PRB: Inconsistent Use of fscanf %x.xf Formatting

 Last reviewed: July 24, 1997
Article ID: Q49823
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, versions 1.0, 1.5 - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

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 );
      }
   }

Keywords : CRTIss kbfasttip
Version : 5.1 6.0 6.0a 6.0ax 7.0 1.0 1.5 2
Platform : MS-DOS NT WINDOWS
Issue type : kbprb

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Last reviewed: July 24, 1997
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