1FENV(3)                    Linux Programmer's Manual                   FENV(3)
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NAME

6       feclearexcept,  fegetexceptflag, feraiseexcept, fesetexceptflag, fetes‐
7       texcept, fegetenv, fegetround, feholdexcept, fesetround, fesetenv,  fe‐
8       updateenv,  feenableexcept,  fedisableexcept,  fegetexcept  - floating-
9       point rounding and exception handling
10

SYNOPSIS

12       #include <fenv.h>
13
14       int feclearexcept(int excepts);
15       int fegetexceptflag(fexcept_t *flagp, int excepts);
16       int feraiseexcept(int excepts);
17       int fesetexceptflag(const fexcept_t *flagp, int excepts);
18       int fetestexcept(int excepts);
19
20       int fegetround(void);
21       int fesetround(int rounding_mode);
22
23       int fegetenv(fenv_t *envp);
24       int feholdexcept(fenv_t *envp);
25       int fesetenv(const fenv_t *envp);
26       int feupdateenv(const fenv_t *envp);
27
28       Link with -lm.
29

DESCRIPTION

31       These eleven functions were defined in C99, and describe  the  handling
32       of  floating-point  rounding  and  exceptions  (overflow,  zero-divide,
33       etc.).
34
35   Exceptions
36       The divide-by-zero exception occurs when an operation on finite numbers
37       produces infinity as exact answer.
38
39       The  overflow exception occurs when a result has to be represented as a
40       floating-point number, but has (much) larger absolute  value  than  the
41       largest (finite) floating-point number that is representable.
42
43       The underflow exception occurs when a result has to be represented as a
44       floating-point number, but has smaller absolute value than the smallest
45       positive normalized floating-point number (and would lose much accuracy
46       when represented as a denormalized number).
47
48       The inexact exception occurs when the rounded result of an operation is
49       not  equal  to  the  infinite  precision result.  It may occur whenever
50       overflow or underflow occurs.
51
52       The invalid exception occurs when there is no well-defined  result  for
53       an operation, as for 0/0 or infinity - infinity or sqrt(-1).
54
55   Exception handling
56       Exceptions  are  represented  in  two  ways: as a single bit (exception
57       present/absent), and these bits correspond in  some  implementation-de‐
58       fined  way  with  bit  positions  in  an integer, and also as an opaque
59       structure that may contain more information about the  exception  (per‐
60       haps the code address where it occurred).
61
62       Each  of  the macros FE_DIVBYZERO, FE_INEXACT, FE_INVALID, FE_OVERFLOW,
63       FE_UNDERFLOW is defined when the implementation  supports  handling  of
64       the  corresponding  exception, and if so then defines the corresponding
65       bit(s), so that one can call exception handling functions, for example,
66       using  the integer argument FE_OVERFLOW|FE_UNDERFLOW.  Other exceptions
67       may be supported.  The macro FE_ALL_EXCEPT is the  bitwise  OR  of  all
68       bits corresponding to supported exceptions.
69
70       The  feclearexcept()  function  clears  the supported exceptions repre‐
71       sented by the bits in its argument.
72
73       The fegetexceptflag() function stores a representation of the state  of
74       the  exception  flags represented by the argument excepts in the opaque
75       object *flagp.
76
77       The feraiseexcept() function raises  the  supported  exceptions  repre‐
78       sented by the bits in excepts.
79
80       The  fesetexceptflag() function sets the complete status for the excep‐
81       tions represented by excepts to the value *flagp.  This value must have
82       been obtained by an earlier call of fegetexceptflag() with a last argu‐
83       ment that contained all bits in excepts.
84
85       The fetestexcept() function returns a word in which the  bits  are  set
86       that  were  set in the argument excepts and for which the corresponding
87       exception is currently set.
88
89   Rounding mode
90       The rounding mode determines how the result  of  floating-point  opera‐
91       tions  is  treated when the result cannot be exactly represented in the
92       significand.  Various rounding modes may be provided: round to  nearest
93       (the  default), round up (toward positive infinity), round down (toward
94       negative infinity), and round toward zero.
95
96       Each of the macros FE_TONEAREST,  FE_UPWARD,  FE_DOWNWARD,  and  FE_TO‐
97       WARDZERO  is  defined when the implementation supports getting and set‐
98       ting the corresponding rounding direction.
99
100       The fegetround() function returns the macro corresponding to  the  cur‐
101       rent rounding mode.
102
103       The  fesetround()  function  sets the rounding mode as specified by its
104       argument and returns zero when it was successful.
105
106       C99 and POSIX.1-2008 specify  an  identifier,  FLT_ROUNDS,  defined  in
107       <float.h>, which indicates the implementation-defined rounding behavior
108       for floating-point addition.  This identifier has one of the  following
109       values:
110
111       -1     The rounding mode is not determinable.
112
113       0      Rounding is toward 0.
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115       1      Rounding is toward nearest number.
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117       2      Rounding is toward positive infinity.
118
119       3      Rounding is toward negative infinity.
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121       Other values represent machine-dependent, nonstandard rounding modes.
122
123       The value of FLT_ROUNDS should reflect the current rounding mode as set
124       by fesetround() (but see BUGS).
125
126   Floating-point environment
127       The entire floating-point environment, including control modes and sta‐
128       tus  flags,  can  be handled as one opaque object, of type fenv_t.  The
129       default environment is denoted by FE_DFL_ENV (of type const  fenv_t *).
130       This is the environment setup at program start and it is defined by ISO
131       C to have round to nearest, all exceptions cleared and a nonstop  (con‐
132       tinue on exceptions) mode.
133
134       The fegetenv() function saves the current floating-point environment in
135       the object *envp.
136
137       The feholdexcept() function does the same, then  clears  all  exception
138       flags,  and sets a nonstop (continue on exceptions) mode, if available.
139       It returns zero when successful.
140
141       The fesetenv() function restores the  floating-point  environment  from
142       the  object *envp.  This object must be known to be valid, for example,
143       the result of a call  to  fegetenv()  or  feholdexcept()  or  equal  to
144       FE_DFL_ENV.  This call does not raise exceptions.
145
146       The feupdateenv() function installs the floating-point environment rep‐
147       resented by the object *envp, except that currently  raised  exceptions
148       are  not  cleared.   After calling this function, the raised exceptions
149       will be a bitwise OR of those previously set with those in  *envp.   As
150       before, the object *envp must be known to be valid.
151

