Question

I’m working with some really large image cubes that are x * y * z in dimension.

Currently I’ve been dealing with them as such

int ***input = malloc(sizeof(int **)*(lines));
int d;
int i;
for(i = 0 ; i<lines ; i++) {
    input[i] = malloc(sizeof(int *)*bands);

    for(d = 0 ; d<bands ; d++) {
        *input[i][d] = malloc(sizeof(int)*(samples));
    }
}

This has worked fine for me, but now I’m rewriting some of the code and would like to be able to pass the array by reference

I thought to do so required me passing such as foo(&input)

where the function looks like:

foo(int ****input) {
    *input = malloc(sizeof(int **)*(lines));
    int d;
    int i;
    for(i = 0 ; i<lines ; i++) {
        *input[i] = malloc(sizeof(int *)*bands);

        for(d = 0 ; d<bands ; d++) {
            *input[i][d] = malloc(sizeof(int)*(samples));
        }

    }
}

However, I appear to receive seg faults after it enters the first for(i...)` loop.
Any suggestions would be very helpful, thank you.

Answer 1

This is fine when input is the pointer to the 3D vector:

/* original */
input[i] = malloc(sizeof(int *)*bands);

When input becomes an int ****: a pointer to the vector pointer, this change is incorrect:

/* original */
*input[i] = malloc(sizeof(int *)*bands);

You want:

/* original */
(*input)[i] = malloc(sizeof(int *)*bands);

In C, *x[y] means *(x[y]).

A much simpler thing would be to use a local variable:

void function(int ****pinput)
{
  int ***input = malloc(/* ... */);
  /*...code stays the same as before...*/
  *pinput = input; /* place it in the location specified by caller */
}

Also, let’s make a few stylistic adjustments to the original. (Ignoring the lack of malloc failure checking):

int ***input = malloc(lines * sizeof *input);

int d;
int i;

for(i = 0 ; i<lines ; i++) {
    input[i] = malloc(bands * sizeof *input[0]);

    /* Noticed an error here: you have *input[i][d] = ... 
       but input[i][d] the pointer to the band;
       *input[i][d] is input[i][d][0]! */
    for(d = 0 ; d<bands ; d++)
        input[i][d] = malloc(samples * sizeof *input[0][0]);
}

I just took out some unnecessary parentheses and changed the sizeof calculation so that instead of repeating (int **), etc, it is based off the type of the pointer expression being assigned to.