The simple answer is to encode it into a uint64 or int64 variable that can be passed around. E.g.,
To encode:
void *vp;
long long *ip;
union {long long theinteger; void *thepointer;} ivp;
:
ivp.theinteger = 0;
ivp.thepointer = vp; // your pointer
plhs[0] = mxCreateNumericMatrix(1,1,mxINT64_CLASS,mxREAL);
ip = (long long *) mxGetData(plhs[0]);
*ip = ivp.theinteger;
To decode:
void *vp;
long long *ip;
union {long long theinteger; void *thepointer;} ivp;
:
ip = (long long *) mxGetData(prhs[0]);
ivp.theinteger = *ip;
vp = ivp.thepointer; // your pointer
However, this raises several issues. If you ever lose track of the pointer at the MATLAB level you will have memory leak and resource tie-up issues. This could be caused for instance if you generate your P inside a function (a local variable) and then some totally unrelated function generates an error and pops you out of the function before you get a chance to call the freestuff(P). Also, it is unclear to me that the memory manager for your createstuff mex function has to necessarily be the same memory manager as your freestuff mex function. It would be if you used the MATLAB Memory Manager for all your allocations, but you don't state any details for what is behind P so I don't know. And then what happens if you inadvertantly clear the mex functions prior to calling freestuff(P)?
To avoid all those issues I would instead advise that you have only one mex function that you call with different options. E.g.,
P = stuff('create'); % return the void pointer
[A B] = stuff('do',P, 13.5,15.6); % Call some mex function that returns two items
C = stuff ('domore',P, A, B); % Do something else
stuff ('free',P);
That way everything is in one routine and you know there will be no memory manager issues. Also you should register a mexAtExit function inside stuff to make sure that P gets free'd if the mex function is cleared. If you can potentially have more than one 'P' floating around in memory at the same time, then you can more easily manage the array of such values inside one mex function as well.
If you really want to have seperate mex functions for all of this, then I would advise that you strongly consider at least having the allocate and deallocate functionality inside one mex routine.
