BUG: The pow() Function Doesn't Set Underflow Condition

• The C Run-Time (CRT), used with:
  • Microsoft Visual C++, 32-bit Editions, versions 5.0, 6.0

SYMPTOMS

The pow() function does not set the errno variable to indicate that an underflow condition has occurred.

RESOLUTION

A potential solution is to check the base of the number that is being raised to a power and check the result of the pow() function. By customizing the _matherr() function, the underflow condition can be trapped and you can display an error message. The program below demonstrates this workaround.

    #include <stdio.h>
    #include <math.h>
    #include <ERRNO.H>
    #include <string.h>
    extern int errno;

    /* The _matherr routine can be customized to handle the specific
        underflow problem that is encountered.  See online help for
        more information on _matherr().
    */ 
    int _matherr( struct _exception *except )
    {
        /* Handle errors caused by pow() function.*/ 
        if(strcmp(except->name,"pow")==0)
        {
            switch(except->type)
            {
                case _DOMAIN:
                {
                    printf("Domain Error: Argument not in domain.\n");
                    return 1;
                    break;
                }
                case _SING:
                {
                    printf("Singularity Error: Argument singularity "
                           "error.\n");
                    return 1;
                    break;
      }
                case _OVERFLOW:
                {
                    printf("Overflow Error: Overflow range error.\n");
                    return 1;
                    break;
                }
                case _PLOSS:
                {
                    printf("Precision Error: Partial loss of "
                           "significance.\n");
                    return 1;
                    break;
                }
                case _TLOSS:
                {
                    printf("Precision Error: Total loss of "
                           "significance.\n");
                    return 1;
                    break;
      }
                case _UNDERFLOW:
      {
                    printf("Underflow Error: The result is too small to "
                           "be represented.\n");
                    return 1;
                    break;
                }
                default:
                {
                    printf("Unknown Error Occurred While Performing Math "
                           "Operation.\n");
                    return 1;
                    break;
      }
       }
        }
        else
            return 0;  /* non-error condition */ 
    }

    void main(void)
    {

    double x = -1.0e+307;
    double y = -1.0e+307;
    double z;

    z = pow(x,y);

    /*
    Check for error conditions.  _matherr() is automatically
    called should there be a math related error.
    */ 

    /*Note that errno is not set properly & _matherr() is called.*/ 
    if (errno != 0)
        printf("ERROR! Errno: %i\n",errno);
    else
        printf("pow( %e, %e ) = %f\n", x, y, z);
   } 

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article.

MORE INFORMATION

The help topic for pow() states:

No error message is printed on overflow or underflow.

Additional query words:

Keywords : kbtool kbVC500bug kbVC600bug
Version : winnt:5.0,6.0
Platform : winnt
Issue type : kbbug