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Generate Code for Global Data

Workflow

To generate C/C++ code from MATLAB® code that uses global data:

  1. Declare the variables as global in your code.

  2. Before using the global data, define and initialize it.

    For more information, see Define Global Data.

  3. Generate code using the MATLAB Coder™ app or using codegen.

If you use global data, you must also specify whether you want to synchronize this data between MATLAB and the generated MEX function. For more information, see Synchronizing Global Data with MATLAB.

Declare Global Variables

When using global data, you must first declare the global variables in your MATLAB code. Consider the use_globals function that uses two global variables AR and B:

function y = use_globals(u)
%#codegen
% Turn off inlining to make 
% generated code easier to read
coder.inline('never');
% Declare AR and B as global variables
global AR;
global B;
AR(1) = u + B(1);
y = AR * 2;

Define Global Data

You can define global data in the MATLAB global workspace, in a MATLAB Coder project, or at the command line. If you do not initialize global data in the project or at the command line, MATLAB Coder looks for the variable in the MATLAB global workspace. If the variable does not exist, MATLAB Coder generates an error.

Defining Global Data in the MATLAB Global Workspace

To generate a MEX function for the use_globals function described in Declare Global Variables using codegen:

  1. In the MATLAB workspace, define and initialize the global data. At the MATLAB prompt, enter:

    global AR B;
    AR = ones(4);
    B = [1 2 3];
    

  2. Generate a MEX file.

    codegen use_globals -args {0}
    % Use the -args option to specify that the input u
    % is a real, scalar, double
    % By default, codegen generates a MEX function,
    % use_globals_mex, in the current folder

Defining Global Data Using the MATLAB Coder App

  1. On the Define Input Types page, automatically define input types or click Let me enter input or global types directly.

    The app displays a table of entry-point inputs.

  2. To add a global variable, click Add global.

    By default, the app names the first global variable in a project g, and subsequent global variables g1, g2, and so on.

  3. Under Global variables, enter a name for the global variable.

  4. Click the field to the right of the global variables name. Specify the type and initial value of the global variable. See Specify Global Variable Type and Initial Value Using the App.

    If you do not specify the type, you must create a variable with the same name in the global workspace.

Defining Global Data at the Command Line

To define global data at the command line, use the codegen -globals option. For example, to compile the use_globals function described in Declare Global Variables, specify two global inputs AR and B at the command line. Use the -args option to specify that the input u is a real, scalar double. By default, codegen generates a MEX function, use_globals_mex, in the current folder.

codegen -globals {'AR',ones(4),'B',[1 2 3]} use_globals -args {0}

Alternatively, specify the type and initial value with the -globals flag using the format -globals {'g', {type, initial_value}}. For cell arrays, you must use this format. See Specify Global Cell Arrays at the Command Line.

Defining Variable-Size Global Data.  To provide initial values for variable-size global data, specify the type and initial value with the -globals flag using the format -globals {'g', {type, initial_value}}. For example, to specify a global variable g1 that has an initial value [1 1] and upper bound [2 2], enter:

codegen foo -globals {'g1', {coder.typeof(0, [2 2],1),[1 1]}}
For a detailed explanation of the syntax, see coder.typeof.

Synchronizing Global Data with MATLAB

Why Synchronize Global Data?

The generated MEX function and MATLAB each have their own copies of global data. To make these copies consistent, you must synchronize their global data whenever the two interact. If you do not synchronize the data, their global variables might differ. The level of interaction determines when to synchronize global data. For more information, see When to Synchronize Global Data.

When global data is constant, you cannot synchronize the global data with MATLAB. By default, the MEX function tests for consistency between the compile-time constant global values and the MATLAB values at function entry and after extrinsic function calls. If the MATLAB values differ from the compile-time constant global values, the MEX function ends with an error. For information about controlling when the MEX function tests for consistency between the compile-time constant global values and the MATLAB values, see Consistency Between MATLAB and Constant Global Data.

When to Synchronize Global Data

By default, synchronization between the MEX function's global data and MATLAB occurs at MEX function entry and exit and for extrinsic calls. Use this synchronization method for maximum consistency between the MEX function and MATLAB.

To improve performance, you can:

  • Select to synchronize only at MEX function entry and exit points.

  • Disable synchronization when the global data does not interact.

  • Choose whether to synchronize before and after each extrinsic call.

The following table summarizes which global data synchronization options to use. To learn how to set these options, see How to Synchronize Global Data.

Global Data Synchronization Options

If you want toSet the global data synchronization mode to:Synchronize before and after extrinsic calls?
Have maximum consistency when all extrinsic calls modify global data.At MEX-function entry, exit and extrinsic calls (default)Yes. Default behavior.
Have maximum consistency when most extrinsic calls modify global data, but a few do not.At MEX-function entry, exit and extrinsic calls (default)

Yes. Use the coder.extrinsic -sync:off option to turn off synchronization for the extrinsic calls that do not change global data.

Have maximum consistency when most extrinsic calls do not modify global data, but a few do.At MEX-function entry and exit

Yes. Use the coder.extrinsic -sync:on option to synchronize only the calls that modify global data.

Maximize performance when synchronizing global data, and none of your extrinsic calls modify global data.At MEX-function entry and exitNo.
Communicate between generated MEX functions only. No interaction between MATLAB and MEX function global data.DisabledNo.

How to Synchronize Global Data

To control global data synchronization, set the global data synchronization mode and select whether to synchronize extrinsic functions. For guidelines on which options to use, see When to Synchronize Global Data.

