Undocumented Matlab
  • SERVICES
    • Consulting
    • Development
    • Training
    • Gallery
    • Testimonials
  • PRODUCTS
    • IQML: IQFeed-Matlab connector
    • IB-Matlab: InteractiveBrokers-Matlab connector
    • EODML: EODHistoricalData-Matlab connector
    • Webinars
  • BOOKS
    • Secrets of MATLAB-Java Programming
    • Accelerating MATLAB Performance
    • MATLAB Succinctly
  • ARTICLES
  • ABOUT
    • Policies
  • CONTACT
  • SERVICES
    • Consulting
    • Development
    • Training
    • Gallery
    • Testimonials
  • PRODUCTS
    • IQML: IQFeed-Matlab connector
    • IB-Matlab: InteractiveBrokers-Matlab connector
    • EODML: EODHistoricalData-Matlab connector
    • Webinars
  • BOOKS
    • Secrets of MATLAB-Java Programming
    • Accelerating MATLAB Performance
    • MATLAB Succinctly
  • ARTICLES
  • ABOUT
    • Policies
  • CONTACT

Better MEX error messages

February 24, 2016 2 Comments

I would like to introduce guest blogger Pavel Holoborodko, the developer of the Advanpix Multiprecision Computing Toolbox for MATLAB. Pavel has already posted here in the past as a guest blogger about undocumented Matlab MEX functions. Today he will discuss another little-known aspect of advanced MEX programming with Matlab.
MEX API provides two functions for proper handling of erroneous situations: the legacy mexErrMsgTxt and the newer mexErrMsgIdAndTxt. Both show error message, interrupt execution of a MEX module and return control to Matlab immediately.
Under the hood, these functions are implemented through C++ exceptions. The reason for this design choice is unclear. Throwing C++ exceptions across module boundary (e.g. from dynamic library to host process) is unsafe and generally considered as bad practice in software design. Exceptions are part of C++ run-time library and different versions of it might have incompatible implementations.
This restricts MEX to use only the same version of C++ run-time and GCC which were used to build that particular version of Matlab itself. This is one of the reasons why TMW distributes its own version of GCC/libstdc++ along with every release of Matlab and pushes developers to use it for MEX compilation.
Such unfortunate design decision have two unpleasant consequences:

  1. developers must use some old version of GCC (forget all the fancy stuff from C++11, C++14, etc.)
  2. compiled MEX modules most likely will not work on older/newer versions of MATLAB (re-compilation is required).

The good news is that both issues are solvable and I will write more about this in the future (it is actually possible to create single binary MEX module, which will work on any GNU Linux flavor regardless of the versions of libc/libstc++/gcc installed in the system or used in Matlab).
Here I propose just first step towards freedom – avoid direct usage of mexErrMsg** functions. Use a simple wrapper instead:

void mxShowCriticalErrorMessage(const char *msg)
{
    mxArray *arg;
    arg = mxCreateString(msg);
    mexCallMATLAB(0,0,1,&arg,"error");
}

void mxShowCriticalErrorMessage(const char *msg) { mxArray *arg; arg = mxCreateString(msg); mexCallMATLAB(0,0,1,&arg,"error"); }


The mxShowCriticalErrorMessage function calls Matlab’s built-in error function via the interpreter, with the error message as input parameter.
In addition to being safe, this approach potentially gives us better control over what additional information is shown together with error messages. Instead of a string, we can use an errorStruct as input argument to Matlab’s error function, with its fields tuned to our requirements (not shown here as I want to keep the example simple).
Even without tuning, output of mxShowCriticalErrorMessage is much more informative and user-friendly:

  • Error message from Matlab’s built-in functionality:

    >> A = magic(3);
    >> A(0)
    Subscript indices must either be real positive integers or logicals.

    Nice one-line message without any distracting information.
     

