Getting access violation writing location 0xCDCDCDCD when trying to make an image bigger on pixel level in C

Question

I'm trying to make an image 4 times its size by making a temp image, with double the original images width and height.Then take information from each pixel on the original image and give it to 4 pixels on the temp image. Something like this.

Original image:

[1] [2]

[3] [4]

To temp image:

[1][1] [2][2]

[3][3] [4][4]

But I get an access violation writing location 0xCDCDCDCD. Here is my function code:

void makeBigger(Image* plain)
{
Image temp;

temp.height = plain->height * 2;
temp.width = plain->width * 2;

temp.pixels = (Pixel**)malloc(sizeof(Pixel*) * temp.height);
for (int i = 0, k = 0; i < temp.height; i+2, k++)
{
    temp.pixels[i] = (Pixel*)malloc(sizeof(Pixel) * temp.width);
    for (int j = 0, g = 0; j < temp.width; j+2, g++)
    {
        temp.pixels[i][j].r = plain->pixels[k][g].r;
        temp.pixels[i][j+1].r = plain->pixels[k][g].r;
        temp.pixels[i+1][j].r = plain->pixels[k][g].r;
        temp.pixels[i+1][j+1].r = plain->pixels[k][g].r;

        temp.pixels[i][j].g = plain->pixels[k][g].g;
        temp.pixels[i][j+1].g = plain->pixels[k][g].g;
        temp.pixels[i+1][j].g = plain->pixels[k][g].g;
        temp.pixels[i+1][j+1].g = plain->pixels[k][g].g;

        temp.pixels[i][j].b = plain->pixels[k][g].b;
        temp.pixels[i][j+1].b = plain->pixels[k][g].b;
        temp.pixels[i+1][j].b = plain->pixels[k][g].b;
        temp.pixels[i+1][j+1].b = plain->pixels[k][g].b;
    }
}

*plain = temp;

}

On the violation seems to occur on the line

temp.pixels[i+1][j].r = plain->pixels[k][g].r;

as that is when the program breaks and the error shows. What is causing this violation and why? What can be done to fix this?

`for (int i = 0, k = 0; i < temp.height; i+2, k++)`: do you mean `i += 2`? Same for the inner loop. — ad absurdum, Dec 30 '16 at 15:07
"On the violation seems to occur on the line" is not an useful information. The i, j, k, g values in that line when the violation occurs will help you. Print them out. — Elan, Dec 30 '16 at 15:11
Off-topic: [You should not cast malloc return](http://stackoverflow.com/q/605845/2173917) — Stargateur, Dec 30 '16 at 15:12
to change where `plain` points, it has to be passed as a point to pointer I.E. `void makeBigger(Image** plain)` and has to be called similar to: `makeBIgger( &plain ); where plain is already a pointer to an image — user3629249, Dec 31 '16 at 09:00

barak manos · Accepted Answer · 2016-12-30T15:26:34.183

4

Inside the outer loop, at every iteration:

You initialize temp.pixels[i] to point to a properly allocated memory block
You attempt to write into the memory block pointed by temp.pixels[i]
You attempt to write into the memory block pointed by temp.pixels[i+1]

But since you do not initialize temp.pixels[i+1] to point to a properly allocated memory block, attempting to access memory with this variable leads to a memory access violation (or more generally, undefined behavior).

edited Dec 30 '16 at 15:26

answered Dec 30 '16 at 15:11

barak manos

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@Stargateur: Yes, that's true.But in either case, OP uses `temp.pixels[i+1]` without initializing it first... – barak manos Dec 30 '16 at 15:21
Thank you! It works now, and yes as @DavidBowling said, I forgot i+=2 as well(so thanks to him too!). – zaylan Dec 30 '16 at 15:26
@Stargateur: In either case, I've made it slightly more general. Thanks for pointing that out :) – barak manos Dec 30 '16 at 15:27

score 3 · Answer 2 · edited May 23 '17 at 12:16

The magic value 0xCDCDCDCD indicates you're accessing uninitialized heap memory:

0xCDCDCDCD : Used by Microsoft's C++ debugging runtime library to mark uninitialised heap memory

Specifically, you're reading that value from a pointer on the heap, and then attempting to dereference it, causing the illegal memory access.

See In Visual Studio C++, what are the memory allocation representations? for more magic values used by MSVC.

Getting access violation writing location 0xCDCDCDCD when trying to make an image bigger on pixel level in C

2 Answers2