pointer to pointer segmentation fault

Guilherme · 06-27-2009, 08:08 PM

Hi
I have been googling trying to fix this ...
basicly I have a (int **) variable, and when I try to pass a (int) value to a specific position, I have seg.fault.

Resuming what I have is:

int ** p;
p[0][0] = 1; //segmentation fault

:\ sorry for this question, but ...

can you please help me ?

Uncle_Theodore · 06-27-2009, 08:10 PM

Well, you need to malloc some memory before you start to write into it...

Guilherme · 06-27-2009, 08:15 PM

ok, I though it was the problem.
but, malloc needs a specific length.
Is it possible with C to use an unlimited array? or ... at least with some kind of undefined length?

Uncle_Theodore · 06-27-2009, 08:19 PM

Not quite, but there are workarounds. First, the length (actually, the size) you request with malloc can be a variable, which is defined at runtime as opposed to compilation time. Second, you can try to realloc() if you really need more.
But it's impossible to request an infinite array from the beginning.

PS. There are also linked lists. Those can be extended at will (when needed).

Here's a small tutorial on malloc
http://ee.hawaii.edu/~tep/EE160/Book...on2.1.2.2.html

Guilherme · 06-27-2009, 08:22 PM

hum

...
ok thanks a lot.
realloc may help a lot in fact.

thanks

Guilherme · 06-27-2009, 09:00 PM

hum ... sorry ... still can't fix it

Code:

int ** matriz_inserir(int dimV, int dimH)
{
        int h, v, tmp;
        int ** matriz;
        matriz = (int**) calloc(dimH*dimV,sizeof(int));
        if(dimH < 1) dimH = 1; 
        if(dimV < 0) dimH *= -1; 
        if(dimV != 0)
        {
                puts(MSG_INSERIR);
                for (h = 0; h < dimH; h++)
                        for (v = 0; v < dimV; v++)
                        {
                                if(dimH > 1) printf(" %d -",h+1);  
                                printf(" %d: ",v+1);
                                scanf("%d",&matriz[h][v]);
                        }
        }
        return matriz;
}

:\

Uncle_Theodore · 06-27-2009, 10:00 PM

In your code the line
matriz = (int**) calloc(dimH*dimV,sizeof(int));
does not create a double array.

Here's a little primer

Code:

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

int main()
{
  int **p;
  int numrows = 3;
  int numcolumns = 4;
  int i, j;

  if(!(p = (int **)malloc(numrows * sizeof(int *)))) return 1;
  for(i=0;i<numrows;i++)
    if(!(p[i] = (int *)malloc(numcolumns * sizeof(int)))) return 1;

  for(i=0;i<numrows;i++)
    for(j=0;j<numcolumns;j++)
      {
	p[i][j] = i+j;
	printf("p[%d][%d]=%d,\n", i,j,p[i][j]);
      };
  for(i=0;i<numrows;i++)
    free(p[i]);
  free(p);
  return 0;
}

Guilherme · 06-28-2009, 09:47 AM

hum ok ...
so it needs to alloc from the "last" pointer to the "first" ... ok ...

thank you a lot indeed