fenv.h(0p) — Linux manual page


fenv.h(0P)              POSIX Programmer's Manual             fenv.h(0P)

PROLOG         top

       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.

NAME         top

       fenv.h — floating-point environment

SYNOPSIS         top

       #include <fenv.h>

DESCRIPTION         top

       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

       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


       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.


       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

       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


       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

       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         top

   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



SEE ALSO         top

       The System Interfaces volume of POSIX.1‐2017, feclearexcept(3p),
       fegetenv(3p), fegetexceptflag(3p), fegetround(3p),
       feholdexcept(3p), feraiseexcept(3p), fetestexcept(3p),

COPYRIGHT         top

       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)