Mex-FIles - memory management

Hi, I have a mex code where the output variable has the same name as the input variable, but it changes size as a result of the operations of the mex code. For instance, I have something like:

A=Function(A) where A in the input is a 100 X 1 vector (much much larger in my simulation) and the output A is a 50 X 1 vector. I want to understand how memory is managed in this situation. After the operation is finished, does A now occupy 50 X 1 worth of space and the rest is free to allocate to other variables?

Thanks!

Siddharth