segmentation fault language c

Question

I get a segmentation fault (core dumped) from this algorithm. Why?

Does it work when using small value of N and K? For example, N=100 and K=10?

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

#define N 400       
#define K 30        

int main()
{
int  i,j,k;
int A[K+1][N][N];
for (i=0;i<K+1;i++)
{
  for (j=0;j<N;j++)
  {
      for (k=0;k<N;k++)
      {

           A[i][j][k]=0; 
      }

  }
}
printf("A[%i][%i][%i] \n", i, j,k);
printf("OK");
return 0;
}

`printf("A[%i][%i][%i] \n", i, j,k);` should be with `%d` (this do not cause the error, just a hint). — pzaenger, Nov 09 '13 at 22:51

Paul Draper · Answer 1 · 2013-11-09T23:14:50.557

6

You are likely exceeding the stack size.

If you are using large values for K and N, consider putting A on the heap.

int *A = malloc((K+1) * N * N * sizeof(int));

To index into it:

A[(i*(K+1) + j)*N + k]=0;

(you make want to make a function or macro for that)

And once you are done with A,

free(A);

edited Nov 09 '13 at 23:14

answered Nov 09 '13 at 22:52

Paul Draper

78,542
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@CharlieBurns, yes, though (surprisingly) you did get two upvotes on your answer ;) – Paul Draper Nov 09 '13 at 22:57
why would it not be : `int *A = malloc(sizeof(int) * (K + 1) * N * N);` ? – ryyker Nov 09 '13 at 22:57
Btw, [do not cast the result of malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). – pzaenger Nov 09 '13 at 22:58
You've probably got somewhere in the region of 16MB max to play with on the stack depending on the platform you run this on. This answer looks highly plausible. – marko Nov 09 '13 at 22:58
@pzaenger, because `malloc` returns `void *`. (It's been awhile since I've used C...the cast is not implicit, right?) – Paul Draper Nov 09 '13 at 22:59
[regarding casting malloc](http://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) – ryyker Nov 09 '13 at 23:01
I'm confused, can you really alloc a 3d array that way? It won't compile as is. – Charlie Burns Nov 09 '13 at 23:09
@CharlieBurns, see my additional info in the answer. – Paul Draper Nov 09 '13 at 23:15
Ok, so it's a 1d array. Should the malloc be 'int *A = malloc((K+1) * N * N * sizeof(int));? As it stands, your malloc line won't compile. Thanks. – Charlie Burns Nov 09 '13 at 23:18

Hhyperion · Answer 2 · 2013-11-09T23:22:29.257

2

You are trying to create an array with size of 19.2 MB (31*400*400 *4) where 4B is the size of type int. It is not possible to statically allocate more than 8.37MB, use malloc instead.

edited Nov 09 '13 at 23:22

answered Nov 09 '13 at 23:05

Hhyperion

666
5
6

Did you mean *not* possible? And stack size varies (widely). – Paul Draper Nov 09 '13 at 23:16
In Windows the stack defaults to 1MB. – Havenard Nov 09 '13 at 23:17
@PaulDraper: I expect he was thinking of `static int A[K+1][N][N];` either at file or function scope. Because they aren't on the stack, the don't blow the stack away, unlike the on-stack (for normal machines) allocation of automatic local variables. – Jonathan Leffler Nov 09 '13 at 23:24

score 0 · Answer 3 · answered Nov 09 '13 at 23:56

Another way is to just move the array off of the stack and into the data segment:

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

#define N 400       
#define K 30        
int A[K+1][N][N];  // ADD THIS LINE

int main()
{
int  i,j,k;
// REMOVE THIS LINE int A[K+1][N][N];
for (i=0;i<K+1;i++)
{
  for (j=0;j<N;j++)
  {
      for (k=0;k<N;k++)
      {

           A[i][j][k]=0; 
      }

  }
}
printf("A[%i][%i][%i] \n", i, j,k);
printf("OK");
return 0;
}

segmentation fault language c

3 Answers3