fenv(3) BSD Library Functions Manual fenv(3)
NAME
fenv -- functions providing access to the floating-point environment
SYNOPSIS
#include <fenv.h>
DESCRIPTION
The header <fenv.h> declares types, macros, and functions to provide
access to the floating-point environment, consisting of any floating-
point status flags and control modes supported by the implementation. A
floating-point status flag is a variable whose value is set (but not
cleared) when a floating-point exception is raised, which occurs as a
side effect of floating-point arithmetic when an exceptional condition is
encountered. A floating-point control mode is a variable whose value may
be set by the user to affect the subsequent behavior of floating-point
arithmetic.
Certain conventions are expected as part of the intended model for inter-
action with the floating-point environment:
o A function does not alter its caller's floating-point control
modes, clear its caller's floating-point status flags, nor
depend on the state of its caller's floating-point status flags
unless the function is so documented.
o A function is assumed to require default floating-point control
modes unless its documentation promises otherwise.
o A function is assumed to have the potential for raising float-
ing-point exceptions unless its documentation promises other-
wise.
Programmers should adhere to these guidelines in their usage of the func-
tions defined in <fenv.h>. Failure to do so may result in bugs which are
extraordinarily hard to diagnose.
Note that any usage of the functions provided by <fenv.h> should occur
within the scope of
The FENV_ACCESS pragma provides a means to inform the compiler that the
program might access the floating-point environment to test status flags
or change the control modes. The pragma shall occur either outside
external declarations (i.e. at file scope) or preceeding all explicit
declarations and statements inside a compound statement. If part of a
program tests floating-point status flags or sets control modes, but was
translated with FENV_ACCESS "off", the behavior of that program is unde-
fined.
The types fenv_t and fexcept_t represent the entire floating-point envi-
ronment and the floating-point status flags, respectively. These should
be treated as opaque types, and be manipulated only via the functions
defined in <fenv.h>.
The macros FE_INVALID, FE_DIVBYZERO, FE_OVERFLOW, FE_UNDERFLOW, and
FE_INEXACT expand to integer constant expressions such that bitwise ORs
of any combination of the macros result in distinct values and bitwise
ANDs of any combinations of the macros results in zero. These macros are
used in functions defined in <fenv.h> to represent floating-point excep-
tions and status flags. Additionally, the macro FE_ALL_EXCEPT is defined
to be the bitwise OR of all floating-point exception macros defined by
the implementation.
The macros FE_TONEAREST, FE_TOWARDZERO, FE_DOWNWARD, and FE_UPWARD expand
to integer constant expressions whose values are distinct nonnegative
numbers. These are intended for use with the fesetround() and
fegetround() functions for control of the floating-point rounding modes.
The macro FE_DFL_ENV represents the default floating-point environ-
ment--the one installed at program startup--and expands to a pointer to a
const fenv_t object. It can be used as an argument to <fenv.h> functions
that manage the floating-point environment.
The feclearexcept(), feraiseexcept(), fetestexcept(), fegetexceptflag(),
and fesetexceptflag() functions provide access to the floating-point sta-
tus flags.
The fegetround() and fesetround() functions provide control of rounding
direction modes.
The fegetenv(), feholdexcept(), fesetenv(), and feupdateenv() functions
provide control of the entire floating-point environment as an abstract
object.
Further documentation for all of these functions is available in their
respective manpages, and also in section 7.6 of the C standard.
SEE ALSO
feclearexcept(3), fegetenv(3), fegetround(3), fegetexceptflag(3),
feholdexcept(3), fesetenv(3), fesetround(3), feraiseexcept(3),
fesetexceptflag(3), feraiseexcept(3), feupdateenv(3)
STANDARDS
These functions conform to ISO/IEC 9899:TC3.
OS X May 9, 2011 OS X
Mac OS X 10.8 - Generated Mon Aug 27 18:09:09 CDT 2012