How do I pass a 2D array into a function with unknown size in C?

Question

I'm trying to read an image into a 2D array and then make modifications to its RGB values and create a resize function next. The dimensions of the image aren't known until the image is loaded so it looks something like this.

I've tried passing in "pixels" into the unsigned char ** and pixels[0][0] to unsigned char* and quite a few other ways I can't remember. I've searched around the web for different methods but none of them seem to work. I've seen that if you define the size of one of the arrays like pixels[][40] it works, but I don't know the size until after the image is loaded.

void invertImg(unsigned char **img, int height, int width)
{
    int h, w;
    for (h = 0; h < height; h++){
        for (w = 0; w < width * 3; w += 3){
            //Blue
            img[h][w] = ~img[h][w];
            //Green
            img[h][w + 1] = ~img[h][w + 1];
            //Red
            img[h][w + 2] = ~img[h][w + 2];
        }
    }
}

    // Get Image name and desired dimensions

    char header[54];
    unsigned char pixels[height][width * 3];

    FILE *in = fopen(img, "rb");
    FILE *out = fopen("out.bmp", "wb");

    fread(header, 1, 54, in);
    fread(pixels, 1, height * width * 3, in);


//function that the 2d array needs to be passed into
invertImg(pixels, height, width);


    fwrite(header, sizeof(char), HEADER_SIZE, out);
    fwrite(pixels, sizeof(char), height * width * 3, out);

    fclose(in);
    fclose(out);

How can I get this unsigned char array into the function to be able to modify its values?

Because C support variable-length arrays, you can use that also for function arguments. But then the dimensions (sizes) needs to be know first, for example by passing them as arguments before the array. — Some programmer dude
– Some programmer dude, Commented Feb 7, 2019 at 17:42
You could cast the argument to type char** and keep your function the way it is. Or, you could dynamically allocate and would have char** nonetheless. — nm_tp
– nm_tp, Commented Feb 7, 2019 at 17:45
The simplest would be to read the file twice - once to get the size and once to read it into your array. Anything else will be much more complex, and therefore be more prone to bugs, and I think would count as premature optimisation unless you have a very strong reason not to. — Euan Smith
– Euan Smith, Commented Feb 7, 2019 at 17:48
@nm_tp An array of arrays is not the same as a pointer to a pointer. — Some programmer dude
– Some programmer dude, Commented Feb 7, 2019 at 17:48

dbush · Accepted Answer · 2019-02-07 17:52:35Z

3

You can pass a multidimensional array with sizes based on other parameters:

void invertImg(int height, int width, unsigned char img[height][width])
{
    ...

A declaration for this function without parameter names would look like this:

void invertImg(int, int, unsigned char img[*][*]);

Then you can call the function like this:

invertImg(height, width, pixels);

answered Feb 7, 2019 at 17:52

dbush

233k27 gold badges261 silver badges334 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Marlin Pierce · Accepted Answer · 2019-02-07 17:48:13Z

0

void invertImg(unsigned char *img, int height, int width)
{
    int h, w;
    for (h = 0; h < height; h++){
        for (w = 0; w < width; w++){
            for (color = 0; color < 3, ++color) {
                img[ 3*width*h + 3*w + color] = ~img[ 3*width*h + 3*w + color]
            }
        }
    }
}

answered Feb 7, 2019 at 17:48

Marlin Pierce

10.1k5 gold badges33 silver badges56 bronze badges

2 Comments

Eugene Sh. Over a year ago

To avoid code duplication I would xor with 0xFF instead. BTW, you are missing semicolon.

Marlin Pierce Over a year ago

Note that we are using just a char pointer, not a pointer to an array. Also, this is an example of how zero based indexing is practical. One based counting would be messier.

Collectives™ on Stack Overflow

How do I pass a 2D array into a function with unknown size in C?

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related