A weird segmentation fault error

Question

Possible Duplicate:
Getting a stack overflow exception when declaring a large array

My system is x86-64 linux and here is my simple code :

#include<stdio.h>
#define N 1024
int main()
{
    int a[N][N];
    int b[N][N];
    printf("hello world\n");
    return 0;
}

And its assembly code from objdump:

00000000004004fc <main>:

4004fc: 55                      push   %rbp
4004fd: 48 89 e5                mov    %rsp,%rbp
400500: 48 81 ec 00 00 80 00    sub    $0x800000,%rsp
400507: bf c4 05 40 00          mov    $0x4005c4,%edi
40050c: e8 cf fe ff ff          callq  4003e0 <puts@plt>
400511: b8 00 00 00 00          mov    $0x0,%eax
400516: c9                      leaveq 
400517: c3                      retq   
400518: 0f 1f 84 00 00 00 00    nopl   0x0(%rax,%rax,1)
40051f: 00

The weird thing is this program will break down when calling printf() function. However, if I define N to be 512, this program works well. I have no idea why. Is there any stack size limitation which limits the memory use by stack?

Does anybody know why? Thanks.

Nemo, I get it. It is ironic to ask this on StackOverflow. :-) — coinsyx, Jan 15 '13 at 08:08

hmatar · Answer 1 · 2013-01-15T06:33:54.890

2

Error due to exceeding stack size. To get rid of this create the arrays in heap. To do that use malloc or other dynamic memory allocation functions.

Like int *a = malloc(N*N*sizeof(int)) this way memory is located in heap.
Also you should test if this memory has been allocated for you by checking:

if(a)
     // do stuff with a

edited Jan 15 '13 at 06:33

answered Jan 15 '13 at 06:23

hmatar

2,437
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[Do not cast the return of malloc in C](http://stackoverflow.com/questions/605845/) – Nemo Jan 15 '13 at 06:29
And there is no `new` in C. – ThiefMaster Jan 15 '13 at 06:32
Edited accordingly. thanks for the reminder @ThiefMaster. – hmatar Jan 15 '13 at 06:35

Theocharis K. · Answer 2 · 2013-01-15T06:42:26.593

Yes there is a maximum stack size which is small, most of the times smaller than a few KBs. You are trying to allocate 1024*1024*sizeof(int)=4194304bytes or 4MBs of data into a single stack allocation which results in a crash.

There are 2 ways to counter this:

1) Allocate the memory outside of the stack

#include<stdio.h>
#define N 1024

int a[N][N];
int b[N][N];    

int main()
{
    printf("hello world\n");
    return 0;
}

or 2) dynamically allocate memory from the heap inside main() using malloc():

    #include<stdio.h>
    #define N 1024

    int main()
    {
        int **a = malloc(N, sizeof(int*));
        int **b = malloc(N, sizeof(int*));
        for (int i=0; i<N; i++)
        {
          a[i]=malloc(N, sizeof(int));
          b[i]=malloc(N, sizeof(int));
         }
        printf("hello world\n");
        return 0;
    }

Note: Do not forget to free() any dynamically allocated memory after you are done with the data it holds, otherwise your program will leak memory.

Well, it was very wrong before you made your edit :-). `a` and `b` in the original code decay to `int *`, not `int **`. — Nemo, Jan 15 '13 at 06:40

score 0 · Answer 3 · answered Jan 15 '13 at 06:23

0

Yes, there is limit on how much you can allocate on stack at a given time. It is compiler dependent. On visual studio default size is 1 MB. Check your compiler settings to know the size. My guess is since you are exceeding this limit it is crashing.

answered Jan 15 '13 at 06:23

Asha

11,002
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score 0 · Answer 4 · edited May 23 '17 at 11:48

0

I suppose I might as well give my answer, borrowed from here:

int (*a)[N] = malloc(N * N * sizeof(int));
int (*b)[N] = malloc(N * N * sizeof(int));

Now you can access them like the 2D arrays in your original code; e.g. a[50][100] = 37;

Do not forget to free(a) and free(b) when you are done with them.

edited May 23 '17 at 11:48

Community

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answered Jan 15 '13 at 07:13

Nemo

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A weird segmentation fault error

4 Answers4