CODEWITHSUNDEEP
carrayscalloc
Martin Indrych
Martin Indrych

Reputation: 31

How to calloc a 2D array without valgrind errors?

I am working on a school project, trying to create a 2d array based on variables. 

int **wagner;
wagner = (int **)calloc((sizeofvstup1 + 1), sizeof(int));
for (int i = 0; i < (sizeofvstup1 + 1); i++) {
    wagner[i] =(int *)calloc((sizeofvstup2 + 1), sizeof(int));
}

I use calloc to get 0 on every array place. But valgrind keeps telling me something like:

Invalid write of size 8 at the "wagner[i]..."

sizeofvstup1 and sizeofvstup2 are variables of the length of the array. How am I supposed to use calloc here? Tried many times to change the lines a bit but never helped... :/

What should the code look like to work properly?

Upvotes: 1

Views: 106

Answers (2)

alk
alk

Reputation: 70971

As others mentioned the code uses the wrong size for the 1st allocation.

To never having to think again which size to use when allocating there is simple trick:

Given that the result of a memory allocation is always assigned to a pointer just simply request to allocate (multiples of) the size this target pointer is pointing to by doing:

int **wagner;
wagner = calloc(sizeofvstup1 + 1, sizeof *wagner);
for (size_t = 0; i < (sizeofvstup1 + 1); ++i) {
    wagner[i] = calloc(sizeofvstup2 + 1, sizeof *wagner[i]);
}

(all other changes to the code are unrelated to this "trick")

This way the code would even survive a change from 

int ** wagner;

to let's say

double ** wagner;

without any modifications.

Upvotes: 0

dbush
dbush

Reputation: 224387

You're not allocating the proper amount of size for the first allocation:

wagner =(int **)calloc((sizeofvstup1+1),sizeof(int));

Here you're allocating space for an array of pointers, but you're passing sizeof(int) for the element size. If an int is smaller than an int *, then you don't have enough space and you end up reading/writing past the end of the array.

Change the element size of the allocation to sizeof(int *). Also, don't cast the return value of calloc:

wagner = calloc((sizeofvstup1 + 1), sizeof(int *));

Upvotes: 3

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