Question

Okay, I have a multi-dimensional array which is statically-allocated. I’d very much like to get a pointer to a portion of it and use that pointer to access the rest of it. Basically, I’d like to be able to do something like this:

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

#define DIM1 4
#define DIM2 4
#define DIM3 8
#define DIM4 64

static char theArray[DIM1][DIM2][DIM3][DIM4] = {0};

int main()
{
    strcpy(theArray[0][0][0], "hello world");

    char** ptr = theArray[0][0];

    printf("%s\n", ptr[0]);

    return 0;
}s

This code results in this error using gcc:

t.c: In function ‘main’:
t.c:17: warning: initialization from incompatible pointer type

char** is apparently not the correct type to use here. I assume it’s because statically-allocated arrays are created as a single block in memory while dynamically allocated ones are separated in memory, with each dimension having a pointer to the next one.

However, as you likely noticed, the number of dimensions here is painfully large, and I’m obviously going to need to use actual variables to index the array rather than the nice, slim character 0, so it’s going to get painfully long to index the array in actual code. I’d very much like to have a pointer into the array to use so that accessing the array is much less painful. But I can’t figure out the correct syntax – if there even is any. So, any help would be appreciated. Thanks.

Answer 1

theArray[0][0] gets rid of the first two dimensions, so you have something of type char [DIM3][DIM4]. The first dimension will drop out when the array decays to a pointer, so the declaration you want is:

char (*ptr)[DIM4] = theArray[0][0];

For what it’s worth, gcc also displays a warning for your array declaration: “warning: missing braces around initializer”. Static variables and global variables will automatically be initialized to 0, so you can fix the warning by getting rid of the initializer:

static char theArray[DIM1][DIM2][DIM3][DIM4];