EXC_BAD_ACCESS memory issue when accessing instance variables

Question

I'm having issues porting a Java project I worked on a while ago to Objective-C code - I get "Program received signal: "EXC_BAD_ACCESS"", on the first line in this code:

-(Point3D *) unit {
  NSLog(@"%@%@", "X: ", x);
  double length = [self length];
  return [[Point3D alloc] initWithX:x/length andY:y/length andZ:z/length];
}

called from here:

-(id) initWithStart:(Point3D *)start andDirection:(Point3D *)dir {

  if ( self = [super init] ) {

    NSLog(@"%@%@", @"Direction:", [dir toString]);

    printf("Trying to find unit length of direction...\n");
    NSLog(@"%@", [[dir unit] toString]);


    self.start = start;
    self.direction = [dir unit];
  }

  return self;
}

Console output is:

2011-12-09 17:20:14.021 RayTracerProject[16607:407] Direction:(0,0,20)
Trying to find unit length of direction...

The toString method of the Point3D looks like this:

-(NSString *) toString {
  NSNumber *xstring = [NSNumber numberWithDouble:self.x];
  NSNumber *ystring = [NSNumber numberWithDouble:self.y];
  NSNumber *zstring = [NSNumber numberWithDouble:self.z];

  NSString * str = @"(";
  str = [str stringByAppendingString:[xstring stringValue]];
  str = [str stringByAppendingString:@","];
  str = [str stringByAppendingString:[ystring stringValue]];
  str = [str stringByAppendingString:@","];
  str = [str stringByAppendingString:[zstring stringValue]];
  str = [str stringByAppendingString:@")"];

  return str;
}

So, from what I can see, my (Point3D *) dir is alive and well when I check what the value is using my [dir toString] call. But when I try to call [dir unit], it seems I no longer have the variables I did in the object, hence the EXC_BAD_ACCESS error.

What am I doing wrong here? I think it's something to do with the way I'm managing (or not) my memory usage, but I don't know what it is.

Duncan Babbage · Accepted Answer · 2011-12-09 07:46:25Z

2

The NSLog in your method 'unit' should be:

NSLog(@"%@ %g", @"X:", x);

Moving the space is optional, but just makes it clearer. The key issues were that you were missing the @ before the "X: ", and you need to use %g rather than %@ because from your other code x is a double rather than an NSObject. Equivalent—but simpler and thus better—would also be:

NSLog(@"X: %g", x);

Apple's documentation provides the definitive guide to string format specifiers and that and the surrounding documentation explain the use of format specifiers like %@ (to reference an Objective C object) and %g (to reference a 64-bit floating-point number) when constructing an NSString.

edited Dec 9, 2011 at 7:46

answered Dec 9, 2011 at 7:00

Duncan Babbage

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3 Comments

Duncan Babbage Over a year ago

I think our edits crossed in the post. I changed my mind about what the issue was! :)

simont Over a year ago

:) Thanks for the response. I haven't yet found something explaining all the %@, %g etc - but I was bashing my head against this all afternoon - all working now.

Duncan Babbage Over a year ago

I've added a link to the Apple documentation that is the definitive guide on those string format specifiers, which are confusing until you get your head around them. :)

Firoze Lafeer · Accepted Answer · 2011-12-09 07:15:08Z

0

It seems this line here:

NSLog(@"%@%@", "X: ", x);

should look more like this:

NSLog(@"%@%lf", @"X: ", self.x);

Note a few changes here:

From your other code, it seems you have a property named 'x'. self.x references that property. (You may or may not also have an instance variable named 'x')
Also from your other code, it seems your property 'x' is a double. So the placeholder for a double is %lf, not %@. %@ is for NSObjects (and descendants)
NSString literals start with @. So @"X: " is an NSString literal. What you had is a C string.

EDIT

Well, I'm slow as usual. :) Ignore what I said and accept Duncan's answer, which was faster and now cleaner, actually.

edited Dec 9, 2011 at 7:15

answered Dec 9, 2011 at 7:09

Firoze Lafeer

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1 Comment

simont Over a year ago

:) Thanks for the response - all afternoon trying to figure it out on my own, and two answers pinpointing my issues within 10 minutes of posting here. Much appreciated!

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