Bug: MATLAB crashes when mexifying the official `timestwo.c`

Update (20220225): Here is the latest update received from the supporting team of MathWorks.

```

Got the confirmation that the crash is actually considered as "undefined behavior" ... But still we are seeking for a way to avoid such crashes, e.g. we can throw an error message ... Many thanks for reporting this issue with detailed reproducing steps. The developers are now aware of it and will have it fixed in a future release (but we can't reveal develop plans to customers so I'm afraid there won't be any specific information about when and how they're gonna fix it).

```

Below is the original post.

This is a bug of MATLAB. MATLAB crashes when executing crash.m shown below. It has been reported to MathWorks and confirmed on the following versions, all under Linux (but not under macOS or Windows).

• R2018b Update 3 (9.5.0.1049112) 64-bit (glnxa64
• R2019b Update 5 (9.7.0.1319299) 64-bit (glnxa64)
• R2020a (9.8.0.1323502) 64-bit (glnxa64)
• R2021a Update 5 (9.10.0.1739362) 64-bit (glnxa64)
• R2021b Update 1 (9.11.0.1809720) (glnxa64) [check how the crash look like on GitHub Actions]

If you are a Linux user and you want to see how the bug looks like, you can copy crash.m given below, and run

crash 

in the MATLAB command window. Then MATLAB will crash. The bug does not affect macOS or Windows: on Windows, MATLAB will raise an error (which is fine) but not crash; on macOS, MATLAB will handle everything nicely without any abnormalty.

The code is also available on GitHub, where you can find much more information, including why does MATLAB crash in this case and how to circumvent the bug before MathWorks fixes it.

Here is crash.m.

function crash(crash_type, crash_indicator, language)
%CRASH is a script to crash MATLAB provided that MEX is configured for `language` and
% `crash_indicator` is absent or set to true. There are two types of crashes:
% 1. if crash_type == 'setup', then `mex('-setup', language)` crashes;
% 2. if crash_type == 'mex', then `mex(timestwo_src, '-outdir', build_dir)` crashes.


% Parse the input.
if nargin < 1 || ~(isa(crash_type, 'char') || isa(crash_type, 'str'))
    crash_type = 'setup';
end

cind = (nargin < 2 || crash_indicator);

if nargin < 3 || ~(isa(language, 'char') || isa(language, 'str'))
    language = 'C';
end


% Locate the official example for MEX provided by MathWorks.
if strcmpi(language, 'Fortran')
    timestwo_src_name = 'timestwo.F';
elseif strcmpi(language, 'C') || strcmpi(language, 'C++')
    timestwo_src_name = 'timestwo.c';
else
    error('Unknown language: %s', language);
end

official_timestwo_src = fullfile(matlabroot, 'extern', 'examples', 'refbook', timestwo_src_name);


% Directories.

% The test directory.
test_dir = pwd();

% The source directory.
src_dir = fullfile(test_dir, 'src');
if exist(src_dir, 'dir')
    rmdir(src_dir, 's');
elseif exist(src_dir, 'file')
    delete(src_dir);
end

% The build directory.
build_dir = fullfile(test_dir, 'build');
if exist(build_dir, 'dir')
    rmdir(build_dir, 's');
elseif exist(build_dir, 'file')
    delete(build_dir);
end


% Conduct the tests.
for itest = 1 : 2
    fprintf('\n************************* Test %d starts. *************************\n', itest);

    mkdir(src_dir);

    fprintf('\nCopy the official `%s` to the source directory ... ', timestwo_src_name);
    timestwo_src = fullfile(src_dir, timestwo_src_name);
    copyfile(official_timestwo_src, timestwo_src, 'f');
    fileattrib(timestwo_src, '+w');
    fprintf('Done.\n');

    if cind
        % Create in the source directory a file with the same name as the mexified `timestwo`.
        fake_mex_file_name = ['timestwo.', mexext];
        fake_mex_file = fullfile(src_dir, fake_mex_file_name);
        fprintf('\nCreate a fake %s file in the source directory ... ', fake_mex_file_name);
        fclose(fopen(fake_mex_file, 'w'));
        fprintf('Done.\n');
    end

    compile(src_dir, build_dir, timestwo_src_name, crash_type);

    fprintf('\nWhich `timestwo`?\n');
    which('timestwo')

    fprintf('\nEvaluate `timestwo(1)` ... ');
    timestwo(1)

    fprintf('************************** Test %d ends. **************************\n\n', itest);
end


end



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



function compile(src_dir, build_dir, timestwo_src_name, crash_type)
%COMPILE is a script that copies the source files from src_dir to build_dir and then compile them.


