multidimensional array faulting

Question

So, I need to generate an array, the struct is set up like so:

struct image
{
    int height, width;
    unsigned char (**data)[3];
};
typedef struct image image_t;

And I am using a function to allocate and initially set all the positions to 0 values (the three slots it actually uses)

image_t create_new_image(int height, int width)
{
 int i, j;
 image_t ret;
 ret.data = malloc(height + 5);
 for (i = 0; i < height; i++)
     ret.data[i] = malloc(width + 5);
 for (i = 0; i < height; i++)
 {
     for (j = 0; j < width; j++) //hit every node
     {
        ret.data[i][j][0] = 0;
        ret.data[i][j][1] = 0;
        ret.data[i][j][2] = 0;
     }
 }
 ret.height = height;
 ret.width = width;
 check_nulls(ret);
 return ret;
}

GDB gives the following:

Breakpoint 2, create_new_image (height=20, width=21) at ansilib.c:28
28      for (i = 0; i < height; i++)
(gdb) c
Continuing.

Program received signal SIGSEGV, Segmentation fault.
0x0000000100001292 in create_new_image (height=20, width=21) at ansilib.c:33
33              ret.data[i][j][1] = 0;
(gdb) p i
$1 = 4
(gdb) p j
$2 = 0
(gdb) p ret.data[0]
$3 = (unsigned char (*)[3]) 0x1001038c0
(gdb) p ret.data[1]
$4 = (unsigned char (*)[3]) 0x1001038e0
(gdb) p ret.data[2]
$5 = (unsigned char (*)[3]) 0x100103900
(gdb) p ret.data[3]
$6 = (unsigned char (*)[3]) 0x100103920
(gdb) p ret.data[4]
$7 = (unsigned char (*)[3]) 0x0
(gdb) p ret.data[5]
$8 = (unsigned char (*)[3]) 0x0

Can somebody help me understand what I am doing wrong? I havn't dealt with multidimensional run-time allocated arrays very much, so it is probably just the way it is allocated, but i'm pretty clueless.

Answer 1

image_t create_new_image(int height, int width){
    int i, j;
    image_t ret;
    ret.data = malloc(height*sizeof(*ret.data));
    for (i = 0; i < height; i++)
        ret.data[i] = malloc(width*sizeof(**ret.data));
    for (i = 0; i < height; i++){
        for (j = 0; j < width; j++){
            ret.data[i][j][0] = 0;
            ret.data[i][j][1] = 0;
            ret.data[i][j][2] = 0;
        }
    }
    ret.height = height;
    ret.width = width;
    check_nulls(ret);
    return ret;
}

Answer 2

Your malloc calls are incorrect. The malloc function expects the number of bytes, not the number of object (and I have no idea what + 5 is supposed to indicate).

If you use the idiom ptr = malloc(N * sizeof *ptr) then it makes things easy:

ret.data = malloc(height * sizeof *ret.data);
for (i = 0; i < height; i++)
    ret.data[i] = calloc(width, sizeof *ret.data[i]);

Then the rest of your code is OK as far as I can see.

Using calloc instead of malloc means that you don't need to loop through setting everything to 0. It doesn't matter which way around the arguments are for calloc.

Answer 3

I guess you want to create RGB or YUB image data.

The problem is this line :

ret.data = malloc(height + 5);

Because you declared

unsigned char (**data)[3];

you can't assign memory to ret.data. It is already allocated as 3-element array.

Try

for(int i=0; i<3; i++) {
     ret.data[i] = malloc() // for height
     for(every line)
          ret.data[i][j] = malloc() // for width
          // and set all elements to zero
}