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nspattak

Member

From: Greece

Registered: 2008-05-21

Posts: 22

Strange pointer problem in C results in segmentation fault

Hallo everyone.

I was experimenting with a toy code and I got some strange segmentation faults. In particular, I am trying to dynamically allocate a 2D array which is a contiguous in memory. I try to do it by declaring

 double **A;

and then allocate space for n pointers to the array's n lines :

A=(double**)malloc(n);

finally  I allocate my array:

A[0]=(double *)malloc((n*n)*sizeof(double));

and finally I assign each of my line pointers to the appropriate matrix line :

for(i=1;i<n;i++) {
      A[i]=A[0]+i*n;
}

I check that everything is ok by manually looking at the addresses. The problem is that if I run it for n>4 it sefaults . As far as I know, it should run.

Are there any ideas ?

This is the complete code I compile:

#include<stdio.h>
#include<stdlib.h>

int main(int argc,char *argv[])
{
   int i,j,n=5;
   double **A;

   A=(double**)malloc(n);

   A[0]=(double *)malloc((n*n)*sizeof(double));

   printf("A[%d]=%d\n",0,&A[0]);
   for(i=1;i<n;i++) {
      A[i]=A[0]+i*n;
      printf("A[%d]=%d\n",i,&A[i]);
   }

   for(i=0;i<n;i++) {
     for(j=0;j<n;j++)
       printf(" %d,%d:%d\t",i,j,&(A[i][j]));
      printf("\n");
   }

   for(i=0;i<n;i++)   {
     for(j=0;j<n;j++)     {
       A[i][j]=i*n+j;
       printf("\t A(%d,%d)=%f- %d",i,j,A[i][j],&A[i][j]);
     }
     printf("\n");
   }
   free(A[0]);free(A);
return 0;
}

markusle

Member

Registered: 2008-11-25

Posts: 4

Re: Strange pointer problem in C results in segmentation fault

A=(double**)malloc(n);

This is probably not what you want since malloc will reserve
a total of n bytes in your code. Presumably, you want n * sizeof(*double)
bytes since A is an array of pointers to double.

nspattak

Member

From: Greece

Registered: 2008-05-21

Posts: 22

Re: Strange pointer problem in C results in segmentation fault

You are absolutely correct! well, almost, it should be

sizeof(double*)

. Thank you very much! Silly me, I should have seen it.

djgera

Developer

From: Buenos Aires - Argentina

Registered: 2008-12-24

Posts: 723

Website

Re: Strange pointer problem in C results in segmentation fault

nspattak: this might interest: http://www.wolfbane.com/fortran/append-c.html

nspattak

Member

From: Greece

Registered: 2008-05-21

Posts: 22

Re: Strange pointer problem in C results in segmentation fault

It is. Thank you very much.

To be honest, when writting scientific codes i do not use dynamically created vectors/arrays. I was trying to do it mostly to improve my understanding of the language and because I saw it as a challenge.

Thanks to both of you.

tomd123

Developer

Registered: 2008-08-12

Posts: 565

Re: Strange pointer problem in C results in segmentation fault

To be honest, when writting scientific codes i do not use dynamically created vectors/arrays.

Because we all know how static, scientific data is.