Protobuf crashes Mex-file

Dear all,

I would like to write a Mex-function that uses protobuf. Unfortunately, my program crashes after calling clear all or recompiling.

In order to illustrate my problem I wrote a reduced version. It consists of three files:

1. An empty MEX-file called protoTest.cpp

#include "mex.hpp"
#include "mexAdapter.hpp"

class MexFunction : public matlab::mex::Function {
    void operator()(matlab::mex::ArgumentList outputs, matlab::mex::ArgumentList inputs) {
    }
};

2. A very simple protobuf file called data3d.proto

syntax = "proto2";

package testproto;

message Data3d {
    required float x = 1;
    required float y = 2;
    required float z = 3;
}

3. A Matlab script for compiling and running the whole thing

% Run protobuf compiler
system('protoc --cpp_out=./ ./*.proto');

% Compile mex file
mex('protoTest.cpp', 'data3d.pb.cc', '-lprotobuf');

% Run mex function
disp('Run..');
protoTest();
disp('ok!');

% Clear all
clear all

% Run mex function again -> Crash
disp('Run again..');
protoTest();
disp('ok!');

I have found similar error descriptions online, but in most cases memory leaks were the cause. I don't see this here.

Some things I have tried so far:

• link the static version of libprotobuf -> didn't manage to get it working
• compile the protobuf file into an object file first and afterwards link it to the mex-file -> no difference
• play around with compiler/linker settings - no success, but I have too little experience with that and my attempts were mostly try-and-error based.

Any suggestions on how to make this work would be greatly appreciated.

Thanks in advance,

Torsten

[Nishanth Kumar]

Hi Torsten Knüppel.

Did you get solution for above problem?

[Torsten Knüppel]

Hi Nishanth,

unfortunately not.

There is something mentioned about problems with clashing symbols on the CMake documentation page for their FindMatlab package (https://cmake.org/cmake/help/v3.14/module/FindMatlab.html#known-issues) - but I'm not sure that this is relevant here. Adding the given linker options didn't solve the problem for me.

Best regards

Torsten

[Sebastian Baur]

I just saw a workaround to the clear all / clear mex problem. Using

mexLock();

in your mex file will make it unresponsive to clear commands. This way you can use Matlab normally after your first call to your mex function.

It reinforces the need for a restart of Matlab for recompiling though, as

mexUnlock();

will just bring you back to crashing after clear.

Answer 1

[Martijn]

It appears that this a more generic limitation of protocol buffers, see for example:

https://github.com/protocolbuffers/protobuf/issues/4126

If that is indeed true and protocol buffers simply has not been designed to be loaded multiple times in a single process by "loading, unloading and then again loading" a dynamic library which was linked against protocol buffers, then I do not think it is feasible you can somehow fix/resolve this limitation in a MEX-file (which is in fact a dynamic library; which is loaded on the first call, unloaded by clear mex and then loaded again on the next call). What you might be able to do however is:

1. Prevent your MEX-file from being unloaded by the call to clear mex which is indeed accomplished by a mexLock() as mentiond in Sebastian's comment.

2. Try running your MEX-file in an external mexhost such that you can completely terminate and restart this mexhost process whenever you need to reload your MEX-file.