[PLUG] [OT] Problem with gcc

[(अमेय) Ameya (पाळंदे) Palande]

> Message: 1
> Date: Thu, 23 Aug 2007 23:50:13 +0530
> From: "Aneesh Mulye" <aneesh.mulye at gmail.com>
> Subject: [PLUG] [OT] Problem with gcc
> To: "Pune GNU/Linux Users Group Mailing List" <plug-mail at plug.org.in>
> Message-ID:
>         <bab07a750708231120k88f3474k17328628dd20d6e6 at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> For some reason, the following code doesn't work:
>
> #include<stdio.h>
> #include<stdlib.h>
> #include<math.h>
>
> /* float determinant(float *a, int n) {{{1 */
> float determinant(float *a, int n)
> {
>     float *b, det=0;
>
>     if(n<2)
>         return 0;
>
>     if(n==2)
>         det=( (*a)*(*(a+3)) - (*(a+1))*(*(a+2)));
>     else
>     {
>         b=malloc((n-1)*(n-1)*sizeof(float));
>
>          int i,j,k,m=0,l=0;
>
>         for(i=0;i<n;i++)
>         {
>             for(j=1;j<n;j++)
>                 for(k=0;k<n;k++)
>                     if(k!=i)
>                     {
>                         *(b + (n-1)*l + m)=
>                             *(a + n*j+ k);
>                         m++;
>                         l+=m/(n-1);
>                         m%=(n-1);
>                     }
>
>             det+=(*(a+i))*determinant(b, n-1)*pow(-1,i);
>         }
>
>         free(b);
>     }
>
>     return det;
> }
> /* 1}}} */
>
> int main()
> {
>     float *a, det;
>     int n,i,j;
>
>     printf("Enter the no. of rows\n# ");
>     scanf("%d", &n);
>     printf("\n\n");
>
>     a=malloc(n*n*sizeof(float));
>
>     for(i=0;i<n;i++)
>         for(j=0;j<n;j++)
>         {
>             printf("Enter element (%d,%d)# ", (i+1), (j+1));
>             scanf("%f", (a + i*n + j));
>         }
>
>     det=determinant(a, n);
>
>     free(a);
>
>     printf("\n\nThe determinant is: %f\n\n", det);
>
>     return 0;
> }
>
> Any help is appreciated.
>

Problem is in line 27 of your code:

*(b + (n-1)*l + m) = *(a + n*j+ k);

For debugging this, use "valgrind" which will tell you invalid memory writes.

Here is the output of debugging:

ameya at morningdew:/tmp/work$ valgrind ./a.out
==6344== Memcheck, a memory error detector.
==6344== Copyright (C) 2002-2006, and GNU GPL'd, by Julian Seward et al.
==6344== Using LibVEX rev 1658, a library for dynamic binary translation.
==6344== Copyright (C) 2004-2006, and GNU GPL'd, by OpenWorks LLP.
==6344== Using valgrind-3.2.1-Debian, a dynamic binary instrumentation
framework.
==6344== Copyright (C) 2000-2006, and GNU GPL'd, by Julian Seward et al.
==6344== For more details, rerun with: -v
==6344==
Enter the no. of rows
# 3


Enter element (1,1)# 1
Enter element (1,2)# 2
Enter element (1,3)# 3
Enter element (2,1)# 4
Enter element (2,2)# 5
Enter element (2,3)# 6
Enter element (3,1)# 7
Enter element (3,2)# 8
Enter element (3,3)# 9
==6344== Invalid write of size 4
==6344==    at 0x80485AB: determinant (test.c:27)
==6344==    by 0x804873D: main (test.c:62)
==6344==  Address 0x41A1090 is 0 bytes after a block of size 16 alloc'd
==6344==    at 0x4021620: malloc (vg_replace_malloc.c:149)
==6344==    by 0x804853C: determinant (test.c:17)
==6344==    by 0x804873D: main (test.c:62)


The determinant is: -6.000000

==6344==
==6344== ERROR SUMMARY: 8 errors from 1 contexts (suppressed: 13 from 1)
==6344== malloc/free: in use at exit: 0 bytes in 0 blocks.
==6344== malloc/free: 2 allocs, 2 frees, 52 bytes allocated.
==6344== For counts of detected errors, rerun with: -v
==6344== All heap blocks were freed -- no leaks are possible.

Cheers,
Ameya.