Thread: different data types

So I have a TIFF raster image, which is 1200x959. Using the libtiff library, I get its dimensions and store them in two variables, w (width) and h (height). But both of these are of type uint32, which I'm pretty sure is the same as unsigned long. I want to manipulate the data and store it in some 2d arrays (Red, Grn, Blu). To do this, I need some loops. So my first question is what does the compiler do when I write something like

Code:

for (i=0; i<h; i++)?

How does it go about comparing i (which is int) and h? I guess I'm not really knowledgeable at all about the difference between various types, and how the computer deals with them.

Here is my code

Code:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <tiffio.h>

int main()
{
    TIFF *tif;
    int i, j;
    uint32 w, h, z, npixels;
    uint32 *raster;
    
    tif = TIFFOpen("c:\\image1.tif", "r");
    TIFFGetField(tif, TIFFTAG_IMAGEWIDTH, &w);
    TIFFGetField(tif, TIFFTAG_IMAGELENGTH, &h);
    npixels = w * h;
    uint32 Red[h][w]; /*SEGFAULT HERE*/
    uint32 Grn[h][w];
    uint32 Blu[h][w];
    
    raster = (uint32*) _TIFFmalloc(npixels * sizeof(uint32));
    TIFFReadRGBAImage(tif, w, h, raster, 0); 
    for (i = 0; i<h; i++)
    {
        for (j = 0; j<w; j++)
        {
            z = raster[i*w + h];
            Red[i][j] = (z & 0xff);
            Grn[i][j] = (z>>8) & 0xff;
            Blu[i][j] = (z>>16) & 0xff;
        }
    }
    _TIFFfree(raster);
    TIFFClose(tif);
    
    return 0;
}

This segfaults, apparently on the line where I declare Red. I know I'm doing something wrong with types, but I'm not sure what. the function TIFFReadRGBAImage takes the image and puts it into raster. Each element of raster holds 4 bytes, One is the amount of Red, and one for Green and one for Blue, (and one for transparency, I think).
Thanks for your time
PS: I know I shouldn't declare Red, Grn, Blu dynamically like that, but I don't want to worry about using malloc and not being sure about my code for that also. I'll change it when everything else is fixed.

The problem causing the segfault is due to the fact that you cannot dynamically allocate memory to an array in the fashoin you are attempting. Look into the use of malloc, calloc and free functions to allocate memory to the arrays at runtime.

Well, I'm not very familiar with VLAs, so I can't really say much about that. But it really does look like the problem to me. *shrug*

[EDIT]
Are you certain the segfault is on that line?

> int i, j;
So make them the same type as your w and h

> uint32 Red[h][w];
4603200 == 1200x959x4
That's over 12MB of stack space for those 3 variables!!!
Many systems impose a limit on the amount of stack space a single process is allowed to use, or set a default size.
Which system are you on?



> I don't know if #0 or #1 mean anything,
They're just counting the nesting level of function calls.
You can navigate up and down the nested set of function calls to examine the state within any function currently in the call chain.

... using malloc(). And don't forget to free the memory when you're done with it.

Do you even need an array? The calculation looks simple enough that you could calculate it on-the-fly when you need a colour value.

Use a function pointer.

Although, if malloc() really does the work, I don't know why you'd want to hide that fact: people (including you) might mistake create() for something you wrote and forget to free() memory, for instance. It seems like you are complicating things for the sake of doing it.

You shouldn't cast malloc(), which was probably mentioned earlier on.

Am I making an ass of myself by asking what OS you're using?

If it's windows just run it and look at your process in the task manager.

Yes, I think I am going to look like an ass...


[EDIT]

Note to self: Read previous posts more thoroughly...