EXC_BAD_ACCESS when passing a pointer-to-pointer in a struct? - objective-c

Question

I've got a c-array of CGPoints in a struct. I need to replace this array when another CGPoint is added. I'd swear I'm doing this right and it seems to work fine a few times but eventually I'll get a EXC_BAD_ACCESS. What am I missing?
Here's the struct, which I've truncated to remove a lot of items that don't pertain.
typedef struct{
CGPoint **focalPoints;
NSUInteger focalPointCount;
CGRect boundingRect;
}FocalPoints;
Here's how I initialize it:
CGPoint *fPoints = (CGPoint *)malloc(sizeof(CGPoint));
FocalPoints focalInfo = {&fPoints, 0, rect};
Note that focalInfo is passed by reference to another function, like so: anotherFunction(&focalInfo).
Now here's the function that replaces the Points array with a new one:
void AddFocalPoint (CGPoint focalPoint, FocalPoints *focal){
if (focalPoint.x == CGFLOAT_MAX) return;
if (!CGRectContainsPoint(focal->boundingRect, focalPoint)) return;
int origCount = focal->focalPointCount;
int newCount = origCount + 1;
CGPoint *newPoints = (CGPoint *) malloc((newCount) * sizeof(CGPoint));
for (int i = 0; i < newCount; i++)
newPoints[i] = (i < origCount) ? *focal->focalPoints[i] : focalPoint; //error occurs here
free(*focal->focalPoints);
*focal->focalPoints = newPoints;
focal->focalPointCount = newCount;
}
The EXC_BAD_ACCESS error occurs in the above code on line 8: newPoints[i] = (i < origCount) ? *focal->focalPoints[i] : focalPoint;. So what exactly am I doing wrong?

Answer 1

This is a bit of a long shot, but maybe there's an issue with operator priority in *focal->focalPoints[i]. Have you try adding parentheses according to what you are trying to achieve ?

Answer 2

I believe the issue comes with where GCPoint *fPoints allocated as &fPoints evaluates to an address of that ... which is no longer valid once the function exits.
(The data to which it points was allocated fine with malloc.)

Answer 3

Aside from the suggestion I made in a comment, of using a linked list/NSMutableArray, my other suggestion would be that you use realloc() instead of constantly using malloc(), copying by hand, and then free()ing the old allocation.
void * realloc(void *ptr, size_t size);
The realloc() function tries to change the size of the allocation pointed to by ptr to size, and returns ptr. If there is not enough room to enlarge the memory allocation pointed to by ptr, realloc() creates a new allocation, copies as much of the old data pointed to by ptr as will fit to the new allocation, frees the old allocation, and returns a pointer to the allocated memory.
This is pretty much exactly what you are doing, but you can let the library handle it for you.
(May I also humbly suggest using the word "focal" slightly less to name variables in your function?) (Also also, I'm not really clear on why focalPoints in your struct is a pointer-to-pointer. You just want an array of structs -- a single pointer should be fine.)
Consider the following (somewhat extensive) rewrite; hope that it's helpful in some way.
typedef struct{
CGPoint *points; // Single pointer
NSUInteger count;
CGRect boundingRect;
} FocalPoints;
// Renamed to match Apple's style, like e.g. CGRectIntersectsRect()
void FocalPointsAddPoint (FocalPoints *, CGPoint);
void FocalPointsAddPoint (FocalPoints *f, CGPoint thePoint){
if (thePoint.x == CGFLOAT_MAX) return;
if (!CGRectContainsPoint(f->boundingRect, thePoint)) return;
NSUInteger origCount = f->count; // |count| is typed as NSUInteger; |origCount|
NSUInteger newCount = origCount + 1; // and |newCount| should be consistent
// Greatly simplified by using realloc()
f->points = (CGPoint *) realloc(f->points, newCount * sizeof(CGPoint));
(f->points)[newCount-1] = thePoint;
f->count = newCount;
}
int main(int argc, const char * argv[])
{
#autoreleasepool {
// Just for testing; any point should be inside this rect
CGRect maxRect = CGRectMake(0, 0, CGFLOAT_MAX, CGFLOAT_MAX);
// Can initialize |points| to NULL; both realloc() and free() know what to do
FocalPoints fp = (FocalPoints){NULL, 0, maxRect};
int i;
for( i = 0; i < 10; i++ ){
FocalPointsAddPoint(&fp, CGPointMake(arc4random() % 100, arc4random() % 100));
NSLog(#"%#", NSStringFromPoint(fp.points[i]));
}
}
return 0;
}