Microsoft KB Archive/167750

Knowledge Base

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

Article ID: 167750

Article Last Modified on 9/23/2003

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APPLIES TO

• The C Run-Time (CRT)
• Microsoft Visual C++ 5.0 Professional Edition
• Microsoft Visual C++ 6.0 Professional Edition
• Microsoft Visual C++ 5.0 Enterprise Edition
• Microsoft Visual C++ 6.0 Enterprise Edition
• Microsoft Visual C++ 6.0 Standard Edition
• Microsoft Visual C++ .NET 2003 Standard Edition
• Microsoft Visual C++ .NET 2002 Standard Edition

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This article was previously published under Q167750

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 that this is a bug in the Microsoft products that are listed at the beginning of this article.

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: kbbug KB167750

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