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Debug Your TLC Code

tlcdebug Tutorial Overview

Objective: Introduces the TLC debugger. You will learn how to set breakpoints and familiarize yourself with TLC debugger commands.

Open the Example:


You can cause the TLC debugger to be invoked whenever the build process is invoked. In this tutorial, you use it to detect a bug in a .tlc file for a model called simple_log. The bug causes the generated code output from the standalone version of the model to differ from its simulation output. The tutorial guides you through following steps:

  1. Getting Started — Run the model and inspect output

  2. Generate and Run Code from the Model — Compare compiled results to original output

  3. Start the Debugger and Use Its Commands — Things you can do with the debugger

  4. Debug timesN.tlc — Find out what went wrong

  5. Fix the Bug and Verify — Easy ways to fix bugs and verify fixes

Getting Started

  1. In the MATLAB® Command Window, create a MEX-file for the S-function:

    mex timesN.c

    This avoids picking up the version shipped with your Simulink® software.


    An error might occur if you have not previously run mex -setup.

  2. Open the model simple_log. The model looks like this.

  3. In the Data Import/Export pane of the Configuration Parameters dialog box, check Time and Output. This causes model variables to be logged to the MATLAB workspace.

  4. Run the model. On the Simulation tab, click Run. Variables tout and yout appear in your MATLAB workspace.

  5. Double-click yout in the Workspace pane of the MATLAB Command Window. The Variable Editor displays the 6x1 array output from simple_log. The display looks like this:

    Column 1 contains discrete pulse output for six time steps (3s and 0s), collected at port out1.

Next, you generate a standalone version of simple_log. You execute it and compare its results to the output from Simulink displayed above.


For the purpose of this exercise, the TLC file provided, timesN.tlc, contains a bug. This version must be in the same folder as the model that uses it.

Generate and Run Code from the Model

  1. Press Ctrl+B.

    The code generator produces, compiles, and links C source code. The MATLAB Command Window shows the progress of the build, which ends with these messages:

    ### Created executable: simple_log.exe  
    ### Successful completion of build procedure 
        for model: simple_log
  2. Run the standalone model just created by typing


    This results in the messages

    ** starting the model **
    ** created simple_log.mat **
  3. Inspect results by placing the variables in your workspace. In the Current Folder pane, double-click simple_log.mat, then double-click rt_yout (the standalone version of variable yout) in the Workspace pane.

    Compare rt_yout with yout. Do you notice differences? Can you surmise what caused values in rt_yout to change?

    A look at the generated C code that TLC placed in your build folder (simple_log_grt_rtw) helps to identify the problem.

  4. Edit simple_log.c and look at its MdlOutputs function, which should appear as shown below:

    /* Model output function */
    static void simple_log_output(void)
      /* DiscretePulseGenerator: '<Root>/Discrete Pulse Generator' */
      simple_log_B.DiscretePulseGenerator = (simple_log_DW.clockTickCounter < 1.0) &&
        (simple_log_DW.clockTickCounter >= 0) ? 1.0 : 0.0;
      if (simple_log_DW.clockTickCounter >= 2.0 - 1.0) {
        simple_log_DW.clockTickCounter = 0;
      } else {
      /* End of DiscretePulseGenerator: '<Root>/Discrete Pulse Generator' */
      /* S-Function (timesN): '<Root>/Gain1st' incorporates:
       *  Outport: '<Root>/Out1'
      /* S-Function Block: <Root>/Gain1st */
      /* Multiply input by 3.0 */
      simple_log_Y.Out1 = simple_log_B.DiscretePulseGenerator * 1;

Note the line near the end:

simple_log_B.first_output = simple_log_B.DiscretePulseGenerator * 1;
How did the incorrect product get assigned to the output when it was supposed to receive a variable that alternates between 3.0 and 0.0? Use the debugger to find out.

Start the Debugger and Use Its Commands

You use the TLC debugger to monitor the code generation process. As it is not invoked by default, you need to request the debugger explicitly.

  1. Set up the TLC debugging environment and start to build the application:

    1. Select the Configuration Parameters > Code Generation pane, and select the options Retain .rtw file and Start TLC debugger when generating code. Click OK.

