1ILOGB(3P) POSIX Programmer's Manual ILOGB(3P)
2
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 delim $$
12
14 ilogb, ilogbf, ilogbl — return an unbiased exponent
15
17 #include <math.h>
18 int ilogb(double x);
20 int ilogbf(float x);
21 int ilogbl(long double x);
22
24 The functionality described on this reference page is aligned with the
25 ISO C standard. Any conflict between the requirements described here
26 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
27 defers to the ISO C standard.
28 These functions shall return the exponent part of their argument x.
30 Formally, the return value is the integral part of $log sub{r}|x|$ as a
31 signed integral value, for non-zero x, where r is the radix of the
32 machine's floating-point arithmetic, which is the value of FLT_RADIX
33 defined in <float.h>.
34 An application wishing to check for error situations should set errno
36 to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
37 functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
38 FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has
39 occurred.
40
42 Upon successful completion, these functions shall return the exponent
43 part of x as a signed integer value. They are equivalent to calling the
44 corresponding logb() function and casting the returned value to type
45 int.
46 If x is 0, the value FP_ILOGB0 shall be returned. On XSI-conformant
48 systems, a domain error shall occur;
49 otherwise, a domain error may occur.
50 If x is ±Inf, the value {INT_MAX} shall be returned. On XSI-conformant
52 systems, a domain error shall occur;
53 otherwise, a domain error may occur.
54 If x is a NaN, the value FP_ILOGBNAN shall be returned. On XSI-confor‐
56 mant systems, a domain error shall occur;
57 otherwise, a domain error may occur.
58 If the correct value is greater than {INT_MAX}, a domain error shall
60 occur and an unspecified value shall be returned. On XSI-conformant
61 systems, a domain error shall occur and {INT_MAX} shall be returned.
62 If the correct value is less than {INT_MIN}, a domain error shall occur
64 and an unspecified value shall be returned. On XSI-conformant systems,
65 a domain error shall occur and {INT_MIN} shall be returned.
66
68 These functions shall fail if:
69 Domain Error
71 The correct value is not representable as an integer.
72 The x argument is zero, NaN, or ±Inf.
74 If the integer expression (math_errhandling & MATH_ERRNO)
76 is non-zero, then errno shall be set to [EDOM]. If the
77 integer expression (math_errhandling & MATH_ERREXCEPT) is
78 non-zero, then the invalid floating-point exception shall
79 be raised.
80 These functions may fail if:
82 Domain Error
84 The x argument is zero, NaN, or ±Inf.
85 If the integer expression (math_errhandling & MATH_ERRNO)
87 is non-zero, then errno shall be set to [EDOM]. If the
88 integer expression (math_errhandling & MATH_ERREXCEPT) is
89 non-zero, then the invalid floating-point exception shall
90 be raised.
91 The following sections are informative.
93
95 None.
96
98 On error, the expressions (math_errhandling & MATH_ERRNO) and
99 (math_errhandling & MATH_ERREXCEPT) are independent of each other, but
100 at least one of them must be non-zero.
101
103 The errors come from taking the expected floating-point value and con‐
104 verting it to int, which is an invalid operation in IEEE Std 754‐1985
105 (since overflow, infinity, and NaN are not representable in a type
106 int), so should be a domain error.
107 There are no known implementations that overflow. For overflow to hap‐
109 pen, {INT_MAX} must be less than LDBL_MAX_EXP*log2(FLT_RADIX) or
110 {INT_MIN} must be greater than LDBL_MIN_EXP*log2(FLT_RADIX) if subnor‐
111 mals are not supported, or {INT_MIN} must be greater than
112 (LDBL_MIN_EXP-LDBL_MANT_DIG)*log2(FLT_RADIX) if subnormals are sup‐
113 ported.
114
116 None.
117
119 feclearexcept(), fetestexcept(), logb(), scalbln()
120 The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of
122 Error Conditions for Mathematical Functions, <float.h>, <math.h>
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125 Portions of this text are reprinted and reproduced in electronic form
126 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
127 -- Portable Operating System Interface (POSIX), The Open Group Base
128 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
129 cal and Electronics Engineers, Inc and The Open Group. (This is
130 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
131 event of any discrepancy between this version and the original IEEE and
132 The Open Group Standard, the original IEEE and The Open Group Standard
133 is the referee document. The original Standard can be obtained online
134 at http://www.unix.org/online.html .
135 Any typographical or formatting errors that appear in this page are
137 most likely to have been introduced during the conversion of the source
138 files to man page format. To report such errors, see https://www.ker‐
139 nel.org/doc/man-pages/reporting_bugs.html .
140IEEE/The Open Group 2013 ILOGB(3P)