1LOG2(3P) POSIX Programmer's Manual LOG2(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
12 log2, log2f, log2l — compute base 2 logarithm functions
13
15 #include <math.h>
16 double log2(double x);
18 float log2f(float x);
19 long double log2l(long double x);
20
22 The functionality described on this reference page is aligned with the
23 ISO C standard. Any conflict between the requirements described here
24 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
25 defers to the ISO C standard.
26 These functions shall compute the base 2 logarithm of their argument x,
28 log2(x).
29 An application wishing to check for error situations should set errno
31 to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
32 functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
33 FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has
34 occurred.
35
37 Upon successful completion, these functions shall return the base 2
38 logarithm of x.
39 If x is ±0, a pole error shall occur and log2(), log2f(), and log2l()
41 shall return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
42 For finite values of x that are less than 0, or if x is -Inf, a domain
44 error shall occur, and either a NaN (if supported), or an implementa‐
45 tion-defined value shall be returned.
46 If x is NaN, a NaN shall be returned.
48 If x is 1, +0 shall be returned.
50 If x is +Inf, x shall be returned.
52
54 These functions shall fail if:
55 Domain Error
57 The finite value of x is less than zero, or x is -Inf.
58 If the integer expression (math_errhandling & MATH_ERRNO)
60 is non-zero, then errno shall be set to [EDOM]. If the
61 integer expression (math_errhandling & MATH_ERREXCEPT) is
62 non-zero, then the invalid floating-point exception shall
63 be raised.
64 Pole Error The value of x is zero.
66 If the integer expression (math_errhandling & MATH_ERRNO)
68 is non-zero, then errno shall be set to [ERANGE]. If the
69 integer expression (math_errhandling & MATH_ERREXCEPT) is
70 non-zero, then the divide-by-zero floating-point exception
71 shall be raised.
72 The following sections are informative.
74
76 None.
77
79 On error, the expressions (math_errhandling & MATH_ERRNO) and
80 (math_errhandling & MATH_ERREXCEPT) are independent of each other, but
81 at least one of them must be non-zero.
82
84 None.
85
87 None.
88
90 feclearexcept(), fetestexcept(), log()
91 The Base Definitions volume of POSIX.1‐2017, Section 4.20, Treatment of
93 Error Conditions for Mathematical Functions, <math.h>
94
96 Portions of this text are reprinted and reproduced in electronic form
97 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
98 table Operating System Interface (POSIX), The Open Group Base Specifi‐
99 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
100 Electrical and Electronics Engineers, Inc and The Open Group. In the
101 event of any discrepancy between this version and the original IEEE and
102 The Open Group Standard, the original IEEE and The Open Group Standard
103 is the referee document. The original Standard can be obtained online
104 at http://www.opengroup.org/unix/online.html .
105 Any typographical or formatting errors that appear in this page are
107 most likely to have been introduced during the conversion of the source
108 files to man page format. To report such errors, see https://www.ker‐
109 nel.org/doc/man-pages/reporting_bugs.html .
110IEEE/The Open Group 2017 LOG2(3P)