Q43217: QuickC: MATH.C Sample Program Contains Incorrect Expressions

Article: Q43217
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_c docerr | mspl13_c
Last Modified: 6-JUN-1989

The sample program MATH.C found in the QuickC Version 2.00 on-line
help system contains the following two statements, which do
not produce the intended results:

   printf( "Mantissa: %2.2lf\tExponent: %d\n", frexp( x, &n ), n );
   printf( "Fraction: %2.2lf\tInteger: %lf\n", modf( x, &y ), y );

The last parameter passed to printf() in each of these statements will
not be displayed correctly when the program is compiled under QuickC
because "n" and "y" are pushed onto the stack before they are altered
in the calls to frexp() and modf().

This example illustrates the danger of relying upon the order of
expression evaluation. When compiled under Microsoft C 5.10 with
default optimizations or optimizations disabled, these statements
function as desired; that is, n and y are altered by frexp() and
modf() before they are pushed onto the stack and displayed by
printf(). When compiled under C 5.10 with maximum optimization,
however, the statements will be pushed before being changed, just as
in QuickC.

Correcting this problem is simply a matter of ensuring that frexp()
and modf() execute prior to the use of n and y, as follows:

   double r;

   r = frexp( x, &n );
   printf( "Mantissa: %2.2lf\tExponent: %d\n", r, n );
   r = modf( x, &y );
   printf( "Fraction: %2.2lf\tInteger: %lf\n", r, y );

The MATH.C program is associated with the following functions:

   exp     pow      sqrt     frexp     log     log10
   ldexp   modf     ceil     floor     fabs    fmod

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