Initialize pointer to 2D array in structure

Question

What I try to get, is a structure, which contains an 2D array of bytes which should store a binary picture.

typedef enum{
    PIC_ID_DOUBLE_UP,
    PIC_ID_DOUBLE_DOWN,
}PICPictureId_t;

typedef struct{
    PICPictureId_t  picId;
    uint8           width;
    uint8           height;
    uint8           (**dataPointer);
}PICPicture_t;

The dimensions (in pixels) are determined by the width and height. Now I tried to initialize an array of pictures. I want to store the whole data in the flash.

PICPicture_t pictures[] = {
    {
        .height       = 3,
        .width        = 16,
        .picId        = PIC_ID_DOUBLE_UP,
        .dataPointer  = ???        
    }
};

How could I initialize the pointer to the data? I get a version which compiles (after studying the answer here: A pointer to 2d array), if I use a pointer to an array in the picture struct and then set the pointer to the first element of the doubleUp 2D array:

typedef struct{
    PICPictureId_t  picId;
    uint8           width;
    uint8           height;
    uint8           (*dataPointer)[2];
}PICPicture_t;

uint8 doubleUp[3][2] = {
    {0x12 ,0x13},
    {0x22 ,0x32},
    {0x22 ,0x32}
};


PICPicture_t pictures[] = {
    {
        .height       = 3,
        .width        = 16,
        .picId        = PIC_ID_DOUBLE_UP,
        .dataPointer  = &(doubleUp[0]),
    }
};

But here I have to declare the dimension of the second array but I want to make the structure idependent of the dimension and use for this the height and width field.

You can't really initialize it in-line like the other members. You have to manually allocate the rows and the columns using malloc. — Some programmer dude
– Some programmer dude, Commented Sep 12, 2016 at 7:53
If the picture is a NVM constant to be used by an embedded system, malloc doesn't make any sense what-so-ever. — Lundin
– Lundin, Commented Sep 12, 2016 at 8:51

2501 · Accepted Answer · 2016-09-12 08:16:07Z

2

Use a pointer to a one-dimensional array and index it manually:

typedef struct{
PICPictureId_t  picId;
uint8           width;
uint8           height;
uint8           *dataPointer;
}PICPicture_t;

The image data will have to change to a single dimension:

uint8 d[6] = {
    0x12 ,0x13,
    0x22 ,0x32,
    0x22 ,0x32
};

You can initialize it:

PICPicture_t s = { 3 , 2, ID , d };

And interpret it as a 2d array:

uint8 x = 1;
uint8 y = 2;
uint8 value = s.dataPointer[y*width+x];

(I changed the width to 2 from 16 so the example is clearer. The idea is the same if you plan to access single bits. )

edited Sep 12, 2016 at 8:16

answered Sep 12, 2016 at 7:57

2501

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