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fenv.h - floating-point environment
Návod, kniha: POSIX Programmer's Manual
#include <fenv.h>
The
<fenv.h> header shall define the following data types through
typedef:
- fenv_t
- Represents the entire floating-point environment. The
floating-point environment refers collectively to any floating-point
status flags and control modes supported by the implementation.
- fexcept_t
- Represents the floating-point status flags collectively,
including any status the implementation associates with the flags. A
floating-point status flag is a system variable whose value is set (but
never cleared) when a floating-point exception is raised, which occurs as
a side effect of exceptional floating-point arithmetic to provide
auxiliary information. A floating-point control mode is a system variable
whose value may be set by the user to affect the subsequent behavior of
floating-point arithmetic.
The
<fenv.h> header shall define the following constants if and
only if the implementation supports the floating-point exception by means of
the floating-point functions
feclearexcept(),
fegetexceptflag(),
feraiseexcept(),
fesetexceptflag(), and
fetestexcept().
Each expands to an integer constant expression with values such that
bitwise-inclusive ORs of all combinations of the constants result in distinct
values.
FE_DIVBYZERO
FE_INEXACT
FE_INVALID
FE_OVERFLOW
FE_UNDERFLOW
The
<fenv.h> header shall define the following constant, which is
simply the bitwise-inclusive OR of all floating-point exception constants
defined above:
The
<fenv.h> header shall define the following constants if and
only if the implementation supports getting and setting the represented
rounding direction by means of the
fegetround() and
fesetround()
functions. Each expands to an integer constant expression whose values are
distinct non-negative vales.
FE_DOWNWARD
FE_TONEAREST
FE_TOWARDZERO
FE_UPWARD
The
<fenv.h> header shall define the following constant, which
represents the default floating-point environment (that is, the one installed
at program startup) and has type pointer to const-qualified
fenv_t. It
can be used as an argument to the functions within the
<fenv.h>
header that manage the floating-point environment.
The following shall be declared as functions and may also be defined as macros.
Function prototypes shall be provided.
int feclearexcept(int);
int fegetexceptflag(fexcept_t *, int);
int feraiseexcept(int);
int fesetexceptflag(const fexcept_t *, int);
int fetestexcept(int);
int fegetround(void);
int fesetround(int);
int fegetenv(fenv_t *);
int feholdexcept(fenv_t *);
int fesetenv(const fenv_t *);
int feupdateenv(const fenv_t *);
The FENV_ACCESS pragma provides a means to inform the implementation when an
application might access the floating-point environment to test floating-point
status flags or run under non-default floating-point control modes. The pragma
shall occur either outside external declarations or preceding all explicit
declarations and statements inside a compound statement. When outside external
declarations, the pragma takes effect from its occurrence until another
FENV_ACCESS pragma is encountered, or until the end of the translation unit.
When inside a compound statement, the pragma takes effect from its occurrence
until another FENV_ACCESS pragma is encountered (including within a nested
compound statement), or until the end of the compound statement; at the end of
a compound statement the state for the pragma is restored to its condition
just before the compound statement. If this pragma is used in any other
context, the behavior is undefined. If part of an application tests
floating-point status flags, sets floating-point control modes, or runs under
non-default mode settings, but was translated with the state for the
FENV_ACCESS pragma off, the behavior is undefined. The default state (on or
off) for the pragma is implementation-defined. (When execution passes from a
part of the application translated with FENV_ACCESS off to a part translated
with FENV_ACCESS on, the state of the floating-point status flags is
unspecified and the floating-point control modes have their default settings.)
The following sections are informative.
This header is designed to support the floating-point exception status flags and
directed-rounding control modes required by the IEC 60559:1989
standard, and other similar floating-point state information. Also it is
designed to facilitate code portability among all systems.
Certain application programming conventions support the intended model of use
for the floating-point environment:
- *
- A function call 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.
- *
- A function call is assumed to require default
floating-point control modes, unless its documentation promises
otherwise.
- *
- A function call is assumed to have the potential for
raising floating-point exceptions, unless its documentation promises
otherwise.
With these conventions, an application can safely assume default floating-point
control modes (or be unaware of them). The responsibilities associated with
accessing the floating-point environment fall on the application that does so
explicitly.
Even though the rounding direction macros may expand to constants corresponding
to the values of FLT_ROUNDS, they are not required to do so.
For example:
#include <fenv.h>
void f(double x)
{
#pragma STDC FENV_ACCESS ON
void g(double);
void h(double);
/* ... */
g(x + 1);
h(x + 1);
/* ... */
}
If the function
g() might depend on status flags set as a side effect of
the first
x+1, or if the second
x+1 might depend on control
modes set as a side effect of the call to function
g(), then the
application shall contain an appropriately placed invocation as follows:
#pragma STDC FENV_ACCESS ON
fexcept_t does not have to be an integer type. Its values must be
obtained by a call to
fegetexceptflag(), and cannot be created by
logical operations from the exception macros. An implementation might simply
implement
fexcept_t as an
int and use the representations
reflected by the exception macros, but is not required to; other
representations might contain extra information about the exceptions.
fexcept_t might be a
struct with a member for each exception
(that might hold the address of the first or last floating-point instruction
that caused that exception). The ISO/IEC 9899:1999 standard makes no
claims about the internals of an
fexcept_t, and so the user cannot
inspect it.
Macros corresponding to unsupported modes and rounding directions are not
defined by the implementation and must not be defined by the application. An
application might use
#ifdef to test for this.
None.
The System Interfaces volume of IEEE Std 1003.1-2001,
feclearexcept(),
fegetenv(),
fegetexceptflag(),
fegetround(),
feholdexcept(),
feraiseexcept(),
fesetenv(),
fesetexceptflag(),
fesetround(),
fetestexcept(),
feupdateenv()
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html
.
