1fenv.h(0P) POSIX Programmer's Manual fenv.h(0P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 fenv.h — floating-point environment
14
16 #include <fenv.h>
17
19 The functionality described on this reference page is aligned with the
20 ISO C standard. Any conflict between the requirements described here
21 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
22 defers to the ISO C standard.
23 The <fenv.h> header shall define the following data types through type‐
25 def:
26 fenv_t Represents the entire floating-point environment. The float‐
28 ing-point environment refers collectively to any floating-
29 point status flags and control modes supported by the imple‐
30 mentation.
31 fexcept_t Represents the floating-point status flags collectively,
33 including any status the implementation associates with the
34 flags. A floating-point status flag is a system variable
35 whose value is set (but never cleared) when a floating-point
36 exception is raised, which occurs as a side-effect of excep‐
37 tional floating-point arithmetic to provide auxiliary infor‐
38 mation. A floating-point control mode is a system variable
39 whose value may be set by the user to affect the subsequent
40 behavior of floating-point arithmetic.
41 The <fenv.h> header shall define each of the following macros if and
43 only if the implementation supports the floating-point exception by
44 means of the floating-point functions feclearexcept(), fegetexcept‐
45 flag(), feraiseexcept(), fesetexceptflag(), and fetestexcept(). The
46 defined macros shall expand to integer constant expressions with values
47 that are bitwise-distinct.
48 FE_DIVBYZERO FE_INEXACT FE_INVALID FE_OVERFLOW FE_UNDERFLOW
50 If the implementation supports the IEC 60559 Floating-Point option, all
52 five macros shall be defined. Additional implementation-defined float‐
53 ing-point exceptions with macros beginning with FE_ and an uppercase
54 letter may also be specified by the implementation.
55 The <fenv.h> header shall define the macro FE_ALL_EXCEPT as the bit‐
57 wise-inclusive OR of all floating-point exception macros defined by the
58 implementation, if any. If no such macros are defined, then the macro
59 FE_ALL_EXCEPT shall be defined as zero.
60 The <fenv.h> header shall define each of the following macros if and
62 only if the implementation supports getting and setting the represented
63 rounding direction by means of the fegetround() and fesetround() func‐
64 tions. The defined macros shall expand to integer constant expressions
65 whose values are distinct non-negative values.
66 FE_DOWNWARD FE_TONEAREST FE_TOWARDZERO FE_UPWARD
68 If the implementation supports the IEC 60559 Floating-Point option, all
70 four macros shall be defined. Additional implementation-defined round‐
71 ing directions with macros beginning with FE_ and an uppercase letter
72 may also be specified by the implementation.
73 The <fenv.h> header shall define the following macro, which represents
75 the default floating-point environment (that is, the one installed at
76 program startup) and has type pointer to const-qualified fenv_t. It
77 can be used as an argument to the functions within the <fenv.h> header
78 that manage the floating-point environment.
79 FE_DFL_ENV
81 The following shall be declared as functions and may also be defined as
83 macros. Function prototypes shall be provided.
84 int feclearexcept(int);
86 int fegetenv(fenv_t *);
87 int fegetexceptflag(fexcept_t *, int);
88 int fegetround(void);
89 int feholdexcept(fenv_t *);
90 int feraiseexcept(int);
91 int fesetenv(const fenv_t *);
92 int fesetexceptflag(const fexcept_t *, int);
93 int fesetround(int);
94 int fetestexcept(int);
95 int feupdateenv(const fenv_t *);
96 The FENV_ACCESS pragma provides a means to inform the implementation
98 when an application might access the floating-point environment to test
99 floating-point status flags or run under non-default floating-point
100 control modes. The pragma shall occur either outside external declara‐
101 tions or preceding all explicit declarations and statements inside a
102 compound statement. When outside external declarations, the pragma
103 takes effect from its occurrence until another FENV_ACCESS pragma is
104 encountered, or until the end of the translation unit. When inside a
105 compound statement, the pragma takes effect from its occurrence until
106 another FENV_ACCESS pragma is encountered (including within a nested
107 compound statement), or until the end of the compound statement; at the
108 end of a compound statement the state for the pragma is restored to its
109 condition just before the compound statement. If this pragma is used in
110 any other context, the behavior is undefined. If part of an application
111 tests floating-point status flags, sets floating-point control modes,
112 or runs under non-default mode settings, but was translated with the
113 state for the FENV_ACCESS pragma off, the behavior is undefined. The
114 default state (on or off) for the pragma is implementation-defined.
