Log Program Execution Results

Multiple techniques are available by which a program generated by the Simulink® Coder™ software can save data to a MAT-file for analysis. A generated executable can save system states, outputs, and simulation time at each model execution time step. The data is written to a MAT-file, named (by default) model.mat, where model is the name of your model. See Log Data for Analysis for a data logging tutorial.


Data logging is available only for system target files that have access to a file system. In addition, only the RSim target executables are capable of accessing MATLAB workspace data.

For MAT-file logging limitations, see the configuration parameter MAT-file logging.

Log Data for Analysis

Set Up and Configure Model

This example shows how data generated by a copy of the model slexAircraftExample is logged to the file myAircraftExample.mat. Refer to Build Process Workflow for Real-Time Systems for instructions on setting up a copy of slexAircraftExample asmyAircraftExample in a working folder if you have not done so already.


When you configure the code generator to produce code that includes support for data logging during execution, the code generator can include text for block names in the block paths included in the log file. If the text includes characters that are unrepresented in the character set encoding for the model, the code generator replaces the characters with XML escape sequences. For example, the code generator replaces the Japanese full-width Katakana letter ア with the escape sequence ア. For more information, see Internationalization and Code Generation.

To configure data logging, open the Configuration Parameters dialog box and select the Data Import/Export pane. The process is the same as configuring a Simulink model to save output to the MATLAB® workspace. For each workspace return variable you define and enable, the Simulink Coder software defines a parallel MAT-file variable. For example, if you save simulation time to the variable tout, your generated program logs the same data to a variable named rt_tout. You can change the prefix rt_ to a suffix (_rt), or eliminate it entirely. You do this by setting model configuration parameter MAT-file variable name modifier.

Simulink lets you log signal data from anywhere in a model. In the Simulink Editor, select the signals that you want to log and then in the Simulation Data Inspector button drop-down, select Log Selected Signals. However, the code generator does not use this method of signal logging in generated code. To log signals in generated code, use the Data Import/Export parameters described below or include To File or To Workspace blocks in your model.


If you enable MAT-file and signal logging (through the Data Import/Export pane) and select signals for logging (through the Simulink Editor), you see the following warning when you build the model:

Warning: MAT-file logging does not support signal logging.
When your model code executes, the signal logging variable 'rt_logsout' will
not be saved to the MAT-file. 
To avoid this warning, clear the model configuration parameter Signal logging.

In this example, you modify the myAircraftExample model so that the generated program saves the simulation time and system outputs to the file myAircraftExample.mat. Then you load the data into the base workspace and plot simulation time against one of the outputs. The myAircraftExample model should be configured as described in Build Process Workflow for Real-Time Systems.

Data Logging During Simulation

To use the data logging feature:

  1. Open the myAircraftExample model if it is not already open.

  2. Open the Configuration Parameters dialog box.

  3. Select the Data Import/Export pane. The Data Import/Export pane lets you specify which outport data is to be saved to the workspace and what variable names to use for it.

  4. Set parameter Format to Structure with time. When you select this format, Simulink saves the model states and outputs in structures that have their names specified in the Save to workspace or file area. By default, the structures are xout for states and yout for output. The structure used to save output has two top-level fields: time and signals. The time field contains a vector of simulation times and signals contains an array of substructures, each of which corresponds to a model output port.

  5. Select Output. This tells Simulink to save output signal data during simulation as a variable named yout. Selecting Output enables the code generator to create code that logs the root Output block (alpha, rad) to a MAT-file.

  6. Set Decimation to 1.

  7. If other parameters are selected, clear them. The figure below shows how the dialog box should appear.

  8. Click Apply and OK to register your changes and close the dialog box.

  9. Save the model.

  10. In the model window, double-click the scope symbol next to the Aircraft Dynamics Model block, then simulate the model. The resulting scope display is shown below.

  11. Verify that the simulation time and outputs have been saved to the base workspace in MAT-files. At the MATLAB prompt, type:

    whos yout
    Simulink displays:

      Name        Size            Bytes  Class     Attributes
      yout        1x1             10756  struct
  12. Verify that alpha, rad was logged by plotting simulation time versus that variable. In the Command Window, type:


    The resulting plot is shown below.

Data Logging from Generated Code

In the second part of this example, you build and run an executable program built by the code generator for model myAircraftExample. The program outputs a MAT-file containing the simulation time and output you previously examined. Even though you have already generated code for the myAircraftExample model, you must now regenerate that code because you have changed the model by enabling data logging. The steps below explain this procedure.

To avoid overwriting workspace data with data from simulation runs, the code generator modifies identifiers for variables logged by Simulink. You can control these modifications.

  1. Set model configuration parameter MAT-file variable name modifier to _rt. This adds the suffix _rt to each variable that you selected to be logged in the first part of this example.

  2. Click Apply and OK to register your changes and close the dialog box.

  3. Save the model.

  4. Build an executable.

  5. When the build concludes, run the executable program with the command:

  6. The program now produces two message lines, indicating that the MAT-file has been written.

    ** starting the model **
    ** created myAircraftExample.mat **
  7. Load the MAT-file data created by the executable and look at the workspace variables from simulation and the generated program by typing:

    load myAircraftExample.mat
    whos yout*

    Simulink displays:

      Name           Size            Bytes  Class     Attributes
      yout           1x1             10756  struct
      yout_rt        1x1             10756  struct

    Note the size and bytes of the structures resulting from the simulation run and generated code are the same.

