GCC Compilation Error on array assignment

There are multiple issues in this code.

1. The allocating storage for string, one must include one byte for a null terminator. Replace:

char store[strlen(text)];

with:

char store[strlen(text) + 1];

Additionally store must be big enough to contain the excess which is up to 3 spaces.

char store[strlen(text) + 3 + 1];

2. In C you cannot use an array as a value. It is converted to a pointer to it's first element in pretty must every context. Therefore it is not possible to return an array directly. It could be workaround by wrapping an array with a struct but it a topic for another day.

As result return arr will be equivalent to return &arr[0] which is int (*)[XXX] a pointer to int array of size XXX.

3. Never ever return a pointer to an object with automatic storage. It's Undefined Behaviour. I know that the intention was returning an array not a pointer to it. Create an object with dynamic storage with malloc-like function to safely return a pointer.

4. Returning Variable Length Array (VLA) by value is not possible because Variably Modified (VM) types cannot be defined at file scope.

5. It looks that indices are swapped in:

printf("%d\n", arr[i][k]);

I guess it should be arr[k][i].

Now... let's solve it.

Returning VLA is tricky. One solution is to pass a pointer to VLA as an argument. See https://stackoverflow.com/a/14088851/4989451. The issue with this solution is that the caller must be able to compute the dimensions.

The other way it to wrap the result of the convert() to a struct. Note that the function and the struct can share the name. The result with have the sizes of VLA as n and m members and the pointer to the data as arr. The caller need to cast it to proper VM type.

To cumbersome casts between the non-trivial pointer types, one can cast via void*.

When all work with the array is done, release it memory with free().

// Converts an input string to its respective ASCII matrix.

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

// Converts the entire string into an multi-dimensional array.
struct convert {
    int n, m;
    int *arr;
} convert(char text[]){

    // copy the input text into a local store.
    size_t textlen = strlen(text);
    char store[textlen + 3 + 1];
    strcpy(store, text);

    // make sure the length of the input string is a multiple of 3 or make it so.
    int excess = textlen % 3;
    char excess_spaces[3] = "   ";
    if (excess != 0) {
      strncat(store, excess_spaces, 3-excess);
    }
    size_t storelen = strlen(store);

    // allocate VLA with dynamic storage
    int (*arr)[storelen / 3] = malloc(3 * sizeof *arr);

    // covert the source into an array
    int steps = storelen / 3;
    for (int i = 0; i < steps; i++) {
      int t[3];
      for (int k = 0; k < 3; k++) {
        t[k] = (int) store[3*i+k];
        arr[k][i] = t[k];
      }
    }

    return (struct convert){ .n = 3, .m = steps, .arr = (int*)arr };
} 

int main() {

  char source[] = "This is the source. "; // placeholder text

  struct convert res = convert(source);
  int n = res.n, m = res.m;
  int (*arr)[m] = (void*)res.arr;

  for (int i = 0; i < n; i++, puts("")) {
    for (int k = 0; k < m; k++) {
      printf("%d ", arr[i][k]); // error occurs at this line.
    }
  }
  free(arr);

  return 0;
}