Q47756: Example of C Function Returning a String to BASIC

Article: Q47756
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom S_C S_QuickC | mspl13_basic
Last Modified: 5-SEP-1990

The two programs shown below demonstrate how a C function can return a
string to a compiled BASIC program.

This information about interlanguage calling applies to QuickBASIC
versions 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft BASIC Compiler
versions 6.00 and 6.00b for MS-DOS and MS OS/2, and to Microsoft BASIC
Professional Development System (PDS) versions 7.00 and 7.10 for
MS-DOS and MS OS/2.

For Microsoft BASIC PDS 7.00 and 7.10, this example applies only to
near strings. If you are using far strings (BC /Fs on compile or when
using QBX.EXE), you must use the string-manipulation routines supplied
with BASIC PDS 7.00 and 7.10 (StringAssign, StringRelease,
StringAddress, and StringLength). For more information about far
strings, see Chapter 13 of "Microsoft BASIC 7.0: Programmer's Guide"
for versions 7.00 and 7.10.

For more information about passing other types of parameters between
BASIC and C and a list of which BASIC and C versions are compatible
with each other, query in the Software/Data Library on the following
word:

   BAS2C

Code Example
------------

The following BASIC program is BSTRF.BAS, which calls the C function
and prints out the returned string and its length:

   DECLARE FUNCTION CFUNC$ CDECL ()
   a$ = CFUNC$
   PRINT a$
   PRINT len(a$)

The following program is CSTRF.C, which builds a string descriptor
that is passed back to the calling BASIC program:

#include <string.h>
struct stringdesc
       {
        int length;        /* length of the string */
        char *string;      /* near pointer to the string */
       };
struct stringdesc *std;
char thestring[18];      /* In the medium memory model this  */
                         /* string will be in DGROUP - which */
                         /* is required for BASIC    */
struct stringdesc *cfunc()
{
  std->length = 18;      /* length of the string */
  strcpy(thestring, "This is the string");
  std->string = thestring;
  return(std);           /* return pointer to string descriptor */
}

To demonstrate these programs from an .EXE program, compile and link
as follows:

   BC BSTRF.BAS;
   CL /c /AM CSTRF.C;
   LINK /NOE BSTRF CSTRF;

BSTRF.EXE produces the following output:

   This is the string
   18

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