fenv.h(7posix)
NAME
fenv.h --- floating-point environment
SYNOPSIS
#include <fenv.h>
DESCRIPTION
The functionality described on this reference page is aligned with the
ISO C standard. Any conflict between the requirements described here
and the ISO C standard is unintentional. This volume of POSIX.12008
defers to the ISO C standard.
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 each of the following macros if and
only if the implementation supports the floating-point exception by
means of the floating-point functions feclearexcept(),
fegetexceptflag(), feraiseexcept(), fesetexceptflag(), and
fetestexcept(). The defined macros shall expand to integer constant
expressions with values that are bitwise-distinct.
FE_DIVBYZERO FE_INEXACT FE_INVALID FE_OVERFLOW FE_UNDERFLOW
If the implementation supports the IEC 60559 Floating-Point option, all
five macros shall be defined. Additional implementation-defined
floating-point exceptions with macros beginning with FE_ and an
uppercase letter may also be specified by the implementation.
The <fenv.h> header shall define the macro FE_ALL_EXCEPT as the
bitwise-inclusive OR of all floating-point exception macros defined by
the implementation, if any. If no such macros are defined, then the
macro FE_ALL_EXCEPT shall be defined as zero.
The <fenv.h> header shall define each of the following macros if and
only if the implementation supports getting and setting the represented
rounding direction by means of the fegetround() and fesetround()
functions. The defined macros shall expand to integer constant
expressions whose values are distinct non-negative values.
FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_UPWARD
If the implementation supports the IEC 60559 Floating-Point option, all
four macros shall be defined. Additional implementation-defined
rounding directions with macros beginning with FE_ and an uppercase
letter may also be specified by the implementation.
The <fenv.h> header shall define the following macro, 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.
FE_DFL_ENV
The following shall be declared as functions and may also be defined as
macros. Function prototypes shall be provided.
int feclearexcept(int);
int fegetenv(fenv_t *);
int fegetexceptflag(fexcept_t *, int);
int fegetround(void);
int feholdexcept(fenv_t *);
int feraiseexcept(int);
int fesetenv(const fenv_t *);
int fesetexceptflag(const fexcept_t *, int);
int fesetround(int);
int fetestexcept(int);
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.
APPLICATION USAGE
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
RATIONALE
The fexcept_t Type 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. Exception and Rounding Macros 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.
FUTURE DIRECTIONS
None.
SEE ALSO
The System Interfaces volume of POSIX.12008, feclearexcept(), fegetenv(), fegetexceptflag(), fegetround(), feholdexcept(), feraiseexcept(), fetestexcept(), feupdateenv()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .
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