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Linux 2.6 - man page for fenv.h (linux section 7posix)

<fenv.h>(P)			    POSIX Programmer's Manual			      <fenv.h>(P)

       fenv.h - floating-point environment

       #include <fenv.h>

       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.

	      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 the following constants if and only if the implementation
       supports the floating-point exception by means of the floating-point functions  feclearex-
       cept(),	fegetexceptflag(),  feraiseexcept(),  fesetexceptflag(), and fetestexcept(). Each
       expands to an integer constant expression with values such that bitwise-inclusive  ORs  of
       all combinations of the constants result in distinct values.


       The  <fenv.h>  header  shall  define  the following constant, which is simply the bitwise-
       inclusive OR of all floating-point exception constants defined above:


       The <fenv.h> header shall define the following constants if and only if the implementation
       supports  getting  and  setting	the  represented  rounding  direction  by  means  of  the
       fegetround() and fesetround() functions. Each expands to an  integer  constant  expression
       whose values are distinct non-negative vales.


       The  <fenv.h>  header  shall  define  the following constant, 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.


       The following shall be declared as functions and may also be defined as	macros.  Function
       prototypes shall be provided.

	      int  feclearexcept(int);
	      int  fegetexceptflag(fexcept_t *, int);
	      int  feraiseexcept(int);
	      int  fesetexceptflag(const fexcept_t *, int);
	      int  fetestexcept(int);
	      int  fegetround(void);
	      int  fesetround(int);
	      int  fegetenv(fenv_t *);
	      int  feholdexcept(fenv_t *);
	      int  fesetenv(const fenv_t *);
	      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  com-
       pound  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 trans-
       lation 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 state-
       ment),  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 appli-
       cation 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	implemen-
       tation-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 set-

       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 float-
       ing-point state information.  Also it is designed to facilitate code portability among all

       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 float-
       ing-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)
		  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  func-
       tion  g(),  then  the application shall contain an appropriately placed invocation as fol-

	      #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.


       The  System  Interfaces volume of IEEE Std 1003.1-2001, feclearexcept(), fegetenv(), fege-
       texceptflag(), fegetround(),  feholdexcept(),  feraiseexcept(),	fesetenv(),  fesetexcept-
       flag(), fesetround(), fetestexcept(), feupdateenv()

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003				      <fenv.h>(P)

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