1 .\" Copyright (c) 2008, Linux Foundation, written by Michael Kerrisk
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24 .TH MATH_ERROR 7 2008-08-11 "Linux" "Linux Programmer's Manual"
26 math_error \- detecting errors from mathematical functions
35 most library functions indicate this fact by returning a special value
37 Because they typically return a floating-point number,
38 the mathematical functions declared in
40 indicate an error using other mechanisms.
41 There are two error-reporting mechanisms:
44 the newer one uses the floating-point exception mechanism (the use of
52 A portable program that needs to check for an error from a mathematical
55 to zero, and make the following call
59 feclearexcept(FE_ALL_EXCEPT);
63 before calling a mathematical function.
65 Upon return from the mathematical function, if
67 is nonzero, or the following call (see
73 fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW |
80 .\" FE_INVALID = 0x01,
81 .\" __FE_DENORM = 0x02,
82 .\" FE_DIVBYZERO = 0x04,
83 .\" FE_OVERFLOW = 0x08,
84 .\" FE_UNDERFLOW = 0x10,
87 then an error occurred in the mathematical function.
89 The error conditions that can occur for mathematical functions
94 occurs when a mathematical function is supplied with an argument whose
95 value falls outside the domain for which the function
96 is defined (e.g., giving a negative argument to
98 When a domain error occurs,
99 math functions commonly return a NaN
100 (though some functions return a different value in this case);
106 floating-point exception is raised.
110 occurs when the mathematical result of a function is an exact infinity
111 (e.g., the logarithm of 0 is negative infinity).
112 When a pole error occurs,
113 the function returns the (signed) value
118 depending on whether the function result type is
123 The sign of the result is that which is mathematically correct for
128 and a "divide-by-zero"
130 floating-point exception is raised.
134 occurs when the magnitude of the function result means that it
135 cannot be represented in the result type of the function.
136 The return value of the function depends on whether the range error
137 was an overflow or an underflow.
141 if the result is finite,
142 but is too large to represented in the result type.
143 When an overflow occurs,
144 the function returns the value
149 depending on whether the function result type is
159 floating-point exception is raised.
163 if the result is too small to be represented in the result type.
164 If an underflow occurs,
165 a mathematical function typically returns 0.0
166 (C99 says a function shall return "an implementation-defined value
167 whose magnitude is no greater than the smallest normalized
168 positive number in the specified type").
174 floating-point exception may be raised.
176 Some functions deliver a range error if the supplied argument value,
177 or the correct function result, would be
179 A subnormal value is one that is nonzero,
180 but with a magnitude that is so small that
181 it can't be presented in normalized form
182 (i.e., with a 1 in the most significant bit of the significand).
183 The representation of a subnormal number will contain one
184 or more leading zeros in the significand.
188 identifier specified by C99 and POSIX.1-2001 is not supported by glibc.
189 .\" See CONFORMANCE in the glibc 2.8 (and earlier) source.
190 This identifier is supposed to indicate which of the two
191 error-notification mechanisms
193 exceptions retrievable via
194 .BR fettestexcept (3))
196 The standards require that at least one be in use,
197 but permit both to be available.
198 The current (version 2.8) situation under glibc is messy.
199 Most (but not all) functions raise exceptions on errors.
204 but don't raise an exception.
205 A very few functions do neither.
206 See the individual manual pages for details.
208 To avoid the complexities of using
213 it is often advised that one should instead check for bad argument
214 values before each call.
215 .\" http://www.securecoding.cert.org/confluence/display/seccode/FLP32-C.+Prevent+or+detect+domain+and+range+errors+in+math+functions
216 For example, the following code ensures that
218 argument is not a NaN and is not zero (a pole error) or
219 less than zero (a domain error):
225 if (isnan(x) || islessequal(x, 0)) {
226 /* Deal with NaN / pole error / domain error */
233 The discussion on this page does not apply to the complex
234 mathematical functions (i.e., those declared by
236 which in general are not required to return errors by C99
242 option causes the executable to employ implementations of some
243 mathematical functions that are faster than the standard
244 implementations, but do not set
251 .IR "-fno-math-errno" .)
252 An error can still be tested for using
253 .BR fetestexcept (3).