How to "dbup" to ORIGINAL error after try/catch and rethrow?

Hi,

When I debug my code, I use "dbstop if error" to inspect the workspace of the function that created an error.

Now, if an error is caught in a try/catch statement and then rethrown, the "dbstop if error" brings me to the workspace of the catch block in which the rethrow happened. From there, I would like to have a way to get back to the workspace in which the original, caught error happened.

Googling suggests that there is no solution (<http://stackoverflow.com/questions/10429229/debugging-back-to-original-error-with-conditional-exception-handling-i-e-rethr here>) but I want to ask here, too. This is a hugely annoying problem and if MathWorks has not addressed it yet, I definitely hope they will do so soon.

If this is indeed impossible, how do I best deal with such a situation? In my case, I need the try/catch to execute some "cleanup" code after an error before Matlab enters debug mode because my error-causing code has control of the keyboard and screen and I cannot use the computer while it is being executed.

Answer 1

[Jan]

Would this help:

dbstop if all error

?

[EDITED, Thanks Per] In modern Matlab versions this is called:

dbstop if caught error

The former syntax seems to be supported for backward compatibility.

[per isakson]

It seems as if dbstop if all error is an undocumented (I fail to find it) alternative to dbstop if caught error

    >> dbclear all
    >> dbstop if all error
    >> dbstatus    
    Stop if caught error.

    >> dbclear all
    >> dbstop if caught error
    >> dbstatus
    Stop if caught error.

[Jan]

It was documented in Matlab 6.5.

[Matthias]

Unfortunately, dbstop if caught error doesn't help, for two reasons:

1. Any function that makes use of exceptions (handled with try/catch) will now throw errors, not just the one I want to debug. Since I am using third-party code that does this, I cannot reasonably use dbstop if caught error.

2. Dbstop if caught error enters debug mode BEFORE executing the code in the catch block. This makes sense, but in my case, the code in the catch block must be executed to give control of the screen and keyboard back to the user (my code blocks user input during execution).

PS: I'm looking for a general solution that's useful for debugging, not a solution for a specific error-type.

[per isakson]

Overloading error with a sub-function in the m-file (see my answer) is a "general solution". Yes indeed, it smells and it requires write permissions to the m-files, but until The MathWorks (or someone else) provides something better.

Answer 2

[per isakson]

"This is a hugely annoying problem". Yes indeed!

I have a hard time to figure out how dbstop if caught error error_id is supposed to be used. This is the best I can think of:

• set dbstop if error and dbstop if caught error error_id
• run test_excc
• Matlab breaks at line 25 sub_test()
• select sub_test() % Step In doesn't work here(?)
• right click and Run Selected
• Matlab breaks at line 39 error('test:error', 'Noooo!');

and the workspace of sub_sub_test() is available for inspection and debugging, i.e. "the ORIGINAL error". Why doesn't Matlab go directly and break at error('test:error', 'Noooo!'); in sub_sub_test() when I run test_excc?

My command window:

    >> dbclear all
    >> dbstop if error
    >> dbstop if caught error test:error
    >> test_excc

    Caught-error breakpoint was hit in test_excc at line 25. The error was:

    Error using test_excc>sub_sub_test (line 39)
    Noooo!

    25          sub_test()
    K>> sub_test
    Error using test_excc>sub_sub_test (line 39)
    Noooo!
    Error in test_excc>sub_test (line 35)
        sub_sub_test(); 
    39          error('test:error', 'Noooo!');
    K>>

where (in one m-file) is copied from your link to stackoverflow

    function test_excc

    %   doc says:
    %       dbstop if error MATLAB:ls:InputsMustBeStrings pauses execution at
    %       the first run-time error outside a try/catch block that has a
    %       message ID of MATLAB:ls:InputsMustBeStrings. 
    %
    %       dbstop if caught error — Run-time error that occurs within the
    %       try portion of a try/catch block. If you want execution to stop 
    %       only if a specific error occurs, specify the message id. See the 
    %       error condition for an example of specifying a message id.
    %
    %       (12) DBSTOP if caught error
    %       (16) DBSTOP if caught error IDENTIFIER

        try
            sub_test()
        catch ME
            if strcmp(ME.identifier, 'test:notsobad')
                fprintf(1, 'Fine\n');
            else
                ME.rethrow();
            end
        end
    end
    function sub_test
        sub_sub_test();
    end
    function sub_sub_test()
        if rand>0.5
            error('test:error', 'Noooo!');
        else
            error('test:notsobad', 'That''OK');
        end
    end

.

[Two hours later:]

Not for faint-hearted!

• Overload the function error temporarily, see below.
• run test_excc
• Matlab breaks in the overloaded function, error
• Step Out takes you to "the ORIGINAL error"

and the workspace of sub_sub_test() is available for inspection and debugging.

My command window:

    >> dbclear all
    >> test_excc
    3   end
    K>>

where

    function    error( varargin )
        dbs
    end

and

    function dbs
    % dbs provides a persistent break-point; debug, dbstop

    %   Based on the FEX contribution kdb 
    %   By Romesh Abeysuriya 15-11-12

        stk = dbstack('-completenames');
        dbstop( 'in', stk(2).file, 'at', num2str( stk(2).line+1 ) )
        pause(0.2)
    end

and don't forget to delete the file, error.m

... or maybe better, add the function error as a sub-function to the file that you are debugging.

Answer 3

[Steven Lord]

Set a breakpoint inside the catch section of the try / catch block. If you only want to enter debug mode when certain errors are thrown and caught, set a conditional breakpoint that stops only if the identifier field of the MException object you caught matches the one you want to debug.

try
    x = ones(3) + ones(4);
catch ME
    fprintf("The error with identifier '%s'\nand message '%s' was thrown.", ...
        ME.identifier, ME.message)
    % or you could use if
    if isequal(ME.identifier, 'MATLAB:sizeDimensionsMustMatch')
        fprintf("You're trying to add arrays with incompatible dimensions.")
    end
end

The error with identifier 'MATLAB:sizeDimensionsMustMatch'
and message 'Arrays have incompatible sizes for this operation.' was thrown.

You're trying to add arrays with incompatible dimensions.