Calling program functions from C

It is possible to call a BDL function from a C-Extension function.

To call a BDL function from a C-Extension function, use the fgl_call macro:

fgl_call ( function-name, nb-params );

In this call, function-name is the name of the program function to call, and nb-params is the number of parameters pushed on the stack for the program function. The function-name must be written in lowercase letters; the fglcomp compiler converts all program function names to lowercase.

The fgl_call() macro is converted to a function that returns the number of values returned on the stack.

Important: Parameters and return values must be pushed/popped on the runtime stack, by using the stack functions. Parameters passed to the BDL function must be pushed in the same order as the BDL parameter list:

push A, B, C =>
FUNCTION fct( A, B, C )

. However, values returned from the BDL function must be popped in the reverse order of the BDL return clause: RETURN A, B, C => pop C, B, A.

The myprog.4gl BDL module defining the MAIN block and the display_item() function to be called from the C extension:

IMPORT mycext

MAIN
    CALL c_fct()
END MAIN

FUNCTION display_item(name, size)
   DEFINE name VARCHAR(30), size INTEGER
   DISPLAY name, size
   RETURN length(name), (size / 100)
END FUNCTION

The mycext.c C extension module calling the BDL function:

#include <stdlib.h>
#include <stdio.h>

#include "f2c/fglExt.h"

int c_fct( int n );

UsrFunction usrFunctions[]={
  {"c_fct",c_fct,0,0},
  {0,0,0,0}
};

int c_fct( int n )
{
   int rc, len;
   float size2;
   if (n != 0) exit(1);
   pushquote("Hand gloves", 11);
   pushint(54);
   rc = fgl_call( display_item, 2 );
   if (rc != 2) exit(1);
   popflo(&size2);
   popint(&len);
   printf(">> %d %f\n", len, size2);
   return 0;
}

Compilation and execution example on a Linux® system:

$ gcc -I $FGLDIR/include -shared -fPIC -o mycext.so mycext.c

$ fglcomp myprog.4gl

$ fglrun myprog.42m
Hand gloves         54
>> 11 0.540000