First, make sure that you have #included `<math.h>`

, and correctly
declared other functions returning `double`

.

If the problem isn't that simple, recall that most digital computers use floating-point formats which provide a close but by no means exact simulation of real number arithmetic. Underflow, cumulative precision loss, and other anomalies are often troublesome.

Don't assume that floating-point results will be exact, and
especially don't assume that floating-point values can be
compared for equality.` `(Don't throw haphazard "fuzz factors"
in, either.)

These problems are no worse for C than they are for any other
computer language.` `Floating-point semantics are usually defined
as "however the processor does them;" otherwise a compiler for a
machine without the "right" model would have to do prohibitively
expensive emulations.

This article cannot begin to list the pitfalls associated with,
and workarounds appropriate for, floating-point work.` `A good
programming text should cover the basics.

References: EoPS Sec. 6 pp. 115-8.

`<math.h>`

;,
but I keep getting "`undefined: _sin`

" compilation errors.
Make sure you're linking with the correct math library.` `For
instance, under Unix, you usually need to use the` -lm `

option,
and at the *end* of the command line, when compiling/linking.
See also question 12.14.

Because few processors have an exponentiation instruction.
Instead, you can `#include <math.h>`

and use the `pow()`

function,
although explicit multiplication is often better for small
positive integral exponents.

References: ANSI Sec. 4.5.5.1 .

The simplest and most straightforward way is with code like

(int)(x + 0.5)

This won't work properly for negative numbers, though.

Many systems with high-quality IEEE floating-point implementations provide facilities (e.g. an isnan() macro) to deal with these values cleanly, and the Numerical C Extensions Group (NCEG) is working to formally standardize such facilities. A crude but usually effective test for NaN is exemplified by

#define isnan(x) ((x) != (x))

although non-IEEE-aware compilers may optimize the test away.

Some compilers for small machines, including Turbo C (and
Ritchie's original PDP-11 compiler), leave out floating point
support if it looks like it will not be needed.` `In particular,
the non-floating-point versions of printf and scanf save space
by not including code to handle `%e`

, `%f`

, and `%g`

.` `It happens that
Turbo C's heuristics for determining whether the program uses
floating point are insufficient, and the programmer must
sometimes insert an extra, explicit call to a floating-point
library routine to force loading of floating-point support.