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Call MATLAB Compiler SDK API Functions from C/C++

Functions in the Shared Library

A shared library generated by MATLAB® Compiler SDK™ contains at least seven functions. There are three generated functions to manage library initialization and termination, one each for printed output and error messages, and two generated functions for each MATLAB file compiled into the library.

To generate the functions described in this section, first copy sierpinski.m, main_for_lib.c, main_for_lib.h, and triangle.c from matlabroot\extern\examples\compilersdk into your directory, and then execute the appropriate MATLAB Compiler SDK command.

Type of Application

For a C Application on Windows

mcc -W lib:libtriangle -T link:lib sierpinski.m
mbuild triangle.c main_for_lib.c libtriangle.lib

For a C Application on UNIX

mcc -W lib:libtriangle -T link:lib sierpinski.m
mbuild triangle.c main_for_lib.c -L. -ltriangle -I.

For a C++ Application on Windows

mcc -W cpplib:libtrianglep -T link:lib sierpinski.m
mbuild triangle.cpp main_for_lib.c libtrianglep.lib

For a C++ Application on UNIX

mcc -W cpplib:libtriangle -T link:lib sierpinski.m
mbuild triangle.cpp main_for_lib.c -L. -ltriangle -I.

These commands create a main program named triangle, and a shared library named libtriangle. The library exports a single function that uses a simple iterative algorithm (contained in sierpinski.m) to generate the fractal known as Sierpinski's Triangle. The main program in triangle.c or triangle.cpp can optionally take a single numeric argument, which, if present, specifies the number of points used to generate the fractal. For example, triangle 8000 generates a diagram with 8,000 points.

In this example, MATLAB Compiler SDK places all of the generated functions into the generated file libtriangle.c or libtriangle.cpp.

Structure of Programs That Call Shared Libraries

All programs that call MATLAB Compiler SDK generated shared libraries have roughly the same structure:

  1. Declare variables and process/validate input arguments.

  2. Call mclInitializeApplication, and test for success. This function sets up the global MATLAB Runtime state and enables the construction of MATLAB Runtime instances.

  3. Call, once for each library, <libraryname>Initialize, to create the MATLAB Runtime instance required by the library.

  4. Invoke functions in the library, and process the results. (This is the main body of the program.)

  5. Call, once for each library, <libraryname>Terminate, to destroy the associated MATLAB Runtime.

  6. Call mclTerminateApplication to free resources associated with the global MATLAB Runtime state.

  7. Clean up variables, close files, etc., and exit.

To see these steps in an actual example, review the main program in this example, triangle.c.

Library Initialization and Termination Functions

The library initialization and termination functions create and destroy, respectively, the MATLAB Runtime instance required by the shared library. You must call the initialization function before you invoke any of the other functions in the shared library, and you should call the termination function after you are finished making calls into the shared library (or you risk leaking memory).

There are two forms of the initialization function and one type of termination function. The simpler of the two initialization functions takes no arguments; most likely this is the version your application will call. In this example, this form of the initialization function is called libtriangleInitialize.

bool libtriangleInitialize(void)

This function creates an MATLAB Runtime instance using the default print and error handlers, and other information generated during the compilation process.

However, if you want more control over how printed output and error messages are handled, you may call the second form of the function, which takes two arguments.

bool libtriangleInitializeWithHandlers(
    mclOutputHandlerFcn error_handler,
    mclOutputHandlerFcn print_handler
)

By calling this function, you can provide your own versions of the print and error handling routines called by the MATLAB Runtime. Each of these routines has the same signature (for complete details, see Print and Error Handling Functions). By overriding the defaults, you can control how output is displayed and, for example, whether or not it goes into a log file.

Note

Before calling either form of the library initialization routine, you must first call mclInitializeApplication to set up the global MATLAB Runtime state. See Call a Shared Library for more information.

On Microsoft® Windows® platforms, MATLAB Compiler SDK generates an additional initialization function, the standard Microsoft DLL initialization function DllMain.

