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
13 log2, log2f, log2l — compute base 2 logarithm functions
14
16 #include <math.h>
17 double log2(double x);
19 float log2f(float x);
20 long double log2l(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 base 2 logarithm of their argument x,
29 log2(x).
30 An application wishing to check for error situations should set errno
32 to zero and call feclearexcept(FE_ALL_EXCEPT) before calling these
33 functions. On return, if errno is non-zero or fetestexcept(FE_INVALID |
34 FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an error has
35 occurred.
36
38 Upon successful completion, these functions shall return the base 2
39 logarithm of x.
40 If x is ±0, a pole error shall occur and log2(), log2f(), and log2l()
42 shall return −HUGE_VAL, −HUGE_VALF, and −HUGE_VALL, respectively.
43 For finite values of x that are less than 0, or if x is −Inf, a domain
45 error shall occur, and either a NaN (if supported), or an implementa‐
46 tion-defined value shall be returned.
47 If x is NaN, a NaN shall be returned.
49 If x is 1, +0 shall be returned.
51 If x is +Inf, x shall be returned.
53
55 These functions shall fail if:
56 Domain Error
58 The finite value of x is less than zero, or x is −Inf.
59 If the integer expression (math_errhandling & MATH_ERRNO)
61 is non-zero, then errno shall be set to [EDOM]. If the
62 integer expression (math_errhandling & MATH_ERREXCEPT) is
63 non-zero, then the invalid floating-point exception shall
64 be raised.
65 Pole Error The value of x is zero.
67 If the integer expression (math_errhandling & MATH_ERRNO)
69 is non-zero, then errno shall be set to [ERANGE]. If the
70 integer expression (math_errhandling & MATH_ERREXCEPT) is
71 non-zero, then the divide-by-zero floating-point exception
72 shall be raised.
73 The following sections are informative.
75
77 None.
78
80 On error, the expressions (math_errhandling & MATH_ERRNO) and
81 (math_errhandling & MATH_ERREXCEPT) are independent of each other, but
82 at least one of them must be non-zero.
83
85 None.
86
88 None.
89
91 feclearexcept(), fetestexcept(), log()
92 The Base Definitions volume of POSIX.1‐2008, Section 4.19, Treatment of
94 Error Conditions for Mathematical Functions, <math.h>
95
97 Portions of this text are reprinted and reproduced in electronic form
98 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
99 -- Portable Operating System Interface (POSIX), The Open Group Base
100 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
101 cal and Electronics Engineers, Inc and The Open Group. (This is
102 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
103 event of any discrepancy between this version and the original IEEE and
104 The Open Group Standard, the original IEEE and The Open Group Standard
105 is the referee document. The original Standard can be obtained online
106 at http://www.unix.org/online.html .
107 Any typographical or formatting errors that appear in this page are
109 most likely to have been introduced during the conversion of the source
110 files to man page format. To report such errors, see https://www.ker‐
111 nel.org/doc/man-pages/reporting_bugs.html .
112IEEE/The Open Group 2013 LOG2(3P)