BUG: Using Intrinsic Versions of log10 and fmod

SYMPTOMS

In some cases, using the intrinsic versions of log10 and fmod together in a program may produce incorrect results. The sample code below illustrates the problem.

When the result of log10 is a negative whole number that is divided by 1.0 using fmod, the result of fmod will be -1.0 rather than 0.0. This only occurs when the compiler generates the intrinsic versions of these functions. You need to use the /Oi compiler option to have the compiler generate intrinsic versions of functions.

   /* Compile options needed: /Oi
   */ 
   #include <math.h>
   #include <stdio.h>

   void main()
   {
       double dValue, theLog, theMod;

       dValue = 0.01;
       theLog = log10( dValue );
       theMod = fmod( theLog, 1.0 );

       printf( "The log10 of %f is %f\n", dValue, theLog );
       printf( "The fmod of (%f,1.0) is %f\n", theLog, theMod );
       printf( "for  all x, fmod(x, 1.0) should always be 0.0\n" );
   }