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FENV(3)                  BSD Library Functions Manual                  FENV(3)

NAME
     fenv -- functions providing access to the floating-point environment

SYNOPSIS
     #include <fenv.h>
     #pragma STDC FENV_ACCESS ON

DESCRIPTION
     The header <fenv.h> declares types, macros, and functions to provide access to the floating-point envi-ronment, environment,
     ronment, consisting of any floating-point status flags and control modes supported by the implementa-tion. implementation.
     tion.  A floating-point status flag is a variable whose value is set (but not cleared) when a floating-point floatingpoint
     point exception is raised, which occurs as a side effect of floating-point arithmetic when an excep-tional exceptional
     tional condition is encountered. A floating-point control mode is a variable whose value may be set by
     the user to affect the subsequent behavior of floating-point arithmetic.

     Certain conventions are expected as part of the intended model for interaction with the floating-point
     environment:
           •   A function 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 is assumed to require default floating-point control modes unless its documenta-tion documentation
               tion promises otherwise.
           •   A function is assumed to have the potential for raising floating-point exceptions unless its
               documentation promises otherwise.
     Programmers should adhere to these guidelines in their usage of the functions defined in <fenv.h>.
     Failure to do so may result in bugs which are extraordinarily hard to diagnose.

     Note that any usage of the functions provided by <fenv.h> should occur within the scope of

           #pragma STDC FENV_ACCESS ON

     The FENV_ACCESS pragma provides a means to inform the compiler that the program might access the float-ing-point floating-point
     ing-point environment to test status flags or change the control modes.  The pragma shall occur either
     outside external declarations (i.e.  at file scope) or preceeding all explicit declarations and state-ments statements
     ments inside a compound statement.  If part of a program tests floating-point status flags or sets con-trol control
     trol modes, but was translated with FENV_ACCESS "off", the behavior of that program is undefined.

     The types fenv_t and fexcept_t represent the entire floating-point environment and the floating-point
     status flags, respectively.  These should be treated as opaque types, and be manipulated only via the
     functions defined in <fenv.h>.

     The macros FE_INVALID, FE_DIVBYZERO, FE_OVERFLOW, FE_UNDERFLOW, and FE_INEXACT expand to integer con-stant constant
     stant expressions such that bitwise ORs of any combination of the macros result in distinct values and
     bitwise ANDs of any combinations of the macros results in zero.  These macros are used in functions
     defined in <fenv.h> to represent floating-point exceptions and status flags.  Additionally, the macro
     FE_ALL_EXCEPT is defined to be the bitwise OR of all floating-point exception macros defined by the
     implementation.

     The macros FE_TONEAREST, FE_TOWARDZERO, FE_DOWNWARD, and FE_UPWARD expand to integer constant expres-sions expressions
     sions whose values are distinct nonnegative numbers.  These are intended for use with the fesetround()
     and fegetround() functions for control of the floating-point rounding modes.

     The macro FE_DFL_ENV represents the default floating-point environment--the one installed at program
     startup--and expands to a pointer to a const fenv_t object.  It can be used as an argument to <fenv.h>
     functions that manage the floating-point environment.

     The feclearexcept(), feraiseexcept(), fetestexcept(), fegetexceptflag(), and fesetexceptflag() func-tions functions
     tions provide access to the floating-point status flags.

     The fegetround() and fesetround() functions provide control of rounding direction modes.

     The fegetenv(), feholdexcept(), fesetenv(), and feupdateenv() functions provide control of the entire
     floating-point environment as an abstract object.

     Further documentation for all of these functions is available in their respective manpages, and also in
     section 7.6 of the C standard.

SEE ALSO
     feclearexcept(3), fegetenv(3), fegetround(3), fegetexceptflag(3), feholdexcept(3), fesetenv(3),
     fesetround(3), feraiseexcept(3), fesetexceptflag(3), feraiseexcept(3), feupdateenv(3)

STANDARDS
     These functions conform to ISO/IEC 9899:TC3.

OS X                              May 9, 2011                             OS X