Q129209: HOWTO: Convert 10-Byte Long Doubles to 8-Byte Doubles

Article: Q129209
Product(s): Microsoft C Compiler
Version(s): 1.0,2.0,2.1,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbCompiler kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Last Modified: 29-JUL-2001

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The information in this article applies to:

- Microsoft C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0, used with:
      - the hardware: Intel x86 
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SUMMARY
=======

With the 16-bit Microsoft C/C++ compilers, long doubles are stored as 80- bit
(10-byte) data types. Under Windows NT, in order to be compatible with other
non-Intel floating point implementations, the 80-bit long double format is
aliased to the 64-bit (8-byte) double format.

This means that 32-bit programs may not be able read back data files written by
16-bit programs because the long double formats are incompatible.

On Intel platforms, the only workaround is to let the floating point processor
handle the conversion from 80-bit to 64-bit doubles. Afterwards, the data can be
stored back into a 64-bit double for use under Win32.

The sample code below illustrates how you could use floating point instructions
in inline assembly to convert from a 10-byte double in a data file to an 8-byte
double.

Sample Code
-----------

  /* Compile options needed: none
  */ 

  #include <stdio.h>

  void main(void)
  {
     FILE *inFile;
     char buffer[10];
     long double Newdbl;

     inFile = fopen("data","rb");
     fread(buffer, 10, 1, inFile);      // reads in 10-byte long double
     fclose(inFile);

     // This moves the contents of the buffer into the floating point
     // register, which then then takes care of the automatic convertion
     // back to a 8-byte long double

     _asm {
        fld TBYTE PTR buffer;
        fstp Newdbl;
     }
  }

Additional query words: 8.00 9.00 9.10

======================================================================
Keywords          : kbCompiler kbVC100 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : :1.0,2.0,2.1,4.0,5.0,6.0
Hardware          : x86
Issue type        : kbhowto

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