fenv.h(3)                  POSIX Programmer's Manual                 fenv.h(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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(3)