% Decide the language being tested.
if endsWith(timestwo_src_name, 'F')
    language = 'Fortran';
elseif endsWith(timestwo_src_name, 'c')
    language = 'C';
else
    error('Unknown source file name %s', timestwo_src_name)
end


% Copy the source files and set up MEX. The order decides what kind of crash will occur.
switch crash_type

case 'setup' % First copy the source files from `src_dir` to `build_dir`, and then set up MEX.

    fprintf('\nCopy the source directory to the build directory ... ');
    copy_src(src_dir, build_dir);
    fprintf('Done.\n');

    fprintf('\nConfigure MEX for compiling %s ...\n', language);
    mex('-setup', language);
    fprintf('Done.\n');

case 'mex'  % First set up MEX, and then copy the source files from `src_dir` to `build_dir`.

    fprintf('\nConfigure MEX for compiling %s ...\n', language);
    mex('-setup', language);
    fprintf('Done.\n');

    fprintf('\nCopy the source directory to the build directory ... ');
    copy_src(src_dir, build_dir);
    fprintf('Done.\n');

otherwise

    error('Unknown crash type %s', crash_type);

end


% Compilation.
fprintf('\nCompile `%s` ... \n', timestwo_src_name);
timestwo_src = fullfile(build_dir, timestwo_src_name);
mex(timestwo_src, '-outdir', build_dir);
addpath(build_dir);
fprintf('Done.\n');


end



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



function copy_src(src_dir, build_dir)

%restoredefaultpath; % `restoredefaultpath` or `rmpath(build_dir)` alone can NOT fix the crash

%clear('timestwo');  % This line alone can fix the crash

%delete(fullfile(src_dir, ['timestwo.', mexext]));  % This line alone can fix the crash

%if exist(build_dir, 'dir'); delete(fullfile(build_dir, ['timestwo.', mexext])); end  % This line alone can fix the crash

copyfile(src_dir, build_dir, 'f');

%clear('timestwo')  % This line alone can NOT fix the crash

%delete(fullfile(src_dir, ['timestwo.', mexext]));  % This line alone can NOT fix the crash

%delete(fullfile(build_dir, ['timestwo.', mexext]));  % This line alone can NOT fix the crash

end

I look forward to this bug fixed. Thanks.

[Stephen23]

@Zaikun Zhang: this is not a bug report forum. If you expect TMW to take any action then you can file a bug report here:

https://www.mathworks.com/support/bug_reports/faq.html

[Zaikun Zhang]

Hi @Stephen . Thank you for your comments. Done with thanks.

Indeed, I believe it may help other MATLAB users by posting the bug here as well.

[James Tursa]

Also note that the timestwo code only works for scalar inputs even though it looks like it is coded for arrays.

[Jan]

I do see a huge effort, a lot of code, Github folders and so on, but I cannot find the information, which command let Matlab crash. Does this happen inside mex() or if you call the compiled function? If this happens in mex() one line of code, or perhaps a 2nd one for copying the source files, would be enough. A description of the actuall crash would be useful also: Is there an error message? A crash dump? Messages in the log files of the OS?

I do not understand, who the files shown in "some workarounds that can circumvent" solve the problem. There are 7 m files - and now?

I am curious: What is the problem?

[Zaikun Zhang]

Hello @Jan, thank you for the comments.

If you are curious and if you are a Linux user, you may copy the compile.m given above and available on GitHub, and then run the following command in you MATLAB command line

crash

Then MATLAB will crash. This bug has been reported to MathWorks and confirmed by MathWorks staff.

If you do not use Linux, then no worry. The bug does not affect macOS or Windows.

I hope the information given in the README of my GitHub repo is helpful to anyone who are interested in this bug.

Look forward to this bug being fixed.

Thank you very much.

[Zaikun Zhang]

Hello @James Tursa, thank you for your comments!

What you said is also how I understand and how I use timestwo. Thanks for raising this insight, which I hope will be helpful to others.

Answer 1

[Zaikun Zhang]

Here is the latest update (20220225) received from the supporting team of MathWorks:

```

Got the confirmation that the crash is actually considered as "undefined behavior" ... But still we are seeking for a way to avoid such crashes, e.g. we can throw an error message ... Many thanks for reporting this issue with detailed reproducing steps. The developers are now aware of it and will have it fixed in a future release (but we can't reveal develop plans to customers so I'm afraid there won't be any specific information about when and how they're gonna fix it).

```