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mex file only found when in working directory, but not when in search path

I would like to use a toolbox containing .m functions and (pre-compiled) .mex files.
The path containing both the function and the mex file are included in the search path. I verified that Matlab can see the function, as help functionname returns the correct help file.
When I try to run some code calling one of the functions, I get the error:
"Invalid MEX-file 'C:\somepath\mexfilename.mexw64': The specified module could not be found."
Note, that it could find the function, but not the mex file located in the same directory as the function.
If I remove the directory from the search path, it (obviously) cannot find the function either, creating a different error:
"Undefined function 'functionname' for input arguments of type 'char'."
The strange thing is, that when I set the working directory to the location where the function and the mex file are, everything works.
But, as mentioned, if the directory is only in the search path, but not currently the working directory, it can find the function, but not the mex file.
Seeing that the code runs perfectly well if the working directory points at the mex fiel, I would exclude all problems related to compilation and/or running of the mex files. I think the problem is with the path / visibility of the file, but then why does it see the function?
I recently switched computers. On my former computer I already used the same toolbox and did not run into this problem (Matlab 2018b on win10 in both cases).

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3 Answers

Answer by Philip Borghesani on 1 Apr 2019
Edited by Philip Borghesani on 2 Apr 2019
 Accepted Answer

Is the mex file dependent on other dlls in the same directory? It sounds to me that Matlab is finding the mex file but not some other dependencies.
If that is the problem you could add the directory to the dos path before starting MATLAB or move the other needed dlls to a directory on the system path when MATLAB is running.

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There are two dll files in the same directory, since they are supplied by the people who wrote the toolbox, I assume they are needed.
Before installing and running the dependency walker, like Jan said, I assumed it's these two dll files that cause the problem and copied the two dll files into a directory included in the system path. Now it works, independent of where the matlab working directory points to.
Thank you so much!!

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Answer by Jan
on 1 Apr 2019
Edited by Jan
on 2 Apr 2019

The message
"The specified module could not be found"
means, that the mex function cannot find a required library, most likely a run-time-lib of the compiler. You can use the DependencyWalker to check this, see e.g. https://www.mathworks.com/matlabcentral/answers/92362-how-do-i-determine-which-libraries-my-mex-file-or-stand-alone-application-requires
Note than "Invalid MEX-file 'C:\somepath\mexfilename.mexw64': ..." implies, that the file has been found successfully. Otherwise it would be impossible for Matlab to determine, that the file is invalid.
"when I set the working directory to the location where the function and the mex file are, everything works"
Yes, this is strange. Maybe the MEX function uses libraries in the same folder, but this folder is not included in the search path of the operating system? This can be examined with the DepencyWalker also.

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That's a good idea. Including plenty of descriptive error messages and/or warnings never hurts. I'll try to contact the developers to tell them.
Thanks again!!
I suspect there is not much the author can do about it. The message/error occurs while loading a dll but before any code in it has run. At one point I tried to improve MATLAB's error message in this situation and there are no simple, robust solutions.
Too bad.
Thank you for letting us know!

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Answer by Bruno Luong
on 1 Apr 2019

If the MEX file is compiled with MSVS you might need to install the appropriate Redistributable on the target PC.

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