MATLAB: Do I get Library was not found Error while using external library in parpool

calllibloadlibraryParallel Computing Toolboxparpool

Hi,

I would like to call external library functions in parallel.

I am using the code below.

clear all
if ~libisloaded('shrlibsample')
   addpath(fullfile(matlabroot,'extern','examples','shrlib'))
   loadlibrary('shrlibsample')
end
struct.p1 = 4; struct.p2 = 7.3; struct.p3 = -290;
parpool('AttachedFiles',{'shrlibsample.mexw64','shrlibsample.h','shrlibsample.c', 'shrhelp.h'}); 
parfor i=1:4
[res,st] = calllib('shrlibsample','addStructByRef',struct);
end
unloadlibrary shrlibsample

But, when running this code, I am getting error- ERROR: Library was not found

How to fix this?

Best Answer

This error means that the library you are trying to call is not available in parallel workers.

Although, you have added required files to the parallel pool using 'AttachedFiles', the reason why this error is thrown is that those libraries are not loaded in parallel workers.

In order to have libraries available in those workers, please load libraries in each worker.

So, to fix this issue, please use loadlibrary to load libraries in each worker.

clear all;
addpath(fullfile(matlabroot,'extern','examples','shrlib'))
struct.p1 = 4; struct.p2 = 7.3; struct.p3 = -290;
parpool('AttachedFiles',{'shrlibsample.mexw64','shrlibsample.h','shrlibsample.c', 'shrhelp.h'}); 
parfor i=1:4
    if ~libisloaded('shrlibsample')
        loadlibrary('shrlibsample')
    end
[res,st] = calllib('shrlibsample','addStructByRef',struct);
end
unloadlibrary shrlibsample

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MATLAB: How to call a DLL or Shared Library function using MEX-files instead of the Generic DLL / Shared Library Interface

This change has been incorporated into the documentation in Release 2009a (R2009a). For previous releases, read below for any additional information:

This is a basic example of calling one of the C Structure examples from the Generic DLL documentation using the MEX interface. Much of the work is doing the data conversion that is normally handled by the CALLLIB, LIBPOINTER, and LIBSTRUCT functions.

This example is configured for 32-bit MATLAB 7.4 (R2007a) and Microsoft Visual Studio 2005. Code will be similar but compiler options files will be different for other configurations. For example, the BAT-file will be a SH-file on Linux. The attached BAT-file is a modified version of the MSVC80OPTS.BAT file which is shipped with MATLAB in the following directory:

 <matlabroot>/bin/win32/mexopts

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To execute this example, please download the attached C- and BAT-files to a working directory, and execute:

copyfile([matlabroot '\extern\examples\shrlib\shrlibsample.c']);
copyfile([matlabroot '\extern\examples\shrlib\shrlibsample.h']);
copyfile([matlabroot '\extern\examples\shrlib\shrhelp.h']);
mex -v -g -f msvc80buildlib.bat shrlibsample.c
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[res,st] = myAddStructByRef(struct)

This copies the Generic DLL example files to your working directory, builds the DLL with all relevant Exports files and import libraries (EXP- and LIB-files), then builds the MEX-file to call addStructByRef(). Finally, it calls the function on a test structure.

This will produce output similar to:

 C Struct:
   p1: 4.000000
   p2: 7
   p3: -290
 Calculated sum: -279.000000
 res = 
     p1: 5.5000
     p2: 1234
     p3: 12345678
 st =
   -279

This calls the same function used in the CALLLIB documentation:

addpath([matlabroot '\extern\examples\shrlib'])
loadlibrary shrlibsample shrlibsample.h
struct.p1 = 4; struct.p2 = 7.3; struct.p3 = -290; 
[res,st] = calllib('shrlibsample','addStructByRef',...
libstruct('c_struct',struct));

Best Answer

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