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Generate Code for Variable-Size Data

Variable-size data is data whose size might change at run time. You can use MATLAB® Coder™ to generate C/C++ code from MATLAB code that uses variable-size data. MATLAB supports bounded and unbounded variable-size data for code generation. Bounded variable-size data has fixed upper bounds. This data can be allocated statically on the stack or dynamically on the heap. Unbounded variable-size data does not have fixed upper bounds. This data must be allocated on the heap. By default, for MEX and C/C++ code generation, support for variable-size data is enabled and dynamic memory allocation is enabled for variable-size arrays whose size is greater than or equal to a configurable threshold.

Disable Support for Variable-Size Data

By default, for MEX and C/C++ code generation, support for variable-size data is enabled. You modify variable sizing settings from the project settings dialog box, the command line, or using dialog boxes.

Using the MATLAB Coder App

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

  2. Click More Settings.

  3. On the Memory tab, select or clear Enable variable-sizing.

At the Command Line

  1. Create a configuration object for code generation. For example, for a library:

    cfg = coder.config('lib');

  2. Set the EnableVariableSizing option:

    cfg.EnableVariableSizing = false;

  3. Using the -config option, pass the configuration object to codegen :

    codegen -config cfg foo
    

Control Dynamic Memory Allocation

By default, dynamic memory allocation is enabled for variable-size arrays whose size is greater than or equal to a configurable threshold. If you disable support for variable-size data (see Disable Support for Variable-Size Data), you also disable dynamic memory allocation. You can modify dynamic memory allocation settings from the project settings dialog box or the command line.

Using the MATLAB Coder App

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

  2. Click More Settings.

  3. On the Memory tab, set Dynamic memory allocation to one of the following options:

    SettingAction
    NeverDynamic memory allocation is disabled. Variable-size data is allocated statically on the stack.
    For all variable-sized arraysDynamic memory allocation is enabled for variable-size arrays. Variable-size data is allocated dynamically on the heap.
    For arrays with max size at or above thresholdDynamic memory allocation is enabled for variable-size arrays whose size is greater than or equal to the Dynamic memory allocation threshold. Variable-size arrays whose size is less than this threshold are allocated on the stack.

  4. Optionally, if you set Dynamic memory allocation to For arrays with maximum size at or above threshold, configure Dynamic memory allocation threshold to fine-tune memory allocation.

At the Command Line

  1. Create a configuration object for code generation. For example, for a MEX function:

    mexcfg = coder.config('mex');

  2. Set the DynamicMemoryAllocation option:

    SettingAction
    mexcfg.DynamicMemoryAllocation='Off';
    Dynamic memory allocation is disabled. Variable-size data is allocated statically on the stack.
    mexcfg.DynamicMemoryAllocation='AllVariableSizeArrays';
    Dynamic memory allocation is enabled for variable-size arrays. Variable-size data is allocated dynamically on the heap.
    mexcfg.DynamicMemoryAllocation='Threshold';
    Dynamic memory allocation is enabled for variable-size arrays whose size (in bytes) is greater than or equal to the value specified using the DynamicMemoryAllocationThreshold parameter. Variable-size arrays whose size is less than this threshold are allocated on the stack.

  3. Optionally, if you set DynamicMemoryAllocation to 'Threshold', configure DynamicMemoryAllocationThreshold to fine tune memory allocation.

  4. Using the -config option, pass the configuration object to codegen:

    codegen -config mexcfg foo
    

Generating Code for MATLAB Functions with Variable-Size Data

Here is a basic workflow that first generates MEX code for verifying the generated code and then generates standalone code after you are satisfied with the result of the prototype.

To work through these steps with a simple example, see Generate Code for a MATLAB Function That Expands a Vector in a Loop

  1. In the MATLAB Editor, add the compilation directive %#codegen at the top of your function.

    This directive:

    • Indicates that you intend to generate code for the MATLAB algorithm

    • Turns on checking in the MATLAB Code Analyzer to detect potential errors during code generation

  2. Address issues detected by the Code Analyzer.

    In some cases, the MATLAB Code Analyzer warns you when your code assigns data a fixed size but later grows the data, such as by assignment or concatenation in a loop. If that data is supposed to vary in size at run time, you can ignore these warnings.

  3. Generate a MEX function using codegen to verify the generated code. Use the following command-line options:

    • -args {coder.typeof...} if you have variable-size inputs

    • -report to generate a code generation report

    For example:

    codegen -report foo -args {coder.typeof(0,[2 4],1)}
    
    This command uses coder.typeof to specify one variable-size input for function foo. The first argument, 0, indicates the input data type (double) and complexity (real). The second argument, [2 4], indicates the size, a matrix with two dimensions. The third argument, 1, indicates that the input is variable sized. The upper bound is 2 for the first dimension and 4 for the second dimension.

    Note

    During compilation, codegen detects variables and structure fields that change size after you define them, and reports these occurrences as errors. In addition, codegen performs a run-time check to generate errors when data exceeds upper bounds.

  4. Fix size mismatch errors:

    CauseHow To FixFor More Information
    You try to change the size of data after its size has been locked.Declare the data to be variable sized.See Diagnosing and Fixing Size Mismatch Errors.
  5. Fix upper bounds errors

    CauseHow To FixFor More Information
    MATLAB cannot determine or compute the upper boundSpecify an upper bound.SeeSpecify Upper Bounds for Variable-Size Arrays and Diagnosing and Fixing Size Mismatch Errors.
    MATLAB attempts to compute an upper bound for unbounded variable-size data.If the data is unbounded, enable dynamic memory allocation. See Control Dynamic Memory Allocation.
  6. Generate C/C++ code using the codegen function.

