PRB: Underflow Caused By exp() Function Fails to Set errno

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

SYMPTOMS

When using the exp() function and an underflow error occurs, the errno variable is not set to a value.

RESOLUTION

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

void main( void )
{
   double x = -1.0e+3,y;

   y = exp( x );

   // The exp function returns the exponential value of the
   // floating-point parameter, x, if successful. On overflow,
   // the function returns INF (infinite) and on underflow, exp
   // returns 0.

   // Check for error conditions.
   // Underflow error or other.
   if(y==0 || (errno!=0))
   {
      if (y==0)
         printf("ERROR: Underflow Error\n");
      else
         perror("ERROR");

      printf("Errno: %i\n",errno);
   }
   // No Errors--print results.
   else
   {
      printf( "exp( %f ) = %f\n", x, y );
   }
} 

STATUS

This behavior is by design.

MORE INFORMATION

The errno variable is a global variable, which is given an integer value upon an error. Math functions, such as exp(), set this value to indicate an error state; library math routines set errno by calling _matherr. However, the errno variable is not set when an underflow occurs. According to the ANSI C specification, whether or not errno acquires the value of the macro ERANGE is implementation-defined.

Steps to Reproduce Behavior

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

void main( void )
{
   double x = -1.0e+3,y;

   y = exp( x );

   // Check for error conditions.
   // perror() returns the error message associated with errno.
   if(errno!=0)
   {
         perror("ERROR");
   }
   // No Errors--print results. But error did occur because we have an
   // underflow error.
   // The underflow goes undetected because errno wasn't set to any
   // value.
   else
   {
      printf( "exp( %f ) = %f\n", x, y );
   }
} 

Additional query words: 5.00

Keywords          : kbcode kbVC500 kbVC600 
Version           : WINNT:5.0
Platform          : winnt 
Issue type        : kbprb 

Last Reviewed: July 20, 1999