  • Error message from MEX using mexErrMsgTxt/mexErrMsgIdAndTxt:

    >> A = mp(magic(3));   % convert matrix to arbitrary precision type, provided by our toolbox
    >> A(0)                % subsref is called from toolbox, it is implemented in mpimpl.mex
    Error using mpimpl
    Subscript indices must either be real positive integers or logicals.
    Error in mp/subsref (line 860)
            [varargout{1:nargout}] = mpimpl(170, varargin{:});

    Intimidating four lines with actual error message lost in the middle. All the additional information is meaningless for end-user and actually misleading.
    The worst thing is that such error message is very different from what user get used to (see above one-liner), which leads to confusion if MEX plugin is properly working at all.
     

  • Error message from MEX using mxShowCriticalErrorMessage:

    >> A = magic(3);
    >> A(0)
    Error using mp/subsref (line 860)
    Subscript indices must either be real positive integers or logicals.

    Now the message is clear and short, with error description in the last line where the user focus is.
     

Related posts:

  1. Checking status of warning messages in MEX – Undocumented Mex functions can be used to extract the state of Matlab warnings in run-time. ...
  2. Setting system tray popup messages – System-tray icons and messages can be programmatically set and controlled from within Matlab, using new functionality available since R2007b....
  3. Sending email/text messages from Matlab – Sending emails and SMS (text) messages from Matlab is easy, once you know a few quirks....
  4. Matlab compilation quirks – take 2 – A few hard-to-trace quirks with Matlab compiler outputs are explained. ...
  5. Parsing mlint (Code Analyzer) output – The Matlab Code Analyzer (mlint) has a lot of undocumented functionality just waiting to be used. ...
  6. Faster csvwrite/dlmwrite – The speed of the builtin csvwrite, dlmwrite functions can be improved dramatically. ...
Mex Pavel Holoborodko Undocumented feature
Print Print
« Previous
Next »
2 Responses
  1. Yaroslav February 25, 2016 at 20:17 Reply

    A very interesting post. Is this problem persists for compiling MEX with c or Fortran, or is there another behaviour altogether?

    • Pavel Holoborodko February 26, 2016 at 13:42 Reply

      I guess it will come down to compatibility of setjmp/longjmp used in LIBC, that you compile your MEX against.
      (C++ exceptions are implemented through these low-level functions with all the additional things for calling class destructors, etc.)

      Cannot say about Fortran though – but probably the situation is similar there.

Leave a Reply
HTML tags such as <b> or <i> are accepted.
Wrap code fragments inside <pre lang="matlab"> tags, like this:
<pre lang="matlab">
a = magic(3);
disp(sum(a))
</pre>
I reserve the right to edit/delete comments (read the site policies).
Not all comments will be answered. You can always email me (altmany at gmail) for private consulting.

Click here to cancel reply.

Useful links
  •  Email Yair Altman
  •  Subscribe to new posts (email)
  •  Subscribe to new posts (feed)
  •  Subscribe to new posts (reader)
  •  Subscribe to comments (feed)
 
Accelerating MATLAB Performance book
Recent Posts

Speeding-up builtin Matlab functions – part 3

Improving graphics interactivity

Interesting Matlab puzzle – analysis

Interesting Matlab puzzle

Undocumented plot marker types

Matlab toolstrip – part 9 (popup figures)

Matlab toolstrip – part 8 (galleries)

Matlab toolstrip – part 7 (selection controls)

Matlab toolstrip – part 6 (complex controls)

Matlab toolstrip – part 5 (icons)

Matlab toolstrip – part 4 (control customization)

Reverting axes controls in figure toolbar

Matlab toolstrip – part 3 (basic customization)

Matlab toolstrip – part 2 (ToolGroup App)

