0xC0000005: Access violation reading location 0x00000000

28
votes

I'm having a very strange issue with a space invaders game I'm working on. Basically I get an access violation error:

Unhandled exception at 0x5edad442 (msvcr100d.dll) in SpaceInvaders.exe: 0xC0000005: Access violation reading location 0x00000000.

when I include the piece of code below. visual studio takes me to "strcmp.asm" when debugging. Note that Im not using strcmp() in any of my code. Is there anything wrong with the code, or is this a problem beyond the scope of what I've included? Thanks for any help

const char* invarray[] = {"invader0.png", "invader1.png", "invader2.png", "invader3.png", "invader4.png"}; 
int i=0;
//Creates 55 invaders
for (int y=0; y<250; y+=50){
    for (int x=0; x<550;x+=50){
        Invader inv(invarray[y/50], x+50, y+550, 15, 15, 1, false, 250);
        invaders[i] = inv;
    }
}

Invader constructor:

Invader::Invader(const char *pic, int x, int y, int w, int h, bool dir, bool des, int point) : MovingObject(pic, x, y, w, h) , direction(dir), destroyed(des), b(0), points(point){};

MovingObject Constructor

MovingObject::MovingObject(const char *pic, int x, int y, int w, int h):picture(pic), positionX(x), positionY(y), width(w), height(h) {};
3
c++access-violation
Show code inside constructor of Invader – Denis Ermolin
The error message suggests that this is a null pointer dereference, but I can't see where this would happen in your code. Apart from the Invader constructor, please show us where/how the invaders array is initialized, too. The copy constructor of Invader might be interesting as well, if it has one. – Medo42
I just updated OP with constructors. @Medo, invader array was just initialized as global variable: "Invader invaders[55];" – Milk
Thanks guys, as Fraser suggested I must have accidentally forgot to increment i. – Milk

3 Answers

36
votes

"Access violation reading location 0x00000000" means that you're derefrencing a pointer that hasn't been initialized and therefore has garbage values. Those garbage values could be anything, but usually it happens to be 0 and so you try to read from the memory address 0x0, which the operating system detects and prevents you from doing.

Check and make sure that the array invaders[] is what you think it should be.

Also, you don't seem to be updating i ever - meaning that you keep placing the same Invader object into location 0 of invaders[] at every loop iteration.

17
votes

This line looks suspicious:

invaders[i] = inv;

You're never incrementing i, so you keep assigning to invaders[0]. If this is just an error you made when reducing your code to the example, check how you calculate i in the real code; you could be exceeding the size of invaders.

If as your comment suggests, you're creating 55 invaders, then check that invaders has been initialised correctly to handle this number.

8
votes

The problem here, as explained in other comments, is that the pointer is being dereference without being properly initialized. Operating systems like Linux keep the lowest addresses (eg first 32MB: 0x00_0000 -0x200_0000) out of the virtual address space of a process. This is done because dereferencing zeroed non-initialized pointers is a common mistake, like in this case. So when this type of mistake happens, instead of actually reading a random variable that happens to be at address 0x0 (but not the memory address the pointer would be intended for if initialized properly), the pointer would be reading from a memory address outside of the process's virtual address space. This causes a page fault, which results in a segmentation fault, and a signal is sent to the process to kill it. That's why you are getting the access violation error.