-03-16
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.
[X]
Welcome to Experts Exchange
Add your voice to the tech community where 5M+ people just like you are talking about what matters.
- Help others & share knowledge
- Earn cash & points
- Learn & ask questions
3-
3
6 Comments
Active today
http://ideone.com/iizCtE
See if this works for you.
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;
}
Active today
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);
Active today
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;
}
Active today
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
Active today
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
Featured Post
Benefit from a mission critical IT monitoring with Monitis Premium or get it FREE for your entry level monitoring needs.
-Over 200,000 users
-More than 300,000 websites monitored
-Used in 197 countries
-Recommended by 98% of users
Question has a verified solution.
If you are experiencing a similar issue, please ask a related question
If you’re thinking to yourself “That description sounds a lot like two people doing the work that one could accomplish,” you’re not alone.
In this post we will learn how to make Android Gesture Tutorial and give different functionality whenever a user Touch or Scroll android screen.
The goal of this video is to provide viewers with basic examples to understand and use while-loops in the C programming language.
The goal of this video is to provide viewers with basic examples to understand and use conditional statements in the C programming language.
Suggested Courses
Course of the Month15 days, 4 hours left to enroll
771 members asked questions and received personalized solutions in the past 7 days.
Join the community of 500,000 technology professionals and ask your questions.