Accessing my malloc'd 2d array with [x][y] - objective-c

Question

I'm updating a class to use member variable instead of #defines to define the bounds of a 2d array. It used to look like:
#define kWidth 3
#define kHeight 100
NSUInteger fields[kWidth][kHeight];
Now kWidth and kHeight are iVars. I switched to malloc convention, because I see no other choice as the bounds can now change. The problem is I cannot access the array using two [] ([][]). See my inline comments. I am sure I've malloc'd correctly. I've done this many times before, and under iOS. Why can't I access this way?
self.kFieldsHeight = 100;
self.kFieldsWidth = 3;
NSUInteger** fields = (NSUInteger**)malloc(sizeof(NSUInteger) * self.kFieldsHeight * self.kFieldsWidth);
memset(fields, 0xFF, self.kFieldsWidth * self.kFieldsHeight * sizeof(NSUInteger));
//// Now with LLDB I can examine the array in one dimension
// p fields[0] // 0xFFFFFFFF
// p fields[299] // 0xFFFFFFFF
// p fields[300] // 0xGARBAGE
//// THIS fails with "error: Couldn't dematerialize struct : Couldn't read a composite type from the target: gdb remote returned an
error: E08"
// p fields[0][0]
// Thus this fails in my code
NSUInteger i = fields[0][0];
What's the deal?
Edit: (more detail)
I've also tried mallocing like this:
fields = (NSUInteger**)malloc(sizeof(NSUInteger*) * self.kFieldsHeight);
if(fields){
for(int i = 0; i < self.kFieldsHeight; i++){
fields[i] = (NSUInteger*)malloc(sizeof(NSUInteger) * self.kFieldsWidth);
}
}
Edit: (even more detail). I swapped the width and height with the same results:
fields = (NSUInteger**)malloc(sizeof(NSUInteger*) * self.kFieldsWidth);
if(fields){
for(int i = 0; i < self.kFieldsWidth; i++){
fields[i] = (NSUInteger*)malloc(sizeof(NSUInteger) * self.kFieldsHeight);
}
}

Answer 1

You need to do this:
NSUInteger **fields = malloc(self.kFieldsHeight * sizeof(NSUInteger*));
for (int i = 0; i < self.kFieldsHeight; i++) {
fields[i] = malloc(sizeof(NSUInteger) * self.kFieldsWidth);
memset(fields, 0xFF, self.kFieldsWidth * sizeof(NSUInteger));
}
But this is not the way most people do the job. Most developers use a big array instead of a 2d array and try to index it themselves. Like this:
NSUInteger *fields = malloc(self.kFieldsHeight * self.kFieldsHeight * sizeof(NSUInteger));
fields[h * width + w] = pixel_value;

Answer 2

try this
fields = (NSUInteger**)malloc(sizeof(NSUInteger*) * self.kFieldsWidth);
if(fields){
for(int i = 0; i < self.kFieldsWidth; i++){
fields+i = (NSUInteger*)malloc(sizeof(NSUInteger) * self.kFieldsHeight);
}
}
no real reason for doing it this way, just another way of saying a[i] is a+i
I am using my C knowledge here and not objective C hope I can help