C String Allocation Wonkiness

Question

Having trouble understanding and getting to work String operations in the following code. Please help, me and my study colleagues are losing our minds over this. ty. This is a simple method to fill a multi dimensional array with custom strings - which for some reason we cannot figure out for the life of us does simply not work - spits out random junk from the memory instead. Also allocation amounts don't seem to be quite right.

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

char*** createKeyPad(int rows, int cols, int num_chars) {

    if(num_chars <= 0) return NULL;

    char needed = 'a';

    char** aptr = NULL;
    char*** rptr = NULL;

    aptr = (char**) malloc(rows * cols * sizeof(char*));
    if(aptr == NULL) {
        return NULL;
    }

    rptr = (char***) malloc(rows * sizeof(char**));
    if(rptr == NULL) {
        free(aptr);
        return NULL;
    }

    for(int row = 0; row < rows; row++) {
        rptr[row] = aptr + (row * cols);
    }

    for(int row = 0; row < rows; row++) {
        for(int col = 0; col < cols; col++) {
            char* string;
            for(int i = 0; i < num_chars; i++) {
                string[i] = needed;
            }
            string[num_chars] = '\0';
            rptr[row][col] = string;
            printf("%s", string);
        }
    }

    printf("%s", "hallo");
    return rptr;

}


int main() {
    printf("Hello, World!\n");
    char*** keypad = createKeyPad(5, 5, 3);

    for(int row = 0; row < 5; row++) {
        for(int col = 0; col < 5; col++) {
            printf("%s", keypad[row][col]);
        }
        printf("\n");
    }
   
}

Answer 1

You have plenty problems in this code.

1. string is a dangling pointer - ie it was not initialized and does not reference a valid char array
2. even if string was referencing a valid object you assign the same pointer to all (pseudo)array elements.
3. Do not use *** pointers.
4. use the correct type for sizes
5. Use positive checks and try to minimize function returns.
6. arrays are indexed from 0 in C and even if the string was referencing an array of num_chars elements, string[num_chars] = '\0'; is accessing an element outside the array bounds.
7. I would use array pointers and use only one allocation to allocate the space for the whole 3D array.
8. Use objects instead of types in sizeofs

int createKeyPad(size_t rows, size_t cols, size_t numChars, char (**pad)[cols][numChars]) 
{
    int result = 0;

    if(numChars > 1)
    {
        *pad = malloc(rows * sizeof(**pad));
        if(*pad)
        {
            result = 1;
            for(size_t row = 0; row < rows; row++) 
            {
                for(size_t col = 0; col < cols; col++) 
                {
                    for(size_t i = 0; i < numChars - 1; i++) 
                    {
                        (*pad)[row][col][i] = row * cols + col + '0';
                    }
                    (*pad)[row][col][numChars - 1] = 0;
                }
            }

        }
    }
    return result;
}


int main(void) 
{
    printf("Hello, World!\n");
    char (*keypad)[5][3];

    if(createKeyPad(5, 5, 3, &keypad))
    {
        for(size_t row = 0; row < 5; row++) 
        {
            for(size_t col = 0; col < 5; col++) 
            {
                printf("%s ", keypad[row][col]);
            }
            printf("\n");
        }
    }
    free(keypad);
}

https://godbolt.org/z/6zY4zbGW3

Answer 2

The main problem is that char* string; followed by string[i] = needed; is dereferencing an invalid pointer because string is not pointing to anything valid.

In the code's current style of allocating one block for each level and dividing the block up, the memory for all the strings could be allocated in one big block:

char* sptr = (char*) malloc(rows * cols * (num_chars + 1) * sizeof(char));

(Note: The (char*) typecast is not required. Also the * sizeof(char) is not required since sizeof(char) is 1 by definition, but I put it in there in case the code is changed to use something other than char at a later date.)

Then the string variable in the nested loop can be initialized as follows:

char* string = sptr + (row * cols + col) * (num_chars + 1);