  8. Plot the generated code output by entering the following command in the Command Window:


    The plot should be identical to the plot that you produced in the previous part of this example.


For UNIX® platforms, run the executable program in the Command Window with the syntax !./executable_name. If preferred, run the executable program from an OS shell with the syntax ./executable_name. For more information, see Run External Commands, Scripts, and Programs (MATLAB).

Configure State, Time, and Output Logging

The Data Import/Export pane enables a generated program to save system states, outputs, and simulation time at each model execution time step. The data is written to a MAT-file, named (by default) model.mat.

Before using this data logging feature, you should learn how to configure a Simulink model to return output to the MATLAB workspace. This is discussed in Export Simulation Data (Simulink).

For each workspace return variable that you define and enable, the code generator defines a MAT-file variable. For example, if your model saves simulation time to the workspace variable tout, your generated program logs the same data to a variable named (by default) rt_tout.

The code generated by the code generator logs the following data:

  • Root Outport blocks

    The default MAT-file variable name for system outputs is rt_yout.

    The sort order of the rt_yout array is based on the port number of the Outport block, starting with 1.

  • Continuous and discrete states in the model

    The default MAT-file variable name for system states is rt_xout.

  • Simulation time

    The default MAT-file variable name for simulation time is rt_tout.

Override Default MAT-File Variable Names

By default, the code generator prefixes the text rt_ to the variable names for system outputs, states, and simulation time to form MAT-file variable names. To change this prefix for a model, set model configuration parameter MAT-file variable name modifier to (rt_), a suffix (_rt), or no modifier (none). Other system target files might not support this parameter.

Override Default MAT-File Name or Buffer Size

You can specify compiler options to override the following MAT-file attributes in generated code:

MAT-File AttributeDefaultCompiler Option
Size of data logging buffer1024 bytes-DDEFAULT_BUFFER_SIZE=n


Valid option syntax can vary among compilers. For example, Microsoft® Visual C++® compilers typically accept /DSAVEFILE=filename as well as -DSAVEFILE=filename.

For a template makefile (TMF) based target, set model configuration parameter Make command to the compiler option. For example:

For a toolchain-based system target file such as GRT or ERT, add the compiler option to model configuration. Set parameter Build configuration to Specify, and add the compiler option to the C Compiler row of the Tool/Options table. For example:

To add the compiler option to a custom toolchain, you can modify and reregister the custom toolchain using the procedures shown in the example Adding a Custom Toolchain (MATLAB Coder). For example, to add the compiler option to the MATLAB source file for the custom toolchain, you could define myCompilerOpts as follows:

optimsOffOpts    = {'/c /Od'};
optimsOnOpts     = {'/c /O2'};
cCompilerOpts    = '$(cflags) $(CVARSFLAG) $(CFLAGS_ADDITIONAL)';
cppCompilerOpts  = '$(cflags) $(CVARSFLAG) $(CPPFLAGS_ADDITIONAL)';
myCompilerOpts   = {' -DSAVEFILE=myCodeLog.mat '};

Then you can add myCompilerOpts to the flags for each configuration and compiler to which it applies, for example:

cfg = tc.getBuildConfiguration('Faster Builds');
cfg.setOption('C Compiler', horzcat(cCompilerOpts, myCompilerOpts, optimsOffOpts));

As shown in Adding a Custom Toolchain (MATLAB Coder), after modifying the custom toolchain, you save the configuration to a MAT-file and refresh the target registry.

Log Data with Scope and To Workspace Blocks

The code generated by the code generator also logs data from these sources:

  • Scope blocks that have the block parameter Log data to workspace enabled

    You must specify the variable name and data format in each Scope block's dialog box.

  • To Workspace blocks in the model

    You must specify the variable name and data format in each To Workspace block's dialog box.

The variables are written to model.mat, along with variables logged from the Workspace I/O pane.

Log Data with To File Blocks

You can also log data to a To File block. The generated program creates a separate MAT-file (distinct from model.mat) for each To File block in the model. The file contains the block time and input variable(s). You must specify the filename, variable names, decimation, and sample time in the To File block dialog box.


Models referenced by Model blocks do not perform data logging in that context except for states, which you can include in the state logged for top models. Code generated by the Simulink Coder software for referenced models does not perform data logging to MAT-files.

Data Logging Differences Between Single- and Multitasking

When logging data in single-tasking and multitasking systems, you will notice differences in the logging of

  • Noncontinuous root Outport blocks

  • Discrete states

In multitasking mode, the logging of states and outputs is done after the first task execution (and not at the end of the first time step). In single-tasking mode, the code generated by the build procedure logs states and outputs after the first time step.

See Data Logging in Single-Tasking and Multitasking Model Execution for more details on the differences between single-tasking and multitasking data logging.


The rapid simulation target (RSim) provides enhanced logging options. See Accelerate, Refine, and Test Hybrid Dynamic System on Host Computer by Using RSim System Target File for more information.

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