1LOGB(3P) POSIX Programmer's Manual LOGB(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
13 logb, logbf, logbl — radix-independent exponent
14
16 #include <math.h>
17 double logb(double x);
19 float logbf(float x);
20 long double logbl(long double x);
21
23 The functionality described on this reference page is aligned with the
24 ISO C standard. Any conflict between the requirements described here
25 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
26 defers to the ISO C standard.
27 These functions shall compute the exponent of x, which is the integral
29 part of logr |x|, as a signed floating-point value, for non-zero x,
30 where r is the radix of the machine's floating-point arithmetic, which
31 is the value of FLT_RADIX defined in the <float.h> header.
32 If x is subnormal it is treated as though it were normalized; thus for
34 finite positive x:
35 1 <= x * FLT_RADIX−logb(x) < FLT_RADIX
37 An application wishing to check for error situations should set errno
39 to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
40 functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
41 FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has
42 occurred.
43
45 Upon successful completion, these functions shall return the exponent
46 of x.
47 If x is ±0, logb(), logbf(), and logbl() shall return −HUGE_VAL,
49 −HUGE_VALF, and −HUGE_VALL, respectively.
50 On systems that support the IEC 60559 Floating-Point option, a pole
52 error shall occur;
53 otherwise, a pole error may occur.
54 If x is NaN, a NaN shall be returned.
56 If x is ±Inf, +Inf shall be returned.
58
60 These functions shall fail if:
61 Pole Error The value of x is ±0.
63 If the integer expression (math_errhandling & MATH_ERRNO)
65 is non-zero, then errno shall be set to [ERANGE]. If the
66 integer expression (math_errhandling & MATH_ERREXCEPT) is
67 non-zero, then the divide-by-zero floating-point exception
68 shall be raised.
69 These functions may fail if:
71 Pole Error The value of x is 0.
73 If the integer expression (math_errhandling & MATH_ERRNO)
75 is non-zero, then errno shall be set to [ERANGE]. If the
76 integer expression (math_errhandling & MATH_ERREXCEPT) is
77 non-zero, then the divide-by-zero floating-point exception
78 shall be raised.
79 The following sections are informative.
81
83 None.
84
86 On error, the expressions (math_errhandling & MATH_ERRNO) and
87 (math_errhandling & MATH_ERREXCEPT) are independent of each other, but
88 at least one of them must be non-zero.
89
91 None.
92
94 None.
95
97 feclearexcept(), fetestexcept(), ilogb(), scalbln()
98 The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of
100 Error Conditions for Mathematical Functions, <float.h>, <math.h>
101
103 Portions of this text are reprinted and reproduced in electronic form
104 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
105 -- Portable Operating System Interface (POSIX), The Open Group Base
106 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
107 cal and Electronics Engineers, Inc and The Open Group. (This is
108 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
109 event of any discrepancy between this version and the original IEEE and
110 The Open Group Standard, the original IEEE and The Open Group Standard
111 is the referee document. The original Standard can be obtained online
112 at http://www.unix.org/online.html .
113 Any typographical or formatting errors that appear in this page are
115 most likely to have been introduced during the conversion of the source
116 files to man page format. To report such errors, see https://www.ker‐
117 nel.org/doc/man-pages/reporting_bugs.html .
118IEEE/The Open Group 2013 LOGB(3P)