'\" et

.TH fenv.h "0P" 2013 "IEEE/The Open Group" "POSIX Programmer's Manual"

.SH PROLOG

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.

.SH NAME

fenv.h

\(em floating-point environment

.SH SYNOPSIS

.LP

.nf

#include <fenv.h>

.fi

.SH 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\(hy2008 defers to the ISO\ C standard.

.P

The

.IR <fenv.h> 

header shall define the following data types through

.BR typedef :

.IP "\fBfenv_t\fR" 10

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.

.IP "\fBfexcept_t\fR" 10

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.

.P

The

.IR <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

\fIfeclearexcept\fR(),

\fIfegetexceptflag\fR(),

\fIferaiseexcept\fR(),

\fIfesetexceptflag\fR(),

and

\fIfetestexcept\fR().

The defined macros shall expand to integer constant expressions with

values that are bitwise-distinct.

.sp

.RS

FE_DIVBYZERO

FE_INEXACT

FE_INVALID

FE_OVERFLOW

FE_UNDERFLOW

.RE

.P

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.

.P

The

.IR <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.

.P

The

.IR <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

\fIfegetround\fR()

and

\fIfesetround\fR()

functions. The defined macros shall expand to integer constant

expressions whose values are distinct non-negative values.

.sp

.RS

FE_DOWNWARD

FE_TONEAREST

FE_TOWARDZERO

FE_UPWARD

.RE

.P

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.

.P

The

.IR <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

.BR fenv_t .

It can be used as an argument to the functions within the

.IR <fenv.h> 

header that manage the floating-point environment.

.sp

.RS

FE_DFL_ENV

.RE

.P

The following shall be declared as functions and may also be defined as

macros. Function prototypes shall be provided.

.sp

.RS 4

.nf

\fB

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 *);

.fi \fR

.P

.RE

.P

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.)

.LP

.IR "The following sections are informative."

.SH "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.

.P

Certain application programming conventions support the intended model

of use for the floating-point environment:

.IP " *" 4

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.

.IP " *" 4

A function call is assumed to require default floating-point control

modes, unless its documentation promises otherwise.

.IP " *" 4

A function call is assumed to have the potential for raising

floating-point exceptions, unless its documentation promises otherwise.

.P

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.

.P

Even though the rounding direction macros may expand to constants

corresponding to the values of FLT_ROUNDS, they are not required to do

so.

.P

For example:

.sp

.RS 4

.nf

\fB

#include <fenv.h>

void f(double x)

{

    #pragma STDC FENV_ACCESS ON

    void g(double);

    void h(double);

    /* ... */

    g(x + 1);

    h(x + 1);

    /* ... */

}

.fi \fR

.P

.RE

.P

If the function

\fIg\fR()

might depend on status flags set as a side-effect of the first

.IR x +1,

or if the second

.IR x +1

might depend on control modes set as a side-effect of the call to

function

\fIg\fR(),

then the application shall contain an appropriately placed invocation

as follows:

.sp

.RS 4

.nf

\fB

#pragma STDC FENV_ACCESS ON

.fi \fR

.P

.RE

.SH RATIONALE

.SS "The fexcept_t Type"

.P

.BR fexcept_t

does not have to be an integer type. Its values must be obtained by a

call to

\fIfegetexceptflag\fR(),

and cannot be created by logical operations from the exception macros.

An implementation might simply implement

.BR fexcept_t

as an

.BR int

and use the representations reflected by the exception macros, but is

not required to; other representations might contain extra information

about the exceptions.

.BR fexcept_t

might be a

.BR 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

.BR fexcept_t ,

and so the user cannot inspect it.

.SS "Exception and Rounding Macros"

.P

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

.BR #ifdef

to test for this.

.SH "FUTURE DIRECTIONS"

None.

.SH "SEE ALSO"

The System Interfaces volume of POSIX.1\(hy2008,

.IR "\fIfeclearexcept\fR\^(\|)",

.IR "\fIfegetenv\fR\^(\|)",

.IR "\fIfegetexceptflag\fR\^(\|)",

.IR "\fIfegetround\fR\^(\|)",

.IR "\fIfeholdexcept\fR\^(\|)",

.IR "\fIferaiseexcept\fR\^(\|)",

.IR "\fIfetestexcept\fR\^(\|)",

.IR "\fIfeupdateenv\fR\^(\|)"

.SH 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 .