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PROLOG | NAME | SYNOPSIS | DESCRIPTION | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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.
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.1‐2017, 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 1003.1-2017, 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(0P)
Pages that refer to this page: feclearexcept(3p), fegetenv(3p), fegetexceptflag(3p), fegetround(3p), feholdexcept(3p), feraiseexcept(3p), fetestexcept(3p), feupdateenv(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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