SDL_GetPixel pointer problems - c++

Question

This is my very first question:
First of these 2 functions you see here below works fine to some extent:
Uint32 AWSprite::get_pixelColor_location(SDL_Surface * surface, int x, int y) {
int bpp = surface->format->BytesPerPixel;
/* Here p is the address to the pixel we want to retrieve */
Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;
switch (bpp) {
case 1:
return *p;
case 2:
return *(Uint16 *)p;
case 3:
if (SDL_BYTEORDER == SDL_BIG_ENDIAN)
return p[0] << 16 | p[1] << 8 | p[2];
else
return p[0] | p[1] << 8 | p[2] << 16;
case 4:
return *(Uint32 *)p;
default:
return 0;
}
}
void AWSprite::set_all_frame_image_actual_size() {
/* This function finds an entire rows that has transparency
then stores the amount of rows to a Frame_image_absolute structure
*/
absolute_sprite = new Frame_image_absolute*[howManyFrames];
for (int f = 0; f < howManyFrames; f++) {
SDL_LockSurface(frames[f]);
int top_gap = 0; int bottom_gap = 0;
int per_transparent_px_count = 1;
for (int i = 0; i < frames[f]->h; i++) {
int per_transparent_px_count = 1;
if (this->get_pixelColor_location(frames[f], j, i) == transparentColour) per_transparent_px_count++;
if (per_transparent_px_count >= frames[f]->w) {
if (i < frames[f]->h / 2) {
per_transparent_px_count = 1;
top_gap++;
} else {
per_transparent_px_count = 1;
bottom_gap++;
}
}
}
}
int realHeight = frames[f]->h - (top_gap + bottom_gap);
absolute_sprite[f] = new Frame_image_absolute();
absolute_sprite[f]->offset_y = top_gap;
absolute_sprite[f]->height = realHeight;
}
}
When i ran this i get:
Unhandled exception at 0x00173746 in SE Game.exe: 0xC0000005: Access violation reading location 0x03acc0b8.
When i when through debuging, i found that it crashes at:
When iterators variable f == 31, i == 38, j = 139
And stops at AWSprite::get_pixelColor_location() in the line at " return *(Uint32 *)p;
I found that if i ran it again and go through debugging line by line then i will works sometime and sometime it dont!!! So i mean that "It crash at randomly when f > 30, i, j iterators value"
What is going on...

Answer 1

I cannot comment on the question yet, but here are some questions:
Where does j come from? Based on the get_pixelColor_location function I would assume that you're iterating over the width of the surface. This part seems to be missing from the code you posted.
Did you validate that i and j are within the bounds of your surface?
Also, you don't seem to Unlock the surface.
Running your function seems to work adequately here so I suspect you're reading outside of your buffer with invalid parameters.