CODEWITHSUNDEEP
carrays
MarcPotts
MarcPotts

Reputation: 23

Segmentation fault with large, contiguous 2D array (with dynamic memory allocation) in C

The code below is supposed to create two 2D arrays (posa and posb) that have contiguous memory locations, and then copy posa over posb using memmove function. Surprisingly, it works well for "small" array sizes (e.g.: size1=size2=100), but I get a segmentation fault (core dump) for larger sizes (e.g.: size1=size2=300). As I am using dynamic memory allocation, I presume this is not a stack overflow issue like here... Can somebody explain what I'm doing wrong ?

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

int ** Allocation_array_x2(int size1, int size2){
    int **ptr;
    int i;
    ptr = malloc(size1 * sizeof(*ptr));
    if(ptr == NULL)  {        
        fprintf(stderr,"Allocation error");
        exit(1);
    }
    ptr[0] = malloc(size1 * size2 * sizeof(**ptr));
    for(i=1 ; i < size1 ; i++){
        ptr[i]=ptr[0]+(i * size2);
        if( ptr[i] == NULL) {
            fprintf(stderr,"Allocation error");
            exit(1);
        }
    }
    return ptr;                         
}

void Free_array_x2(int **ptr){
    free(ptr);
    ptr=NULL;
}

int main(){
    int size1=300, size2=300;
    int **posa, **posb;
    posa=Allocation_array_x2(size1,size2);
    posb=Allocation_array_x2(size1,size2);

    /* array_filling */
    for( int j = 0 ; j<size1 ; j++ ) {
        for( int k = 0 ; k<size2 ; k++ ) {
            posa[j][k] = 2*j+3*k+1;     
            posb[j][k] = 1000;
        }
    }

    memmove(posb,posa,size1*size2*sizeof(int));

    for(int i = 0 ; i<size1 ; i++ ) {
        for(int j = 0 ; j<size2 ; j++ ) {
            printf("%d\t%d\n",posa[i][j],posb[i][j]);
        }
    }


    Free_array_x2(posa);
    Free_array_x2(posb);

}

Upvotes: 2

Views: 153

Answers (2)

4pie0
4pie0

Reputation: 29724

Error is here:

memmove(posb,posa,size1*size2*sizeof(int));

because you pass pointers to pointers to integer, but you shoud pass pointer to integer instead:

memmove(*posb, *posa, size1 * size2 * sizeof(int));

You have also errors in Free_array_x2 method because you don't release all memory you have taken from the system with malloc

void Free_array_x2(int **ptr){
    free(ptr);
    ptr=NULL;
}

This should go backward to what you do in allocation routine:

void Free_array_x2(int **ptr)
{
    free(ptr[0]);
    ptr[0] = NULL;
    free(ptr);
    ptr = NULL;
}

Upvotes: 1

Chris Beck
Chris Beck

Reputation: 16204

Changing the code to this fixed it for me:

memmove(*posb,*posa,size1*size2*sizeof(int));

The issue is that your main block of memory is actually at posa[0] and posb[0], not posa and posb. posa and posb just point to the secondary allocations.

Speaking of which, should point out that you are leaking memory -- your create function contains 2 malloc calls, but your free function only frees one of them.

Upvotes: 3

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