Section: FreeMat External Interface
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:
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 $N imes N$
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.
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 --> quit