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fenv.h(0P) POSIX Programmer's Manual fenv.h(0P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
fenv.h — floating-point environment
#include <fenv.h>
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.1‐2008 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.
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
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.
None.
The System Interfaces volume of POSIX.1‐2008, feclearexcept(3p),
fegetenv(3p), fegetexceptflag(3p), fegetround(3p), feholdexcept(3p),
feraiseexcept(3p), fetestexcept(3p), feupdateenv(3p)
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 .
IEEE/The Open Group 2013 fenv.h(0P)
Pages that refer to this page: feclearexcept(3p), fegetenv(3p), fegetexceptflag(3p), fegetround(3p), feholdexcept(3p), feraiseexcept(3p), fetestexcept(3p), feupdateenv(3p)