115 (When execution passes from a part of the application translated with
116 FENV_ACCESS off to a part translated with FENV_ACCESS on, the state of
117 the floating-point status flags is unspecified and the floating-point
118 control modes have their default settings.)
119 The following sections are informative.
121
123 This header is designed to support the floating-point exception status
124 flags and directed-rounding control modes required by the
125 IEC 60559:1989 standard, and other similar floating-point state infor‐
126 mation. Also it is designed to facilitate code portability among all
127 systems.
128 Certain application programming conventions support the intended model
130 of use for the floating-point environment:
131 * A function call does not alter its caller's floating-point control
133 modes, clear its caller's floating-point status flags, nor depend
134 on the state of its caller's floating-point status flags unless the
135 function is so documented.
136 * A function call is assumed to require default floating-point con‐
138 trol modes, unless its documentation promises otherwise.
139 * A function call is assumed to have the potential for raising float‐
141 ing-point exceptions, unless its documentation promises otherwise.
142 With these conventions, an application can safely assume default float‐
144 ing-point control modes (or be unaware of them). The responsibilities
145 associated with accessing the floating-point environment fall on the
146 application that does so explicitly.
147 Even though the rounding direction macros may expand to constants cor‐
149 responding to the values of FLT_ROUNDS, they are not required to do so.
150 For example:
152 #include <fenv.h>
154 void f(double x)
155 {
156 #pragma STDC FENV_ACCESS ON
157 void g(double);
158 void h(double);
159 /* ... */
160 g(x + 1);
161 h(x + 1);
162 /* ... */
163 }
164 If the function g() might depend on status flags set as a side-effect
166 of the first x+1, or if the second x+1 might depend on control modes
167 set as a side-effect of the call to function g(), then the application
168 shall contain an appropriately placed invocation as follows:
169 #pragma STDC FENV_ACCESS ON
171
173 The fexcept_t Type
174 fexcept_t does not have to be an integer type. Its values must be
175 obtained by a call to fegetexceptflag(), and cannot be created by logi‐
176 cal operations from the exception macros. An implementation might sim‐
177 ply implement fexcept_t as an int and use the representations reflected
178 by the exception macros, but is not required to; other representations
179 might contain extra information about the exceptions. fexcept_t might
180 be a struct with a member for each exception (that might hold the
181 address of the first or last floating-point instruction that caused
182 that exception). The ISO/IEC 9899:1999 standard makes no claims about
183 the internals of an fexcept_t, and so the user cannot inspect it.
184 Exception and Rounding Macros
186 Macros corresponding to unsupported modes and rounding directions are
187 not defined by the implementation and must not be defined by the appli‐
188 cation. An application might use #ifdef to test for this.
189
191 None.
192
194 The System Interfaces volume of POSIX.1‐2008, feclearexcept(),
195 fegetenv(), fegetexceptflag(), fegetround(), feholdexcept(), feraiseex‐
196 cept(), fetestexcept(), feupdateenv()
197
199 Portions of this text are reprinted and reproduced in electronic form
200 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
201 -- Portable Operating System Interface (POSIX), The Open Group Base
202 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
203 cal and Electronics Engineers, Inc and The Open Group. (This is
204 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
205 event of any discrepancy between this version and the original IEEE and
206 The Open Group Standard, the original IEEE and The Open Group Standard
207 is the referee document. The original Standard can be obtained online
208 at http://www.unix.org/online.html .
209 Any typographical or formatting errors that appear in this page are
211 most likely to have been introduced during the conversion of the source
212 files to man page format. To report such errors, see https://www.ker‐
213 nel.org/doc/man-pages/reporting_bugs.html .
214IEEE/The Open Group 2013 fenv.h(0P)