RETURN VALUE

153       These  functions  return  zero  on  success and nonzero if an error oc‐
154       curred.
155

VERSIONS

157       These functions first appeared in glibc in version 2.1.
158

ATTRIBUTES

160       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
161       tributes(7).
162
163       ┌────────────────────────────────────────────┬───────────────┬─────────┐
164Interface                                   Attribute     Value   
165       ├────────────────────────────────────────────┼───────────────┼─────────┤
166feclearexcept(), fegetexceptflag(),         │ Thread safety │ MT-Safe │
167feraiseexcept(), fesetexceptflag(),         │               │         │
168fetestexcept(), fegetround(), fesetround(), │               │         │
169fegetenv(), feholdexcept(), fesetenv(),     │               │         │
170feupdateenv(), feenableexcept(),            │               │         │
171fedisableexcept(), fegetexcept()            │               │         │
172       └────────────────────────────────────────────┴───────────────┴─────────┘
173

CONFORMING TO

175       IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
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NOTES

178   Glibc notes
179       If possible, the GNU C Library defines a macro FE_NOMASK_ENV which rep‐
180       resents  an  environment  where every exception raised causes a trap to
181       occur.  You can test for this macro using #ifdef.  It is  defined  only
182       if  _GNU_SOURCE  is defined.  The C99 standard does not define a way to
183       set individual bits in the floating-point mask, for example, to trap on
184       specific  flags.  Since version 2.2, glibc supports the functions feen‐
185       ableexcept() and fedisableexcept()  to  set  individual  floating-point
186       traps, and fegetexcept() to query the state.
187
188       #define _GNU_SOURCE         /* See feature_test_macros(7) */
189       #include <fenv.h>
190
191       int feenableexcept(int excepts);
192       int fedisableexcept(int excepts);
193       int fegetexcept(void);
194
195       The  feenableexcept()  and fedisableexcept() functions enable (disable)
196       traps for each of the exceptions represented by excepts and return  the
197       previous  set  of enabled exceptions when successful, and -1 otherwise.
198       The fegetexcept() function returns the set of all currently enabled ex‐
199       ceptions.
200

BUGS

202       C99  specifies  that  the value of FLT_ROUNDS should reflect changes to
203       the current rounding mode, as set  by  fesetround().   Currently,  this
204       does not occur: FLT_ROUNDS always has the value 1.
205

SEE ALSO

207       math_error(7)
208

COLOPHON

210       This  page  is  part of release 5.12 of the Linux man-pages project.  A
211       description of the project, information about reporting bugs,  and  the
212       latest     version     of     this    page,    can    be    found    at
213       https://www.kernel.org/doc/man-pages/.
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217Linux                             2021-03-22                           FENV(3)
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