C - Problems stepping through a struct pointer

This is fairly elementary and I'm probably missing something really obvious, but this one has me a bit stumped. Basically, let's say I'm trying to turn an entire PPM image red (that's not actually what I'm doing, but for the sake of troubleshooting, we'll go with that). I have a struct that I'm using to store the pixel values in.

typedef struct pixel_type
{
   unsigned char r;
   unsigned char g;
   unsigned char b;
}  pixel_t;

The pointer that I am using to point to that struct is "buffer."

pixel_t *buffer;

Which I have also allocated the amount of space I need for (the width of the image * the height of the image * the number of pixel values [3] )

buffer = malloc(WIDTH*HEIGHT*sizeof(pixel_t));

I then have a 'for' loop which I am using to step through the buffer pointer and convert every pixel value to red.

int i;
for (i=0;i<(WIDTH*HEIGHT);i++){
   buffer->r=255;
   buffer->g=0;
   buffer->b=0;
   buffer++;} //this part appears to be the problem

The problem here is the output is just a black image with a few garbage pixels right at the top. I have also tried taking out the 'buffer++," and the first pixel appears to be converted to red with no issues. However, when I put it back in, NONE of the pixels are red (not even the first one).

I am not sure what I'm missing. I know that when you create a pointer, you can step through to the next address of the pointer simply by doing the pointer (without a * dereference) with a '++' at the end. Nevertheless, this seems to be the issue. Can someone tell me what I'm doing wrong?

Thanks again,

Austin

Solution

I think the problem is that you are not storing the initial value of the buffer pointer that you have allocated. You increment the pointer in a loop as you go, so buffer points to the end of the buffer after the loop is over.

Use a different pointer in your loop to fix this problem:

int i;
pixel_t *ptr;
for (i=0, ptr = buffer;i<(WIDTH*HEIGHT);i++){
   ptr->r=255;
   ptr->g=0;
   ptr->b=0;
   ptr++;}