1MATH_ERROR(7)              Linux Programmer's Manual             MATH_ERROR(7)


6       math_error - detecting errors from mathematical functions


9       #include <math.h>
10       #include <errno.h>
11       #include <fenv.h>


14       When  an  error  occurs,  most  library functions indicate this fact by
15       returning a special value (e.g., -1 or NULL).  Because  they  typically
16       return  a floating-point number, the mathematical functions declared in
17       <math.h> indicate an error  using  other  mechanisms.   There  are  two
18       error-reporting  mechanisms:  the  older  one sets errno; the newer one
19       uses the floating-point exception  mechanism  (the  use  of  feclearex‐
20       cept(3) and fetestexcept(3), as outlined below) described in fenv(3).
22       A portable program that needs to check for an error from a mathematical
23       function should set errno to zero, and make the following call
25           feclearexcept(FE_ALL_EXCEPT);
27       before calling a mathematical function.
29       Upon return from the mathematical function, if errno is nonzero, or the
30       following call (see fenv(3)) returns nonzero
32           fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
33                        FE_UNDERFLOW);
35       then an error occurred in the mathematical function.
37       The  error  conditions  that  can  occur for mathematical functions are
38       described below.
40   Domain error
41       A domain error occurs when a mathematical function is supplied with  an
42       argument whose value falls outside the domain for which the function is
43       defined (e.g., giving a negative argument to log(3)).   When  a  domain
44       error  occurs,  math functions commonly return a NaN (though some func‐
45       tions return a different value in this case); errno is set to EDOM, and
46       an "invalid" (FE_INVALID) floating-point exception is raised.
48   Pole error
49       A  pole  error  occurs when the mathematical result of a function is an
50       exact infinity (e.g., the logarithm of 0 is negative infinity).  When a
51       pole  error  occurs,  the function returns the (signed) value HUGE_VAL,
52       HUGE_VALF, or HUGE_VALL, depending on whether the function result  type
53       is double, float, or long double.  The sign of the result is that which
54       is mathematically correct for the function.  errno is  set  to  ERANGE,
55       and  a  "divide-by-zero"  (FE_DIVBYZERO)  floating-point  exception  is
56       raised.
58   Range error
59       A range error occurs when the magnitude of the  function  result  means
60       that  it cannot be represented in the result type of the function.  The
61       return value of the function depends on whether the range error was  an
62       overflow or an underflow.
64       A  floating  result overflows if the result is finite, but is too large
65       to represented in the result type.  When an overflow occurs, the  func‐
66       tion  returns the value HUGE_VAL, HUGE_VALF, or HUGE_VALL, depending on
67       whether the function result type is  double,  float,  or  long  double.
68       errno  is set to ERANGE, and an "overflow" (FE_OVERFLOW) floating-point
69       exception is raised.
71       A floating result underflows if the result is too small  to  be  repre‐
72       sented  in  the  result  type.   If an underflow occurs, a mathematical
73       function typically returns 0.0 (C99 says a function  shall  return  "an
74       implementation-defined  value  whose  magnitude  is no greater than the
75       smallest normalized positive number in the specified type").  errno may
76       be  set  to  ERANGE,  and  an  "overflow" (FE_UNDERFLOW) floating-point
77       exception may be raised.
79       Some functions deliver a range error if the supplied argument value, or
80       the  correct function result, would be subnormal.  A subnormal value is
81       one that is nonzero, but with a magnitude that  is  so  small  that  it
82       can't  be presented in normalized form (i.e., with a 1 in the most sig‐
83       nificant bit of the significand).  The representation  of  a  subnormal
84       number will contain one or more leading zeros in the significand.


87       The  math_errhandling  identifier  specified  by C99 and POSIX.1 is not
88       supported by glibc.  This identifier is supposed to indicate  which  of
89       the  two  error-notification  mechanisms (errno, exceptions retrievable
90       via fettestexcept(3)) is in use.  The standards require that  at  least
91       one  be  in use, but permit both to be available.  The current (version
92       2.8) situation under glibc is messy.   Most  (but  not  all)  functions
93       raise  exceptions on errors.  Some also set errno.  A few functions set
94       errno, but don't raise an exception.  A very few functions do  neither.
95       See the individual manual pages for details.
97       To  avoid the complexities of using errno and fetestexcept(3) for error
98       checking, it is often advised that one should  instead  check  for  bad
99       argument  values  before  each  call.   For example, the following code
100       ensures that log(3)'s argument is not a NaN and is  not  zero  (a  pole
101       error) or less than zero (a domain error):
103           double x, r;
105           if (isnan(x) || islessequal(x, 0)) {
106               /* Deal with NaN / pole error / domain error */
107           }
109           r = log(x);
111       The  discussion on this page does not apply to the complex mathematical
112       functions (i.e., those declared by <complex.h>), which in  general  are
113       not required to return errors by C99 and POSIX.1.
115       The  gcc(1)  -fno-math-errno  option  causes  the  executable to employ
116       implementations of some mathematical functions that are faster than the
117       standard  implementations,  but do not set errno on error.  (The gcc(1)
118       -ffast-math option also enables -fno-math-errno.)  An error  can  still
119       be tested for using fetestexcept(3).


122       gcc(1),  errno(3),  fenv(3),  fpclassify(3), INFINITY(3), isgreater(3),
123       matherr(3), nan(3)
125       info libc


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135Linux                             2017-09-15                     MATH_ERROR(7)