Matlab: avoiding memory allocation in mex

Question

I'm trying to make my mex library avoid all memory allocation what so even.
Until now, the mex got an input, created some matrices using mxCreate...() and returned this output.
But now I'd like to modify this interface so that the mex itself would not do any allocations.
What I had in mind is that the mexFunction will get as input the matrix to fill values into and return this very same matrix as an output.

Is this supposed to be possible?

The slight alarm that got me thinking if this is at all something I need to be doing is that the left hand arguments come to the mexFunction as const and the right hand argument are non-const. to return the input matrix as an output I'll need to remove this const.

Answer 1

Modifying the right-hand arguments would be a bad idea. Those inputs can be reference counted, and if you modify them when the reference count is greater than one, then you will be silently modifying the value stored in other variables as well.

Unfortunately, I don't believe there is a way to do what you want given the existing MEX API.

Answer 2

Funnily enough I was just looking at this the other day. The best info I found was threads here and here and also this.

Basically it is generally considered a very bad thing in Matlab world... but at the same time, nothing stops you so you can do it - try some simple examples and you will see that the changes are propogated. Just make changes to the data you get from prhs (you don't need to return anything - since you changed the raw data it will be reflected in the variable in the workspace).

However as pointed out in the links, this can have strange consequences, because of Matlabs copy-on-write semantics. Setting format debug can help a lot with getting intuition on this. If you do a=b then you will see a and b have different 'structure addresses' or headers, representing the fact that they are different variables, but the data pointer, pr, points to the same area in memory. Normally, if you change y in Matlab, copy-on-write kicks in and the data area is copied before being changed, so after y has a new data pointer. When you change things in mex this doesn't happen, so if you changed y, x would also change.

I think it's OK to do it - it's incredibly useful if you need to handle large datasets, but you need to keep an eye out for any oddness - try to make sure the data your putting in isn't shared among variables. Things get even more complicated with struct and cell arrays so I would be more inclined to avoid doing it to those.