You can control the global data synchronization mode from the project settings dialog box, the command line, or a MEX configuration dialog box. You control the synchronization of data with extrinsic functions using the coder.extrinsic -sync:on and -sync:off options.

Controlling the Global Data Synchronization Mode Using the MATLAB Coder App

  1. To open the Generate dialog box, on the Generate Code page, click the Generate arrow .

  2. Set Build type to MEX.

  3. Click More Settings.

  4. On the Memory tab, set Global data synchronization mode to At MEX-function entry and exit or Disabled, as applicable.

Controlling the Global Data Synchronization Mode from the Command Line

  1. In the MATLAB workspace, define the code generation configuration object. At the MATLAB command line, enter:

    mexcfg = coder.config('mex');

  2. At the MATLAB command line, set the GlobalDataSyncMethod property to SyncAtEntryAndExits or NoSync, as applicable. For example:

    mexcfg.GlobalDataSyncMethod = 'SyncAtEntryAndExits';
    

  3. When compiling your code, use the mexcfg configuration object. For example, to generate a MEX function for function foo that has no inputs:

    codegen -config mexcfg foo

Controlling Synchronization for Extrinsic Function Calls.  To control whether synchronization between MATLAB and MEX function global data occurs before and after you call an extrinsic function, use the coder.extrinsic-sync:on and -sync:off options.

By default, global data is:

  • Synchronized before and after each extrinsic call, if the global data synchronization mode is At MEX-function entry, exit and extrinsic calls. If you are sure that certain extrinsic calls do not change global data, turn off synchronization for these calls using the -sync:off option. For example, if functions foo1 and foo2 do not change global data, turn off synchronization for these functions:

    coder.extrinsic('-sync:off', 'foo1', 'foo2');

  • Not synchronized, if the global data synchronization mode is At MEX-function entry and exit. If the code has a few extrinsic calls that change global data, turn on synchronization for these calls using the -sync:on option. For example, if functions foo1 and foo2 change global data, turn on synchronization for these functions:

    coder.extrinsic('-sync:on', 'foo1', 'foo2');

  • Not synchronized, if the global data synchronization mode is Disabled. When synchronization is disabled, you cannot use the -sync:on option to control the synchronization for specific extrinsic calls.

Clear Global Data

Because MEX functions and MATLAB each have their own copies of global data, you must clear both copies to ensure that consecutive MEX runs produce the same results. The clear global command removes only the copy of the global data in the MATLAB workspace. To remove both copies of the data, use the clear global and clear mex commands together. The clear all command also removes both copies.

Define Constant Global Data

If you know that the value of a global variable does not change at run time, you can reduce overhead in the generated code by specifying that the global variable has a constant value. You cannot write to the constant global variable.

Define Constant Global Data Using the MATLAB Coder App

  • On the Define Input Types page, automatically define input types or click Let me enter input or global types directly.

    The app displays a table of entry-point inputs.

  1. To add a global variable, click Add global.

    By default, the app names the first global variable in a project g, and subsequent global variables g1, g2, and so on.

  2. Under Global Variables, enter a name for the global variable.

  3. Click the field to the right of the global variable name.

  4. Select Define Constant Value.

  5. In the field to the right of the global variable, enter a MATLAB expression.

Define Constant Global Data at the Command Line

To specify that a global variable is constant using the codegen command, use the -globals option with the coder.Constant class.

  1. Define a configuration object for the code generation output type that you want. For example, define a configuration object for MEX code generation:

    cfg = coder.config('mex');
    

  2. Use coder.Constant to specify that a global variable has a constant value. For example, the following code specifies that the global variable g has initial value 4 and that global variable gc has the constant value 42.

    global_values = {'g', 4, 'gc', coder.Constant(42)};
  3. Generate the code using the -globals option. For example, generate code for myfunction specifying that the global variables are defined in the cell array global_values.

    codegen -config cfg -globals global_values myfunction

Consistency Between MATLAB and Constant Global Data

By default, the generated MEX function verifies that the values of constant global data in the MATLAB workspace are consistent with the compile-time values in the generated MEX. It tests for consistency at function entry and after calls to extrinsic functions. If the MEX function detects an inconsistency, it ends with an error. To control when the MEX function tests for consistency, use the global synchronization mode and the coder.extrinsic synchronization options.

The following table shows how the global data synchronization mode and the coder.extrinsic synchronization option setting determine when a MEX function verifies consistency between the compile-time constant global data values and MATLAB.

Global Data Synchronization Mode (Project)GlobalDataSyncMethod (MEX Configuration Object)Verify Consistency of Constant Global Values at MEX Function Entrycoder.extrinsic synchronization optionVerify Consistency of Constant Global Values After Extrinsic Function Call

At MEX-function entry, exit and extrinsic calls (default)

'SyncAlways'

yes

'sync:on' (default)

yes

'sync:off'

no

At MEX-function entry and exit

'SyncAtEntryAndExits'

yes

'sync:on'

yes

'sync:off' (default)

no

Disabled

'NoSync'

no

N/A

N/A

Constant Global Data in a Code Generation Report

The code generation report provides the following information about a constant global variable:

  • Type of Global on the Variables tab.

  • Highlighted variable name in the Function pane.

See MATLAB Variables.

Global Data Limitations for Generated Code

  • Global structure variables cannot contain handle objects or sparse arrays.

  • You cannot apply coder.cstructname directly to a global variable. To name the structure type to use with a global variable, use coder.cstructname to create a type object that names the structure type. Then, when you run codegen, specify that the global variable has that type. See Name the C Structure Type to Use With a Global Structure Variable.

See Also

Related Topics