Generate Code for a MATLAB Function That Expands a Vector in a Loop

About the MATLAB Function myuniquetol

This example uses the function myuniquetol. This function returns in vector B a version of input vector A, where the elements are unique to within tolerance tol of each other. In vector B, abs(B(i) - B(j)) > tol for all i and j. Initially, assume input vector A can store up to 100 elements.

function B = myuniquetol(A, tol)
A = sort(A);
B = A(1);
k = 1;
for i = 2:length(A)
   if abs(A(k) - A(i)) > tol
      B = [B A(i)];
      k = i;
   end
end

Step 1: Add Compilation Directive for Code Generation

Add the %#codegen compilation directive at the top of the function:

function B = myuniquetol(A, tol) %#codegen
A = sort(A);
B = A(1);
k = 1;
for i = 2:length(A)
   if abs(A(k) - A(i)) > tol
      B = [B A(i)];
      k = i;
   end
end

Step 2: Address Issues Detected by the Code Analyzer

The Code Analyzer detects that variable B might change size in the for-loop. It issues this warning:

The variable 'B' appears to change size on every loop iteration.
Consider preallocating for speed.

In this function, you expect vector B to expand in size because it adds values from vector A. Therefore, you can ignore this warning.

Step 3: Generate MEX Code

It is a best practice to generate MEX code before you generate C/C++ code. Generating MEX code can identify code generation issues that are harder to detect at run time.

  1. Generate a MEX function for myuniquetol:

    codegen -report myuniquetol -args {coder.typeof(0,[1 100],1),coder.typeof(0)}

     What do these command-line options mean?

    Code generation is successful. codegen does not detect issues. In the current folder, codegen generates a MEX function for myuniquetol and provides a link to the code generation report.

  2. Click the View report link.

  3. In the code generation report, select the Variables tab.

    The size of A is 1x:100 because you specified that A is variable size with an upper bound of 100. The size of variable B is 1x:?, indicating that it is variable size with no upper bounds.

Step 4: Generate C Code

Generate C code for variable-size inputs. By default, codegen allocates memory statically for data whose size is less than the dynamic memory allocation threshold of 64 kilobytes. If the size of the data is greater than or equal to the threshold or is unbounded, codegen allocates memory dynamically on the heap.

  1. Create a configuration option for C library generation:

    cfg=coder.config('lib');
    
  2. Issue this command:

    codegen -config cfg -report myuniquetol -args {coder.typeof(0,[1 100],1),coder.typeof(0)}

    codegen generates a static library in the default location, codegen\lib\myuniquetol and provides a link to the code generation report.

  3. Click the View report link.

  4. In the list of generated files, click myuniquetol.h.

    The function declaration is:

    extern void myuniquetol(const double A_data[], const int A_size[2], double tol,
      emxArray_real_T *B);

    codegen computes the size of A and, because its maximum size is less than the default dynamic memory allocation threshold of 64k bytes, allocates this memory statically. The generated code contains:

    • double A_data[]: the definition of A.

    • int A_size[2]: the actual size of the input.

    The code generator determines that B is variable size with unknown upper bounds. It represents B as emxArray_real_T. MATLAB provides utility functions for creating and interacting with emxArrays in your generated code. For more information, see Use C Arrays in the Generated Function Interfaces.

Step 5: Specify an Upper Bound for the Output Vector

You specified that the input A is variable size with an upper bound of 100. Therefore, you know that the output B cannot be larger than 100 elements.

  • Use coder.varsize to indicate that B is variable size with an upper bound of 100.

    function B = myuniquetol(A, tol) %#codegen
    A = sort(A);
    coder.varsize('B', [1 100], [0 1]);
    B = A(1);
    k = 1;
    for i = 2:length(A)
       if abs(A(k) - A(i)) > tol
          B = [B A(i)];
          k = i;
       end
    end

  • Generate code.

    codegen -config cfg -report myuniquetol -args {coder.typeof(0,[1 100],1),coder.typeof(0)}  

    The function declaration is:

    extern void myuniquetol(const double A_data[], const int A_size[2], double tol,
      double B_data[], int B_size[2]);

    The code generator statically allocates the memory for B. It stores the size of B in int B_size[2].

Step 6: Change the Dynamic Memory Allocation Threshold

In this step, you reduce the dynamic memory allocation threshold and generate code for an input that exceeds this threshold. This step specifies that the second dimension of A has an upper bound of 10000.

  1. Change the upper bound of B to match the upper bound of A.

    function B = myuniquetol(A, tol) %#codegen
    A = sort(A);
    coder.varsize('B', [1 10000], [0 1]);
    B = A(1);
    k = 1;
    for i = 2:length(A)
       if abs(A(k) - A(i)) > tol
          B = [B A(i)];
          k = i;
       end
    end

  2. Set the dynamic memory allocation threshold to 4 kilobytes and generate code where the size of input A exceeds this threshold.

    cfg.DynamicMemoryAllocationThreshold=4096;
    codegen -config cfg -report myuniquetol -args {coder.typeof(0,[1 10000],1),coder.typeof(0)} 

  3. View the generated code in the report. Because the maximum size of A and B now exceed the dynamic memory allocation threshold, codegen allocates A and B dynamically on the heap. In the generated code, A and B have type emxArray_real_T.

    extern void myuniquetol(const emxArray_real_T *A, double tol, emxArray_real_T *B);

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