Bug#279044: marked as done (libc6: qsort() doesn't sort for large size)
Your message dated Sun, 31 Oct 2004 12:05:39 -0500
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and subject line Bug#279044: libc6: qsort() doesn't sort for large size
has caused the attached Bug report to be marked as done.
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From: Thomas Koenig <Thomas.Koenig@online.de>
To: Debian Bug Tracking System <submit@bugs.debian.org>
Subject: libc6: qsort() doesn't sort for large size
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Date: Sun, 31 Oct 2004 10:35:02 +0100
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Package: libc6
Version: 2.3.2.ds1-18
Severity: normal
$ cat mysort.c
#include <stdio.h>
#include <stdlib.h>
#define DATASIZE 1800
#define NDAT 100000
typedef struct {
int key;
char data [DATASIZE];
} mydata;
int mycomp(void const *p1, void const *p2) {
mydata const *v1, *v2;
v1 = p1;
v2 = p2;
return (v1->key) < (v2->key);
}
int main() {
int i;
mydata *p;
p = calloc(NDAT, sizeof(mydata));
for (i=0; i<NDAT; i++) {
p[i].key = random();
}
qsort(p, NDAT, sizeof(*p), mycomp);
for (i=0; i<10; i++) {
printf("%d\n",p[i].key);
}
return 0;
}
$ gcc -g -Wall -o mysort mysort.c
$ ./mysort
1804289383
585681566
200863687
1979603839
2085853640
1489258466
556712446
1788149182
1893664349
43517041
which is pretty unsorted, if you ask me :-)
It's OK for a smaller struct, though:
$ cat mysort-works.c
#include <stdio.h>
#include <stdlib.h>
#define DATASIZE 20
#define NDAT 100000
typedef struct {
int key;
char data [DATASIZE];
} mydata;
int mycomp(void const *p1, void const *p2) {
mydata const *v1, *v2;
v1 = p1;
v2 = p2;
return (v1->key) < (v2->key);
}
int main() {
int i;
mydata *p;
p = calloc(NDAT, sizeof(mydata));
for (i=0; i<NDAT; i++) {
p[i].key = random();
}
qsort(p, NDAT, sizeof(*p), mycomp);
for (i=0; i<10; i++) {
printf("%d\n",p[i].key);
}
return 0;
}
$ gcc -g -Wall -o mysort-works mysort-works.c
$ ./mysort-works
2147469841
2147466242
2147414141
2147378031
2147317028
2147316088
2147311484
2147278491
2147248040
2147230544
$
-- System Information:
Debian Release: 3.1
APT prefers unstable
APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: i386 (i686)
Kernel: Linux 2.6.8-1-k7
Locale: LANG=C, LC_CTYPE=de_DE@euro (charmap=ISO-8859-15)
Versions of packages libc6 depends on:
ii libdb1-compat 2.1.3-7 The Berkeley database routines [gl
-- no debconf information
---------------------------------------
Received: (at 279044-done) by bugs.debian.org; 31 Oct 2004 17:06:11 +0000
>From jwnimmer@alum.mit.edu Sun Oct 31 09:06:11 2004
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for <279044-done@bugs.debian.org>; Sun, 31 Oct 2004 12:05:39 -0500
Subject: Re: Bug#279044: libc6: qsort() doesn't sort for large size
From: Jeremy Nimmer <jwnimmer@alum.mit.edu>
To: 279044-done@bugs.debian.org
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Thomas,
In general, it's unlikely that anything you think is a libc bug is
really a libc bug, especially for something like a severe functional
problem like "qsort() doesn't sort".
In this case, it's your bug:
On Sun, 2004-10-31 at 04:35, Thomas Koenig wrote:
> int mycomp(void const *p1, void const *p2) {
> mydata const *v1, *v2;
> v1 = p1;
> v2 = p2;
> return (v1->key) < (v2->key);
> }
The above is an incorrect comparison routine. Did you read the qsort
man page? Is says, in part:
The comparison function must return an integer less than, equal
to, or greater than zero if the first argument is considered to
be respectively less than, equal to, or greater than the second.
So using "<" is totally wrong. I'd suggest "(v1->key) - (v2->key)" if
you want ascending order.
Thanks,
- Jeremy
p.s. I'm not a debian developer; I'm just trying to save their time for
the real problems.
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