IMPORT Foreign Function Import

Section: FreeMat External Interface

Usage

The import function allows you to call functions that are compiled into shared libraries, as if they were FreeMat functions. The usage is 
  import(libraryname,symbol,function,return,arguments)

The argument libraryname is the name of the library (as a string) to import the function from. The second argument symbol (also a string), is the name of the symbol to import from the library. The third argument function is the the name of the function after its been imported into Freemat. The fourth argument is a string that specifies the return type of the function. It can take on one of the following types:

The fourth argument is more complicated. It encodes the arguments of the imported function using a special syntax. In general, the argument list is a string consisting of entries of the form: 
  type[optional bounds check] {optional &}name

Here is a list of various scenarios (expressed in 'C'), and the corresponding entries, along with snippets of code. \emph{Scalar variable passed by value:} Suppose a function is defined in the library as 

  int fooFunction(float t),

i.e., it takes a scalar value (or a string) that is passed by value. Then the corresponding argument string would be 

  'float t'

For a C-string, which corresponds to a function prototype of 

  int fooFunction(const char *t),

the corresponding argument string would be 

  'string t'

Other types are as listed above. Note that FreeMat will automatically promote the type of scalar variables to the type expected by the C function. For example, if we call a function expecting a float with a double or int16 argument, then FreeMat will automatically apply type promotion rules prior to calling the function. \emph{Scalar variable passed by reference:} Suppose a function is defined in the library as 

  int fooFunction(float *t),

i.e., it takes a scalar value (or a string) that is passed as a pointer. Then the corresponding argument string would be 

  'float &t'

If the function fooFunction modifies t, then the argument passed in FreeMat will also be modified. \emph{Array variable passed by value:} In C, it is impossible to distinguish an array being passed from a simple pointer being passed. More often than not, another argument indicates the length of the array. FreeMat has the ability to perform bounds-checking on array values. For example, suppose we have a function of the form 

  int sum_onehundred_ints(int *t),

where sum_onehundred_ints assumes that t is a length 100 vector. Then the corresponding FreeMat argument is 

  'float32[100] t'.

Note that because the argument is marked as not being passed by reference, that if sub_onehundred_ints modifies the array t, this will not affect the FreeMat argument. Note that the bounds-check expression can be any legal scalar expression that evaluates to an integer, and can be a function of the arguments. For example to pass a square $N \times N$ matrix to the following function: 

  float determinantmatrix(int N, float *A),

we can use the following argument to import: 

  'int32 N, float[N*N] t'.

\emph{Array variable passed by reference:} If the function in C modifies an array, and we wish this to be reflected in the FreeMat side, we must pass that argument by reference. Hence, consider the following hypothetical function that squares the elements of an array (functionally equivalent to $x.^2$): 

  void squarearray(int N, float *A)

we can use the following argument to import: 

  'int32 N, float[N] &A'.

Note that to avoid problems with memory allocation, external functions are not allowed to return pointers. As a result, as a general operating mechanism, the FreeMat code must allocate the proper arrays, and then pass them by reference to the external function. \emph{Sending text to the FreeMat console:} Starting with FreeMat 4.0, it is possible for external code that is called using the import statement to send text to the FreeMat console. To do so, you must define in each library that wants to send text a function with the name freemat_io_handler that captures its argument (a function pointer), and stores it for use by functions in the library. That function pointer takes a standard C string argument. Here is a snippet of code to demonstrate how this works: 

  /* just to make it readable */
  typedef void (*strfunc)(const char*); 

  /* The name we want to use for the function */
  strfunc FreeMatText;                  

  /* this name is case sensitive and must not be mangled (i.e., use extern "C") */
  void freemat_io_handler(strfunc printFunc) {
     FreeMatText = printFunc;
  }

  double SomeImportedFunction(double t) {
     FreeMatText("I am going to double that argument!\n");
     return (t*2);
  }

In this case, once SomeImportedFunction is called from within FreeMat, the text "I am going to double that argument" will appear in the FreeMat console. Your freemat_io_handler function is automatically called when your library is loaded by FreeMat, which happens with the first import statement that references it.

Example

Here is a complete example. We have a C function that adds two float vectors of the same length, and stores the result in a third array that is modified by the function. First, the C code: 

     addArrays.c
void addArrays(int N, float *a, float *b, float *c) {
  int i;
 
  for (i=0;i<N;i++)
   c[i] = a[i] + b[i];
}

We then compile this into a dynamic library, say, add.so. The import command would then be: 

  import('add.so','addArrays','addArrays','void', ...
         'int32 N, float[N] a, float[N] b, float[N] &c');

We could then exercise the function exactly as if it had been written in FreeMat. The following only works on systems using the GNU C Compiler: 

--> if (strcmp(computer,'MAC')) system('gcc -bundle -flat_namespace -undefined suppress -o add.so addArrays.c'); end;
--> if (~strcmp(computer,'MAC')) system('gcc -shared -fPIC -o add.so addArrays.c'); end;
--> import('add.so','addArrays','addArrays','void','int32 N, float[N] a, float[N] b, float[N] &c');
--> a = [3,2,3,1];
--> b = [5,6,0,2]; 
--> c = [0,0,0,0];
--> addArrays(length(a),a,b,c)

ans = 
  []
--> c

ans = 
 8 8 3 3