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

• The C Run-time (CRT) included with: - Microsoft Visual C++, 32-bit Editions, version 5.0

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. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

MORE INFORMATION

The help topic for pow() states:

"No error message is printed on overflow or underflow."

However, an overflow does set errno to the value of 34 indicating "Result too large". When an underflow occurs, the returned value is 0.

Keywords          : CRTIss vcbuglist500 kbtool kbbuglist
Version           : 5.0
Platform          : NT WINDOWS
Issue type        : kbbug