creating a 2d array using double pointer

Question

i have made a program that creates 2d array. the code below works well on visual studio & turbo c:

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


int **allocarr(int **arr, int row, int col){
    int i;
    arr=calloc(row, sizeof(int));
    for(i=0; i<row; i++){
        arr[i]=calloc(col, sizeof(int));


    }

    return arr;
}

void freearr(int **arr, int row, int col){
    int i;
    for(i=0; i<row; i++) free(arr[i]);
    free(arr);


}

void printarr(int **arr, int row, int col){
    int i, j;
    for(i=0; i<row; i++){
        for(j=0;j<col;j++){
            printf("\t%d", arr[i][j]);
        }
        printf("\n\n");
    }


}



int main(){
    int **arr=NULL;
    arr=allocarr(arr, 3, 3);
    arr[2][2]=8;
    printarr(arr, 3, 3);
    freearr(arr,3, 3);
    return 0;
}

but it does not work on mac os x gcc and mingw gcc. everything compiles fine without any warning and errors(-Wall & -Wextra), but it crashes on runtime... in gdb and lldb, it says that free() is freeing a non-allocated memory.

after allocating a pointer array, when i allocate memory after that, exactly 2 of the memory block(is this the right word?) will have random values that cannot be initialized.

i tried allocating another array before i assign it to the pointer array. it prints fine(no 2 random values), but will still occasionally crash when assigning values on it. why is this happening?

score 2 · Accepted Answer · edited May 23 '17 at 12:29

2

This

arr=(int **)calloc(row, sizeof(int));

should be

arr = calloc(row, sizeof(int*));

(note that the cast is not required in C and actually discouraged)

So on implementations where the size of a pointer is not the size of int (e.g. on x64 machines), you will invoke undefined behavior when you read outside the bounds of allocated memory.

You might consider not using an explicit type as the operand of sizeof.

arr = calloc(row, sizeof*arr);

This lets the compiler deduce the type and will help you avoid errors like this.

edited May 23 '17 at 12:29

Community

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answered Oct 15 '15 at 01:16

PC Luddite

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i know that. but it still doesnt change the fact that its not working. – hoholee12 Oct 15 '15 at 01:17
@hoholee12 It works fine on my side, no leaks detected – vsoftco Oct 15 '15 at 01:19
@PC Luddite examples please. where can i find the UB in the code? – hoholee12 Oct 15 '15 at 01:22
@hoholee12I think this should correct the error. Are you sure you changed `sizeof(int)` to `sizeof(int*)`? – PC Luddite Oct 15 '15 at 01:23
@hoholee12 Anytime you read or write to the upper-indices of `arr`, you will invoke undefined behavior. You haven't allocated enough memory in `arr` to store all `int*` when `sizeof(int*) > sizeof(int)`. – PC Luddite Oct 15 '15 at 01:27
@PCLuddite ah, didn't notice the sizeof(int*). thanks. – hoholee12 Oct 15 '15 at 01:42

creating a 2d array using double pointer

1 Answers1