BOOL WINAPI DllMain(HINSTANCE hInstance, DWORD dwReason,  
                    void *pv)

The generated DllMain performs a very important service; it locates the directory in which the shared library is stored on disk. This information is used to find the deployable archive, without which the application will not run. If you modify the generated DllMain (not recommended), make sure you preserve this part of its functionality.

Library termination is simple.

void libtriangleTerminate(void)

Call this function (once for each library) before calling mclTerminateApplication.

Print and Error Handling Functions

By default, MATLAB Compiler SDK generated applications and shared libraries send printed output to standard output and error messages to standard error. MATLAB Compiler SDK generates a default print handler and a default error handler that implement this policy. If you'd like to change this behavior, you must write your own error and print handlers and pass them in to the appropriate generated initialization function.

You may replace either, both, or neither of these two functions. The MATLAB Runtime sends all regular output through the print handler and all error output through the error handler. Therefore, if you redefine either of these functions, the MATLAB Runtime will use your version of the function for all the output that falls into class for which it invokes that handler.

The default print handler takes the following form.

static int mclDefaultPrintHandler(const char *s)

The implementation is straightforward; it takes a string, prints it on standard output, and returns the number of characters printed. If you override or replace this function, your version must also take a string and return the number of characters “handled.” The MATLAB Runtime calls the print handler when an executing MATLAB file makes a request for printed output, e.g., via the MATLAB function disp. The print handler does not terminate the output with a carriage return or line feed.

The default error handler has the same form as the print handler.

static int mclDefaultErrorHandler(const char *s)

However, the default implementation of the print handler is slightly different. It sends the output to the standard error output stream, but if the string does not end with carriage return, the error handler adds one. If you replace the default error handler with one of your own, you should perform this check as well, or some of the error messages printed by the MATLAB Runtime will not be properly formatted.

Caution

The error handler, despite its name, does not handle the actual errors, but rather the message produced after the errors have been caught and handled inside the MATLAB Runtime. You cannot use this function to modify the error handling behavior of the MATLAB Runtime -- use the try and catch statements in your MATLAB files if you want to control how a MATLAB Compiler SDK generated application responds to an error condition.

Note

If you provide alternate C++ implementations of either mclDefaultPrintHandler or mclDefaultErrorHandler, then functions must be declared extern "C". For example:

extern "C" int myPrintHandler(const char *s); 

Functions Generated from MATLAB Files

For each MATLAB file specified on the MATLAB Compiler SDK command line, the product generates two functions, the mlx function and the mlf function. Each of these generated functions performs the same action (calls your MATLAB file function). The two functions have different names and present different interfaces. The name of each function is based on the name of the first function in the MATLAB file (sierpinski, in this example); each function begins with a different three-letter prefix.

Note

For C shared libraries, MATLAB Compiler SDK generates the mlx and mlf functions as described in this section. For C++ shared libraries, the product generates the mlx function the same way it does for the C shared library. However, the product generates a modified mlf function with these differences:

  • The mlf before the function name is dropped to keep compatibility with R13.

  • The arguments to the function are mwArray instead of mxArray.

mlx Interface Function

The function that begins with the prefix mlx takes the same type and number of arguments as a MATLAB MEX-function. (See the External Interfaces documentation for more details on MEX-functions.) The first argument, nlhs, is the number of output arguments, and the second argument, plhs, is a pointer to an array that the function will fill with the requested number of return values. (The “lhs” in these argument names is short for “left-hand side” -- the output variables in a MATLAB expression are those on the left-hand side of the assignment operator.) The third and fourth parameters are the number of inputs and an array containing the input variables.

void mlxSierpinski(int nlhs, mxArray *plhs[], int nrhs,
                   mxArray *prhs[])

mlf Interface Function

The second of the generated functions begins with the prefix mlf. This function expects its input and output arguments to be passed in as individual variables rather than packed into arrays. If the function is capable of producing one or more outputs, the first argument is the number of outputs requested by the caller.

void mlfSierpinski(int nargout, mxArray** x, mxArray** y,
                   mxArray* iterations, mxArray* draw)

In both cases, the generated functions allocate memory for their return values. If you do not delete this memory (via mxDestroyArray) when you are done with the output variables, your program will leak memory.