Matlab toolstrip – part 1

Categories
  • Desktop (45)
  • Figure window (59)
  • Guest bloggers (65)
  • GUI (165)
  • Handle graphics (84)
  • Hidden property (42)
  • Icons (15)
  • Java (174)
  • Listeners (22)
  • Memory (16)
  • Mex (13)
  • Presumed future risk (394)
    • High risk of breaking in future versions (100)
    • Low risk of breaking in future versions (160)
    • Medium risk of breaking in future versions (136)
  • Public presentation (6)
  • Semi-documented feature (10)
  • Semi-documented function (35)
  • Stock Matlab function (140)
  • Toolbox (10)
  • UI controls (52)
  • Uncategorized (13)
  • Undocumented feature (217)
  • Undocumented function (37)
Tags
ActiveX (6) AppDesigner (9) Callbacks (31) Compiler (10) Desktop (38) Donn Shull (10) Editor (8) Figure (19) FindJObj (27) GUI (141) GUIDE (8) Handle graphics (78) HG2 (34) Hidden property (51) HTML (26) Icons (9) Internal component (39) Java (178) JavaFrame (20) JIDE (19) JMI (8) Listener (17) Malcolm Lidierth (8) MCOS (11) Memory (13) Menubar (9) Mex (14) Optical illusion (11) Performance (78) Profiler (9) Pure Matlab (187) schema (7) schema.class (8) schema.prop (18) Semi-documented feature (6) Semi-documented function (33) Toolbar (14) Toolstrip (13) uicontrol (37) uifigure (8) UIInspect (12) uitools (20) Undocumented feature (187) Undocumented function (37) Undocumented property (20)
Recent Comments
  • Jianfei (30 days 21 hours ago): I have tried the MathWorks CheckBoxList in Matlab 2015b. For whatever the reason, I can’t change the font properties. I can change the font color, but changing font properties...
  • Donato Coppola (36 days 16 hours ago): Hi Yair, I have a problem with the double format. When I run the treeTable function, the numbers in double format cells are displayed with comma as decimal separator. How can...
  • Kim (42 days 6 hours ago): Yair, the following didn’t work for me either: jh.setBorderPainted(false); Regards, Kim
  • Adil (44 days 8 hours ago): Thank you for the blog, it was useful for me. I have a file named App_project.mlapp.zip and when I extract it through Winzip it gives all the files exactly as you described above. I...
  • Mr Ashley Trowell (46 days 18 hours ago): Thank you so much for this analysis. Also, I find it somewhat horrifying. The take away seems to be to use && / || and NOT and / or. Thanks a bunch! ~Ashley
  • Matt (51 days 19 hours ago): Late reply I know, but you can call custom shortcuts with alt-#. Hold down Alt to see what number is assigned to the shortcuts you’ve created. Now if there was a way to create a...
  • James (59 days 14 hours ago): Is there a way to change the location of the window? They pop up at random locations and the user has to chase them down.
  • Robin (59 days 22 hours ago): Hi Yair, This has been a great help. With the removal of com.mathworks sometime in the future (as announced in R2020b), is there an alternative? For me, just getting a preference...
  • Eric Dziekonski (81 days 20 hours ago): This is fantastic! Thanks everyone! To get access to CEF on 2019b, I had to take the advise of Jeffery Devereux and Xiangrui Li. warning off MATLAB:structOnObject % suppress...
  • Yair Altman (82 days 2 hours ago): jPanel1.setParent(jPanel2) will reparent jPanel1 (and similarly any Java component/container) inside jPanel2 (any Java container). While this might work, it seems to me to be an...
  • Eivind (82 days 16 hours ago): Hi, Thanks for a great function! I was trying to put the main panel of my figure into a scrollable panel with the goal of zooming into the panel using shift+scroll and scrolling up...
  • Yair Altman (97 days 3 hours ago): There is no direct way to replicate the CW in a GUI window, you’ll need to display the text in the GUI using your program logic. Here is one possible implementation that...
  • Sunki Reddy Gunugu (99 days 0 hours ago): hi 1.i would like to know how the error in command window of matlab is captured and displayed in the GUI with the same color in CW 2. Is their any way to replicate the...
  • Matthias Brenneis (101 days 19 hours ago): Hi, thank you so much for the inspiring work and comments! My issue is: As soon as the panel gets an axes, the panel is no longer transparent: f =...
  • Collin (121 days 16 hours ago): Yair, In 2020a Mathworks added this semi-useful class matlab.ui.internal.JavaMigrati onTools Which contains static methods to call javacomponent, actxcontrol or to get an...
Contact us
Undocumented Matlab © 2009 - Yair Altman
Scroll to top