Q52103: Casting Large Double to Float Gives Compile or Run-Time Error

Article: Q52103
Product(s): See article
Version(s): 5.10   | 5.10
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | s_quickc | mspl13_c
Last Modified: 10-JAN-1990

Casts of double numbers (greater than 3.4E+38) to float numbers can
cause serious problems at compile time or at run time. Although this
is expected behavior, it is inconsistent with typical overflow casting
in that no errors or warnings are produced by the compiler when
nonfloat overflow casts are made.

In addition, overflowing a float by casting it to a double constant
may cause the compiler to generate several lines of random characters.

Microsoft has confirmed this random character generation to be a
problem with C Version 5.10. We are researching this problem and will
post new information as it becomes available.

If a constant that is greater than 3.4E+38 (the maximum float) is cast
to a float, the compiler will fail with the following error:

   fatal error C1045: floating point overflow

The following code sample demonstrates the compile error:

     /* This code when compiled will produce compiler error code
        C1045. */

     void main(void)
     {
          float f;

          f = (float)6e39;
          /* All numbers up to 3.4e38 work fine */
     }

If a double return value (greater than 3.4E+38) is cast to a float,
the following run-time error message is displayed:

   run-time error M6104: MATH
   -floating-point error: overflow

This following code sample demonstrates the run-time error:

     /* This code will compile fine but produces a run-time error
        M6104. */

     #include <stdlib.h>

     void main(void)
     {
         float f;

         f = (float)atof("6e39");
         /* Once again, all numbers up to 3.4e38 work fine */
     }

On the other hand, the following integer cast overflow demonstrates
that on a nonfloating point cast overflow, no errors are produced:

     /* The following code produces no compiler errors, warnings
        (even at warning level 3), or run-time errors. */

     #include <stdio.h>

     void main(void)
     {
         int i;

         i = (int)70000;
         /* The signed integer i is overflowed in this case and will
            probably produce unintended results. */
     }

There is no workaround regarding the casting of double constants to
floats at compile time other than eliminating the offending cast from
the code. On the other hand, the problem with the floating point
overflow error at run time can be caught by using the signal()
function to trap floating point exceptions. However, both problems can
be avoided if you ensure that a value greater than 3.4E+38 (maximum
float value as determined by the IEEE standard for 4 byte floating
point numbers) is never assigned to a float.

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