    2. Build the model.

      The MATLAB Command Window describes the building process. The build stops at the timesN.tlc file and displays the command prompt:

  2. Type help to list the TLC debugger commands. Here are some things you can do in the debugger.

    • View and query various entities in the TLC scope.

      TLC-DEBUG> whos CompiledModel
      TLC-DEBUG> print CompiledModel.NumSystems
      TLC-DEBUG> print TYPE(CompiledModel.NumSystems)
    • Examine the statements in your current context.

      TLC-DEBUG> list
      TLC-DEBUG> list 10,40
    • Move to the next line of code.

      TLC-DEBUG> next
    • Step into a function.

      TLC-DEBUG> step
    • Assign a constant value to a variable, such as the input signal %<u>.

      TLC-DEBUG> assign u = 5.0
    • Set a breakpoint where you are or in some other part of the code.

      TLC-DEBUG> break timesN.tlc:10
    • Execute until the next breakpoint.

      TLC-DEBUG> continue
    • Clear breakpoints you have established.

      TLC-DEBUG> clear 1
      TLC-DEBUG> clear all
  3. If you have tried the TLC debugger commands, execute the remaining code to finish the build process, then build simple_log again. The build stops at the timesN.tlc file and displays the command prompt:


Debug timesN.tlc

Now look around to find out what is wrong with the code:

  1. Set a breakpoint on line 20 of timesN.tlc.

    TLC-DEBUG> break timesN.tlc:20
  2. Instruct the TLC debugger to advance to your breakpoint.

    TLC-DEBUG> continue

    TLC processes input, reports its progress, advances to line 20 in timesN.tlc, displays the line, and pauses.

    ### Loading TLC function libraries
    ### Initial pass through model to cache user defined code
    ### Caching model source code
    Breakpoint 1
    00020:   %roll idx = RollRegions, lcv = RollThreshold, block, "Roller", rollVars
  3. Use the whos command to see the variables in the current scope.

    TLC-DEBUG> whos
    Variables within: <BLOCK_LOCAL>
    gain                           Real
    rollVars                       Vector
    block                          Resolved
    system                         Resolved
  4. Inspect the variables using the print command (names are case sensitive).

    TLC-DEBUG> print gain
    TLC-DEBUG> print rollVars
    [U, Y]
  5. Execute one step.

    TLC-DEBUG> step
    00021:     %<LibBlockOutputSignal(0, "", lcv, idx)> = \
  6. Because it is a built-in function, advance via the next command.

    TLC-DEBUG> next
    00022:     %<LibBlockInputSignal(0, "", lcv, idx)> * 1;

    This is the origin of the C statement responsible for the erroneous constant output, simple_log_B.first_output = simple_log_B.DiscretePulseGenerator * 1;.

  7. Abandon the build by quitting the TLC debugger. Type

    TLC-DEBUG> quit

    An error message is displayed showing that you stopped the build by using the TLC debugger quit command. Close the error window.

Fix the Bug and Verify

The problem you identified is caused by evaluating a constant rather than a variable inside the TLC function FcnEliminateUnnecessaryParams(). This is a typical coding error and is easily repaired. Here is the code you need to fix.

%function Outputs(block, system) Output
   %assign gain =SFcnParamSettings.myGain
  /* %<Type> Block: %<Name> */
  /* Multiply input by %<gain> */
  %assign rollVars = ["U", "Y"]
  %roll idx = RollRegions, lcv = RollThreshold, block, "Roller", rollVars
    %<LibBlockOutputSignal(0, "", lcv, idx)> = \
    %<LibBlockInputSignal(0, "", lcv, idx)> * 1;


%% [EOF] timesN.tlc
  1. To fix the coding error, edit timesN.tlc. The line

    %<LibBlockInputSignal(0, "", lcv, idx)> * 1;
    multiplies the evaluated input by 1. Change the line to
    %<LibBlockInputSignal(0, "", lcv, idx)> * %<gain>;

    Save timesN.tlc.

  2. Build the standalone model again. Complete the build by typing continue at each TLC-DEBUG> prompt.

  3. Execute the standalone model by typing

    A new version of simple_log.mat is created containing its output.

  4. Load simple_log.mat and compare the workspace variable rt_yout with yout, as you did before. The values in the first column should now correspond.

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