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26 .TH MATH_ERROR 7 2021-03-22 "Linux" "Linux Programmer's Manual"
28 math_error \- detecting errors from mathematical functions
37 most library functions indicate this fact by returning a special value
39 Because they typically return a floating-point number,
40 the mathematical functions declared in
42 indicate an error using other mechanisms.
43 There are two error-reporting mechanisms:
46 the newer one uses the floating-point exception mechanism (the use of
54 A portable program that needs to check for an error from a mathematical
57 to zero, and make the following call
61 feclearexcept(FE_ALL_EXCEPT);
65 before calling a mathematical function.
67 Upon return from the mathematical function, if
69 is nonzero, or the following call (see
75 fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
82 .\" FE_INVALID = 0x01,
83 .\" __FE_DENORM = 0x02,
84 .\" FE_DIVBYZERO = 0x04,
85 .\" FE_OVERFLOW = 0x08,
86 .\" FE_UNDERFLOW = 0x10,
89 then an error occurred in the mathematical function.
91 The error conditions that can occur for mathematical functions
96 occurs when a mathematical function is supplied with an argument whose
97 value falls outside the domain for which the function
98 is defined (e.g., giving a negative argument to
100 When a domain error occurs,
101 math functions commonly return a NaN
102 (though some functions return a different value in this case);
108 floating-point exception is raised.
112 occurs when the mathematical result of a function is an exact infinity
113 (e.g., the logarithm of 0 is negative infinity).
114 When a pole error occurs,
115 the function returns the (signed) value
120 depending on whether the function result type is
125 The sign of the result is that which is mathematically correct for
130 and a "divide-by-zero"
132 floating-point exception is raised.
136 occurs when the magnitude of the function result means that it
137 cannot be represented in the result type of the function.
138 The return value of the function depends on whether the range error
139 was an overflow or an underflow.
143 if the result is finite,
144 but is too large to represented in the result type.
145 When an overflow occurs,
146 the function returns the value
151 depending on whether the function result type is
161 floating-point exception is raised.
165 if the result is too small to be represented in the result type.
166 If an underflow occurs,
167 a mathematical function typically returns 0.0
168 (C99 says a function shall return "an implementation-defined value
169 whose magnitude is no greater than the smallest normalized
170 positive number in the specified type").
176 floating-point exception may be raised.
178 Some functions deliver a range error if the supplied argument value,
179 or the correct function result, would be
181 A subnormal value is one that is nonzero,
182 but with a magnitude that is so small that
183 it can't be presented in normalized form
184 (i.e., with a 1 in the most significant bit of the significand).
185 The representation of a subnormal number will contain one
186 or more leading zeros in the significand.
190 identifier specified by C99 and POSIX.1 is not supported by glibc.
191 .\" See CONFORMANCE in the glibc 2.8 (and earlier) source.
192 This identifier is supposed to indicate which of the two
193 error-notification mechanisms
195 exceptions retrievable via
196 .BR fetestexcept (3))
198 The standards require that at least one be in use,
199 but permit both to be available.
200 The current (version 2.8) situation under glibc is messy.
201 Most (but not all) functions raise exceptions on errors.
206 but don't raise an exception.
207 A very few functions do neither.
208 See the individual manual pages for details.
210 To avoid the complexities of using
215 it is often advised that one should instead check for bad argument
216 values before each call.
217 .\" http://www.securecoding.cert.org/confluence/display/seccode/FLP32-C.+Prevent+or+detect+domain+and+range+errors+in+math+functions
218 For example, the following code ensures that
220 argument is not a NaN and is not zero (a pole error) or
221 less than zero (a domain error):
227 if (isnan(x) || islessequal(x, 0)) {
228 /* Deal with NaN / pole error / domain error */
235 The discussion on this page does not apply to the complex
236 mathematical functions (i.e., those declared by
238 which in general are not required to return errors by C99
243 .I "\-fno\-math\-errno"
244 option causes the executable to employ implementations of some
245 mathematical functions that are faster than the standard
246 implementations, but do not set
253 .IR "\-fno\-math\-errno" .)
254 An error can still be tested for using
255 .BR fetestexcept (3).