Control Dynamic Memory Allocation for Fixed-Size Arrays

The code generated by MATLAB Coder allocates memory to the program stack for fixed-size arrays, whose size is less than a threshold. For example, in the following code, the array Z is a fixed-size array with a first dimension of size 1 and a second dimension of size 4.

function Z = myfcn()
Z = zeros(1,4);
end

Dynamic memory allocation allocates memory on the heap for fixed-size arrays, instead of the program stack. Consider dynamically allocating fixed-size arrays when you have large arrays that could exhaust stack memory.

Dynamic memory allocation might result in slower execution of the generated code.

Enable Dynamic Memory Allocation for Fixed-Size Arrays

By default, dynamic memory allocation for fixed-size arrays is disabled. To enable it:

• In a configuration object for code generation, set EnableDynamicMemoryAllocation and DynamicMemoryAllocationForFixedSizeArrays options to true.

• In the MATLAB^® Coder™ app, in the Memory settings, select Enable dynamic memory allocation and Enable dynamic memory allocation for fixed-sized arrays.

Dynamic Memory Allocation Threshold for Fixed-Size Arrays

When DynamicMemoryAllocationForFixedSizeArrays is enabled, the code generator allocates memory dynamically on the heap for fixed-size arrays whose size (in bytes) is

greater than or equal to DynamicMemoryAllocationThreshold.

The default dynamic memory allocation threshold is 64 kilobytes. To configure the threshold:

• In a configuration object for code generation, set a value for the DynamicMemoryAllocationThreshold.

• In the MATLAB Coder app, in the Memory settings, set a value for Dynamic memory allocation threshold.

Generating Code for Fixed-Size Arrays

Consider the following MATLAB function which calculates the product of two large fixed-size arrays a and b.

function y = tlargeSize(a,b)
  y = a*b;
end

To generate C code, run these commands:

cfg = coder.config('lib');
cfg.VerificationMode="SIL";
cfg.DynamicMemoryAllocationForFixedSizeArrays = true;
t=coder.typeof(0,[1e4 1e4]);
codegen tlargeSize -args {t,t} -config cfg - report

By enabling the DynamicMemoryAllocationForFixedSizeArrays option, arrays a and b are dynamically allocated on the heap preventing stack overflow.

Generated Code

...
void tlargeSize(const emxArray_real_T *a, const emxArray_real_T *b,
                emxArray_real_T *y)
{
  const double *a_data;
  const double *b_data;
  double *y_data;
...

Usage Notes and Limitations

When enabling DynamicMemoryAllocationForFixedSizeArrays:

• Input and output variables of the function changes to:

  • coder::array for C++ code generation. See, Use Dynamically Allocated C++ Arrays in Generated Function Interfaces.

  • emxArray for C code generation. See, Use C Arrays in the Generated Function Interfaces

• Ensure C/C++ function signatures are updated when passing struct with fixed-size arrays to coder.ceval or Code Replacement Library (CRL).

Note

Enabling DynamicMemoryAllocationForFixedSizeArrays is not supported for GPU code generation.