MATLAB Central Newsreader - Using mexGetVariablePtr and mexGetVariable

Using mexGetVariablePtr and mexGetVariable

lee — Sat, 14 Jun 2008 18:32:01 -0400

hello, I have some very simple code to get a variable from
the matlab workspace into my mex function via
mexGetVariablePtr. Problem is that it seems to be getting a
0 all the time even though the workspace value (hullo=5 in
the matlab workspace) is not zero. To debug it I'm
printf-ing the initial value returned from
mexGetVariablePtr, and the numbers I'm deriving from that in
trying to get the 'hullo' value.

When I delete the hullo variable fromthe matlab workspace
and run the mex file matlab has a 'segmentation violation
detection'. This also happens if I change the variable name
from 'hullo' in my mex file to a nonexisting variable name.
-So- it seems as if the mex file is getting something
related to the 'hullo' variable, but I can't get it's actual
value,... here are some exampled printf results from when
I've run the program (hullo=5 in matlab):
myarray 658681640 | alpha 20379184 | ho1 0
myarray 658690856 | alpha 20379264 | ho1 0
myarray 658601072 | alpha 20361152 | ho1 0

here's the code:

#include "mex.h"

void mexFunction(int nlhs, mxArray *plhs,
int nrhs, mxArray *prhs[])
{
int ThreadNr;
double *alpha;
double ho1;
const mxArray *myarray;

myarray = mexGetVariablePtr("base", "hullo"); //can be base
caller or global

alpha = mxGetPr(myarray);
ho1 = *alpha;

mexPrintf("myarray %d | alpha %d | ho1 %d", myarray,
alpha, ho1);
}

Re: Using mexGetVariablePtr and mexGetVariable

Sebastiaan — Sat, 14 Jun 2008 19:58:01 -0400

>
> #include "mex.h"
>
> void mexFunction(int nlhs, mxArray *plhs,
> int nrhs, mxArray *prhs[])
First of all, which Matlab version are you using? The
correct function call is:

void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const
mxArray* prhs[])

(at least, for a number of versions).
> mexPrintf("myarray %d | alpha %d | ho1 %d", myarray,
> alpha, ho1);

The bug is here. The variable is recorded correctly, but it
is a double, while printed as a integer. Use %f instead.

Btw: to prevent your code from crashing, check if
myarray!=0, e.g.:

if (myarray==NULL)
mexErrMsgTxt("Variable hullo not initialized\n")

Greetings,
Sebastiaan

Re: Using mexGetVariablePtr and mexGetVariable

lee — Sat, 14 Jun 2008 23:12:01 -0400

"Sebastiaan " <s.breedveld@erasmusmc.REMOVE.BOO.BOO.nl>
wrote in message <g317s9$3sm$1@fred.mathworks.com>...
> >
> > #include "mex.h"
> >
> > void mexFunction(int nlhs, mxArray *plhs,
> > int nrhs, mxArray *prhs[])
> First of all, which Matlab version are you using? The
> correct function call is:
>
> void mexFunction(int nlhs, mxArray* plhs[], int nrhs, const
> mxArray* prhs[])
>
> (at least, for a number of versions).
> > mexPrintf("myarray %d | alpha %d | ho1 %d", myarray,
> > alpha, ho1);
>
> The bug is here. The variable is recorded correctly, but it
> is a double, while printed as a integer. Use %f instead.
>
> Btw: to prevent your code from crashing, check if
> myarray!=0, e.g.:
>
> if (myarray==NULL)
> mexErrMsgTxt("Variable hullo not initialized\n")
>
> Greetings,
> Sebastiaan
>

thanks, that was it, I just posted some instructions on mex
file multithreading (I needed the above help for a
multithreading prog I was writing) here:
http://www.instructables.com/id/Matlab-Multithreading-EASY/