Why this program crashes?

Question

This small C program is crashing every single time.

It is supposed to allocate a chunk of memory under the form of a 3D grid composed of many structures (cells) disposed in memory in a 3D-friendly pattern. The structs will be populated with position data.

I don't know why it crashes. It returns this number: c0000005.

#include <stdio.h>
#include <malloc.h>

typedef struct {
int coords[3];
} cell;

int main() {

    int x=4, y=8, z=6;

    int volume=x*y*z;

    cell *arr=(cell*)calloc(volume,sizeof(cell));

    int u=0,v=0,w=0;
    int index;

    for (w=0; w<z; w++) {
    for (v=0; v<y; v++) {
    for (u=0; u<x; u++) {

        //printf("%d %d %d\n", u, v, w);

        index=u+v*y+w*y*z;

        arr[index].coords[0]=u;
        arr[index].coords[1]=v;
        arr[index].coords[2]=w;

        //getchar();

    }}}

    printf("All done.\n");

    return 0;
}

[Don't cast the result of malloc (and friends)](http://stackoverflow.com/q/605845) — Deduplicator, Sep 19 '14 at 19:18
Compile your program with all warnings & debug info (`gcc -Wall -g`). Then **use the debugger** (`gdb`) — Basile Starynkevitch, Sep 19 '14 at 19:19
Surely u+v*y+w*y*z becomes greater than x*y*z, so you're indexing into something you've not allocated. — nos, Sep 19 '14 at 19:20
@Deduplicator If I don't cast the calloc, the comp returns an error that states: "invalid conversion from void* to cell*" — user2464424, Sep 19 '14 at 19:24
@user2464424: Which is a sure indicator that you ain't using a C compiler. I'm guessing you are actually using C++. — Deduplicator, Sep 19 '14 at 19:26
@Deduplicator yes because i forgot to switch to C when isolating the issue. Im very tired. — user2464424, Sep 19 '14 at 19:30

score 2 · Accepted Answer · answered Sep 19 '14 at 19:20

2

The problem is index=u+v*y+w*y*z;.

It should be index=u+v*x+w*y*x;.

So @nos is right. It triggers a segmentation fault because 6=z>x=4 and index becomes too large.

answered Sep 19 '14 at 19:20

francis

9,525
2
25
41

Why this program crashes?

1 Answers1