The input parameters are passed to C functions using the stack. They can be retrieved by the pop functions — for instance, popint.
This file is located in the %LYCIA_DIR\include directory. This directory is referenced by -I flag when a C source is compiled.
Now suppose we want the following C function to be callable from our 4GL program:
/* my_c_src.c */
#include "fglsys.h"
int my_c_func(int argc) {
int n;
popint(&n);
pushint(n/2);
pushint(n*2);
return(2);
}