Passing array of float pointers into VBO - objective-c

Question

I'm creating a particle system renderer, the problem is that all my particle positions are encapsulated into classes that integrate them over time and do some other stuff. Instead of copying the values into a separate array to render each frame, I created a point struct like this that I can use to point to all the values once:
typedef struct
{
float *x, *y, *z;
} point;
I then malloc an array of these the same size as my array of particles.
Declaration at the top:
point *points;
Malloc:
points = malloc(sizeof(point) * [particles count]);
I then loop through all the particles and copy the addresses of each position to the corresponding pointer so:
for (int i = 0; i < [particles count]; ++i)
{
points[i].x = &[[particles objectAtIndex:i] getPosition].x;
points[i].y = &[[particles objectAtIndex:i] getPosition].y;
points[i].z = &[[particles objectAtIndex:i] getPosition].z;
}
My get position function just returns a vector struct (float x, y, z).
The first problem I encountered with this is that when I display the values like so:
NSLog(#"%f", *points[0].x);
Instead of reading of the value (say: 0.5), it displays it rounded and negated (so: -1.0).
I then get the data into my VBO like this:
glBufferData(GL_ARRAY_BUFFER, sizeof(point) * [particles count], &points[0], GL_DYNAMIC_DRAW);
I think this is also wrong but can't find any information on how to pass pointers into VBOs.

Answer 1

What you're doing is wrong because in C everything is passed by value, so when you dereference the .x member of the returned struct, you're doing it on a copy of the position struct, which will be popped from the stack if not assigned to any value.
You're design is wrong, you don't need a struct like this one:
typedef struct
{
float *x, *y, *z;
} point;
Rather one like this:
typedef struct
{
float x,y,z;
}
If you still want to assign it to it's original value, in a way that if the original struct's values change, also this struct is changed, then you need a getter who will return a pointer to the struct, and hold a pointer to it:
points[i] = [[particles objectAtIndex:i] getPositionPointer];
This way you're sure that your struct contains the same values of the original struct, but you're violating encapsulation. Which makes me think that maybe you don't really need to hold a pointer to original struct, but if there's a reason to do then I've shown you the way.