Phoenix and Scilab/C Interfacing

2005-08-09T06:25:00

Phoenix and Scilab/C interfacing

The Phoenix project is a step towards modernizing Physics education in our country. I had written an LG Article about it some time back. The basic idea is to build a low cost device connected to the PC parallel port using which students can control, capture and analyse real world signals. Companies such as Vernier are in the business of manufacturing interface equipments for teaching Chemistry, Physics and Biology (yes, Biology has progressed a LOT) - what is unique about Phoenix is that it is low cost, made with locally available components and is perhaps the only product of its kind which exploits the power of GNU/Linux. Scilab is a powerful Matlab like program available as Free Software. Ajith Kumar, the designer of Phoenix, had this idea of interfacing the Phoenix box with Scilab so that the signals captured by Phoenix can be analysed (applying filters, taking fft) numerically. It seemed to be real cool! Let's see how this is done (for a simple introduction to Scilab, see this article).

Writing a Scilab-callable C routine

As is the case with most instances of foreign function interfacing, your C function should be `wrapped' by another function which does all kinds of nitty-gritty stuff to massage the high-level language datatypes (like lists) to simple C types. Fortunately, Scilab comes with a program called `intersci' which creates the wrapper functions automagically from a text file with some rather simple interface specifications. Let's say we wish to write a function which takes in two numbers and returns its sum. Here is the C function (in a file called add.c):
int add_(int *a, int *b, int *c)

{
	*c = *a + *b;
	return 0;
}
And here is the interface specification (in a file add_wrap.desc):
add a b
  a vector 1
  b vector 1
  c vector 1

add  a b c
  a integer
  b integer
  c integer

out sequence c
The first part of the interface spec specifies what the Scilab function takes as arguments (a and b, both vectors of length 1). The second part specifies what the C function takes as arguments (a, b and c, all of which should be pointers to integers, but which are simply written as `integer' in the interface spec). The third part says that `c' is an output variable, ie, a value `returned' from the Scilab function. Note that the address of the object to be returned from the Scilab function is passed as an argument to the C function. You visualize a Scilab invocation:
c = add(a, b)
getting transformed to a C function invocation of the form:
add(&a, &b, &c);
Here are the steps to make this C function Scilab callable:
  1. Run the command `intersci-n' with add_wrap.desc as argument. You should see several files, two of which are `add_wrap.c' and `add_wrap_builder.sce'.
  2. Run scilab and type the following at the Scilab prompt:
    
    files = ['add.o', 'add_wrap.o']
    libs = []
    exec add_wrap_builder.sce
    exec loader.sce
Now you can invoke add(1,2) at the Scilab prompt and you will get the proper answer. (Note that Scilab doesn't differentiate between a 1 element vector and a scalar quantity; thus [1] is same as 1).

Adding two vectors

Here is another simple example. Our C function is:

int addvect_(int *n, double *a, double *b, double *c)
{
	int i;
	for(i = 0; i < *n; i++)
		c[i] = a[i] + b[i];
	return 0;
}
The interface spec looks like this:

addvect a b
  a vector n
  b vector n
  c vector n

addvect  n a b t
  n integer
  a double
  b double
  c double

out sequence c
********************
We can call addvect from the Scilab prompt as shown below:

addvect([1,2,3], [4,5,6])

Returning two vectors

Here is a sample C function:
int addmulvect_(int *n, double *a, double *b, double *c, double *d)
{
	int i;
	for(i = 0; i < *n; i++)
		c[i] = a[i] + b[i];

	for(i = 0; i < *n; i++)
		d[i] = a[i] * b[i];

	return 0;
}
And here is the interface description:

addmulvect a b
  a vector n
  b vector n
  c vector n
  d vector n

addmulvect  n a b c d
  n integer
  a double
  b double
  c double
  d double

out sequence c d
The function can be called like this:

[p, q] = addmulvect([1,2,3], [4,5,6])
We note that this function returns two vectors which get stored in `p' and `q'.

selvakumar

Sat Feb 9 05:14:22 2008

dear sir the below website have a serial tool box for scilab http://www.weizmann.ac.il/home/fesegre/scistuff.html can experiments in pheonix kit can be accessed with scilab??


Tim Reilly

Thu Jul 10 13:06:48 2008

I have been looking for some help doing this. Thank you very much! Best Regards, Tim

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