Your program may call whichever of these functions is more convenient, as they both invoke your MATLAB file function in an identical fashion. Most programs will likely call the mlf form of the function to avoid managing the extra arrays required by the mlx form. The example program in triangle.c calls mlfSierpinski.

mlfSierpinski(2, &x, &y, iterations, draw);

In this call, the caller requests two output arguments, x and y, and provides two inputs, iterations and draw.

If the output variables you pass in to an mlf function are not NULL, the mlf function will attempt to free them using mxDestroyArray. This means that you can reuse output variables in consecutive calls to mlf functions without worrying about memory leaks. It also implies that you must pass either NULL or a valid MATLAB array for all output variables or your program will fail because the memory manager cannot distinguish between a non-initialized (invalid) array pointer and a valid array. It will try to free a pointer that is not NULL -- freeing an invalid pointer usually causes a segmentation fault or similar fatal error.

Using varargin and varargout in a MATLAB Function Interface

If your MATLAB function interface uses varargin or varargout, you must pass them as cell arrays. For example, if you have N varargins, you need to create one cell array of size 1-by-N. Similarly, varargouts are returned back as one cell array. The length of the varargout is equal to the number of return values specified in the function call minus the number of actual variables passed. As in the MATLAB software, the cell array representing varagout has to be the last return variable (the variable preceding the first input variable) and the cell array representing varargins has to be the last formal parameter to the function call.

For information on creating cell arrays, refer to the C MEX function interface in the External Interfaces documentation.

For example, consider this MATLAB file interface:

[a,b,varargout] = myfun(x,y,z,varargin)

The corresponding C interface for this is

void mlfMyfun(int numOfRetVars, mxArray **a, mxArray **b, 
              mxArray **varargout, mxArray *x, mxArray *y,
              mxArray *z, mxArray *varargin)

In this example, the number of elements in varargout is (numOfRetVars - 2), where 2 represents the two variables, a and b, being returned. Both varargin and varargout are single row, multiple column cell arrays.

Caution

The C++ shared library interface does not support varargin with zero (0) input arguments. Calling your program using an empty mwArray results in the compiled library receiving an empty array with nargin = 1. The C shared library interface allows you to call mlfFOO(NULL) (the compiled MATLAB code interprets this as nargin=0). However, calling FOO((mwArray)NULL) with the C++ shared library interface causes the compiled MATLAB code to see an empty array as the first input and interprets nargin=1.

For example, compile some MATLAB code as a C++ shared library using varargin as the MATLAB function's list of input arguments. Have the MATLAB code display the variable nargin. Call the library with function FOO() and it won't compile, producing this error message:

... 'FOO' : function does not take 0 arguments
Call the library as:
  mwArray junk; 
  FOO(junk); 
or
 FOO((mwArray)NULL);
At runtime, nargin=1. In MATLAB, FOO() is nargin=0 and FOO([]) is nargin=1.

C++ Interfaces for MATLAB Functions Using varargin and varargout.  The C++ mlx interface for MATLAB functions does not change even if the functions use varargin or varargout. However, the C++ function interface (the second set of functions) changes if the MATLAB function is using varargin or varargout.

For examples, view the generated code for various MATLAB function signatures that use varargin or varargout.

Note

For simplicity, only the relevant part of the generated C++ function signature is shown in the following examples.

 function varargout = foo(varargin)

 function varargout = foo(i1, i2, varargin)

 function [o1, o2, varargout] = foo(varargin)

 function [o1, o2, varargout] = foo(i1, i2, varargin)

Retrieving MATLAB Runtime State Information While Using Shared Libraries

When using shared libraries, you may call functions to retrieve specific information from the MATLAB Runtime state. For details, see Set and Retrieve MATLAB Runtime Data for Shared Libraries.

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