Q44137: CALLing C to Return Single-Precision Gives Incorrect Results

Article: Q44137
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist4.00 buglist4.00b buglist4.50 B_BasicCom S_QuickC S_C | mspl13_basic
Last Modified: 8-NOV-1990

From within a BASIC program, invoking a Microsoft C or QuickC function
that attempts to return a single-precision number may give
unpredictable results. Returning any other numeric type from a C
function gives correct results.

To work around the problem, you can pass the single-precision number
as a parameter (whose value is set inside the C function and passed
back).

This information applies to Microsoft C Compiler versions 5.00, 5.10,
and 6.00, and to Microsoft QuickC versions 1.00, 1.01, 2.00, 2.01,
2.50, and 2.51.

Microsoft has confirmed this to be a problem in QuickBASIC versions
4.00, 4.00b, and 4.50 for MS-DOS, in Microsoft BASIC Compiler versions
6.00 and 6.00b (buglist6.00, buglist6.00b) for MS-DOS and OS/2, and in
Microsoft BASIC Professional Development System (PDS) versions 7.00
and 7.10 for MS-DOS and MS OS/2 (buglist7.00, buglist7.10). We are
researching this problem and will post new information here as it
becomes available.

Note that only certain versions of BASIC link with certain versions of
C, as explained in a separate article found by searching in this
Knowledge Base for the following words:

   BASIC and C and QuickC and link and compatible and pass and calling

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

'BASIC code to show incorrect return of a single
DECLARE FUNCTION doublefun# CDECL ALIAS "_doublefun"
DECLARE FUNCTION singlefun! CDECL ALIAS "_singlefun"
DECLARE SUB floatchange CDECL (BYVAL offset%)

x# = doublefun#                   'Works correctly
y! = singlefun!                   'Returns wrong value
z! = 0
CALL floatchange (VARPTR(z!))     'Workaround - pass as parameter
PRINT x#
PRINT y!
PRINT z!
END

/* C code, compile with /AM (medium memory model)*/
double doublefun()        /* returns a double */
{
  double x;
  x = 1.45;
  printf("%f\n",x);
  return x;
}

float singlefun()         /* returns a single */
{
  float x;
  x = 1.45;
  printf("%f\n",x);
  return x;
}

void floatchange (x)     /* Takes a single as a parameter */
float *x;
{
  *x = 1.45;
  printf("%f\n",x);
}

Program Output
--------------

  1.450000
  1.450000              <--These are the C PRINTF statements.
  1.450000

  1.45
  2                     <--These are the BASIC PRINT statements.
  1.45

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