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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.1‐2017 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.1‐2017, feclearexcept(), fegetenv(), fegetexceptflag(),
fegetround(), feholdexcept(), feraiseexcept(), fetestexcept(), feupdateenv()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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 2017 fenv.h(7POSIX)