Constant, random and inexplicable (to me, anyway) EXC BAD ACCESS in Objective C app

Question

I have Automatic Reference Counting and Zombies enabled...

I keep getting EXC BAD ACCESS to different points in the code, most of the time with no further information coming from zombies.

The objective is to draw a rectangle to the screen with a texture on it that is loaded from an image. It does actually work! But often it is corrupted (the image and the vectors) and then often I just get the exc bad access warning.

I have this sort of structure...

App Delegate Class Declaration:

GWBackgroundScene* background;
EAGLContext *context;
GLKView *view;
GLKViewController *controller;
UIWindow *window;

and then window is made into a property and synthesized.

The did Finish Launching With Options:

context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
[EAGLContext setCurrentContext:context];

view = [[GLKView alloc] initWithFrame:[[UIScreen mainScreen] bounds] context:context];
view.delegate = self;

controller = [[GLKViewController alloc] init];
[controller shouldAutorotateToInterfaceOrientation:UIInterfaceOrientationPortrait];
controller.delegate = self;
controller.view = view;

self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.window.rootViewController = controller;
[self.window makeKeyAndVisible];

background = [[GWBackgroundScene alloc] initWithImage:[UIImage imageNamed:@"DSC_0059.jpg"]];

The AppDelegate acts as an OpenGL delegate and OpenGL calls a function called 'render' in the class which then calls [background render];

My GWBackgroundScene class:

@interface GWBackgroundScene : NSObject
{
    GLKTextureInfo *texture;
    NSMutableData* vertexData;
    NSMutableData* textureCoordinateData;
}  
@property(readonly) GLKVector2 *vertices;
@property(readonly) GLKVector2 *textureCoordinates;
-(void) render;
-(id) initWithImage: (UIImage*)image;

Initialises with:

self = [super init];

if(self != nil)
{
  texture = [GLKTextureLoader textureWithCGImage:image.CGImage options:[NSDictionary  dictionaryWithObject:[NSNumber   numberWithBool:YES]                                                                                        forKey:GLKTextureLoaderOriginBottomLeft]  error:&error];

    vertexData = [NSMutableData dataWithLength:4];
    textureCoordinateData = [NSMutableData dataWithLength:4];

    self.vertices[0] = GLKVector2Make(-2.0,3.0);     
    ...

    self.textureCoordinates[0] = GLKVector2Make(0,0);
    ...
}

return self;

and has these two functions for dealing with the vector and texture information

- (GLKVector2 *)vertices {
  return [vertexData mutableBytes];
}
- (GLKVector2 *)textureCoordinates {
return [textureCoordinateData mutableBytes];
}

and then the render function (which is called by OpenGL via its delegate (the App delegate) uses:

1. texture:

   GLKBaseEffect *effect = [[GLKBaseEffect alloc] init];
   effect.texture2d0.envMode = GLKTextureEnvModeReplace;
   effect.texture2d0.target = GLKTextureTarget2D;
   effect.texture2d0.name = texture.name; 
   

2. self.vertices:

   glVertexAttribPointer(GLKVertexAttribPosition, 2, GL_FLOAT, GL_FALSE, 0, self.vertices);
   

3. self.textureCordinates

   glVertexAttribPointer(GLKVertexAttribTexCoord0, 2, GL_FLOAT, GL_FALSE, 0, self.textureCoordinates);
   

What are the obvious memory issues with what im doing?

Thanks very much

Answer 1

You create an NSMutableData object with size 4 bytes, but the data type GLKVector2 is larger than 1 byte. If you're expecting to store a few objects in there you should do

vertexData = [NSMutableData dataWithLength:4 * sizeof(GLKVector2)];

And similarly for textureCoordinateData.