How to declare a pointer to a pointer array?

My code (which is adopted from C) necessitates me to declare a pointer to a pointer array. While it is typically from a C code, I'm trying to incorporate something like that in Matlab. Since this pointer has to be passed as an argument in a function that is from a shared C library, my pointer which essentially points to a Matrix cannot imitate 'pointer of pointer array' behavior. For a better understanding I'll type in the code from both C and Matlab that I have at the moment.

C code:

void* target[max]; 
for(c=0,c<max,c++)  
{  
      target[c] = malloc(samples*bytes_per_sample);  
}
Myfuncfromsharlib(target,....(other arguments)); /** signature of this function being Myfuncfromsharlib( void** , ...) **/

MATLAB code:

target = libpointer('int16Ptr',int16(zeros(samples,2)));
calllib('sharedlib','Myfruncfromsharlib',target,.. (other arguments)... );

By doing this, whenever I have only one element in my pointer array, I have the answer. But when I have two elements, I have only the second one as the output. For this reason, I tried using a structure of pointers, and also pointer arrays, but they return an error.

Error using calllib

Parameter must be a scalar handle

You guys have any ideas?

Thank You!

[Stephen23]

Tell us what you are actually trying to achieve, rather than your attempt to treat the high-level MATLAB language as if were a low-level language like C:

http://xyproblem.info/

http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem

[RakJuk]

I'm trying to acquire data from an external device that has in-built C libraries which can be made use of. Also, let me tell you that these libraries are easier to access from Matlab in Windows OS as they have a wrapper around the libraries, which in turn creates mex files. However, I have to use this set up on Linux. So, my question is the same as what I asked. I have a parsing/data representation problem since I move to matlab, but still use C functions. I hope my question is better now.

[Philip Borghesani]

If you can get the source code to the Windows wrapper it should not be difficult to port it to Linux. I expect it does something similar to what I suggested in my answer.

Answer 1

[Philip Borghesani]

MATLAB can't create and manage an array of pointers to data buffers for external use. My suggestion is to create a mex file or new shared library that simplifies and helps with the interface to the shared library you are attempting to use. I might start with these functions:

void** create_target(int max, int samples, int bytes_per_sample)
{
   void* target[max]; 
   for(c=0,c<max,c++)  
   {  
      target[c] = malloc(samples*bytes_per_sample);  
   }
   return target;
}

 void * GetScan(void **target, int scan)
 {
      return target[scan)]
  }

% MATLAB code:

   %loadlibrarys
   target=calllib('mylib','create_target',max,samples , bytes_per_sample)
   calllib('sharedlib','Myfruncfromsharlib',target,.. (other arguments)... );

   for n=1:num_scans
        scans(n)=calllib('mylib','GetScan',target,n) % probably want more code here to retrieve pointer value and put scans in matrix or cell array...
   end

It may be possible to avoid writing GetScan and in MATLAB use pointer addition:

scanptr=libpointer('int16Ptr',target);
for n=1:num_scans
     scanptr=libpointer('int16Ptr',target+n-1);
      scanptr.reshape(samples,1);
      scans(:,n)=scanptr.value;
 end

Answer 2

[Thorsten]

Hm. I think it would be best to use genuine Matlab to solve your problem, instead of fiddling with pointers. I'd like to do that when using C, but have never used pointers in Matlab. If you could say what you want to do, there might me a Matlab one-liner to achieve it. Otherwise I fear that we run into an XY problem.

For example, to allocate an array of size samples*bytes_per_sample x max, you can use

target(samples*bytes_per_sample, cmax) = 0;

and you can store Matlab indices in target; indices correspond pretty much t pointers.