DOCUMENT:Q49823  22-JUL-2001  [visualc]
TITLE   :PRB: Inconsistent Use of fscanf %x.xf Formatting
PRODUCT :Microsoft C Compiler
PROD/VER:5.1 6.0 6.0a 6.0ax 7.0 1.0 1.5 2
OPER/SYS:
KEYWORDS:kbCRT kbVC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600

======================================================================

-------------------------------------------------------------------------------
The information in this article applies to:

 - The C Run-Time (CRT), included with:
    - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax 
    - Microsoft C/C++ for MS-DOS, version 7.0 
    - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5 
    - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

Using the Microsoft C run-time library function fscanf() with the format
specifier %x.xf to read floating-point values usually produces undesirable
results. This happens especially if the file contains floating point information
that was not recorded using the same %x.x specifier that is being used to read.

The sample code below can be used as an example of expected behavior. The first
output line shows the input values, the second shows what an incorrect fscanf()
format specifier reads, and the third is the correct data read by fscanf(). The
output from the program is as follows:

   Values: 3.104000, 34.235340, 834.343201, 5968.394043
   After Read: 3.100000, 419561472.000000, 0.000003, 4.227506
   After Rewind: 3.104000, 34.235340, 834.343201, 5968.394043

The output of the second line (After Read) may differ between compilers and may
depend upon the compiler options, but is still not the expected values.

CAUSE
=====

This is not a problem with the fscanf() function; it is due to using a different
floating point specifier to read the value than was used to write the value.

In the sample code below, the first value is read correctly. After the first
value is read, the file pointer points to the second 0 (zero) in 3.104000. When
the second value is read, the information in the file does not conform to
floating-point format, and zeros are read after that. The compiler has no way of
knowing how far to move the file pointer to get to the next value.

RESOLUTION
==========

Read floating point values with the same specifier that was used to write them.
Once a value has been read correctly, it may be truncated or formatted as
desired.

MORE INFORMATION
================

For more information on the scanf() family of functions, please refer to the
Visual C++ Books Online.

The following sample code demonstrates the importance of reading floating point
values with the same type specifier that was used in writing them. Refer to the
SUMMARY above for the expected program output.

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

   /* Compile options needed: none
   */ 

   #include<stdio.h>

   void main( void )
   {
      float a = 3.104f, b = 34.23534f, c = 834.3432f, d = 5968.394f;
      float ar, br, cr, dr;
      FILE *stream;

      if( (stream = fopen( "file.tst", "a" )) == NULL )
         printf( "ERROR:  Unable to open output file" );
      else
         fprintf( stream, "%f,%f,%f,%f\n", a, b, c, d );
      fclose( stream );
      if( (stream = fopen( "file.tst", "r" )) == NULL )
         printf( "ERROR:  Unable to open input file" );
      else
                       /* NOTE:  %x.x format specifier */ 
      {                /*  |  */ 
         printf("Values: %f, %f, %f, %f\n", a, b, c, d );
         fscanf( stream, "%4f,%4f,%4f,%4f", &ar, &br, &cr, &dr );
         printf("After Read: %f, %f, %f, %f\n", ar, br, cr, dr );
         rewind(stream);
         fscanf( stream, "%f,%f,%f,%f", &ar, &br, &cr, &dr );
         printf("After Rewind: %f, %f, %f, %f\n", ar, br, cr, dr );
      }
   }

Additional query words:

======================================================================
Keywords          : kbCRT kbVC kbVC100 kbVC150 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCRT
Version           : 5.1 6.0 6.0a 6.0ax 7.0 1.0 1.5 2
Issue type        : kbprb

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2001.