log(3) - f38

1log(3)                     Library Functions Manual                     log(3)
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NAME

6       log, logf, logl - natural logarithmic function
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LIBRARY

9       Math library (libm, -lm)
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SYNOPSIS

12       #include <math.h>
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14       double log(double x);
15       float logf(float x);
16       long double logl(long double x);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       logf(), logl():
21           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
22               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
23               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

26       These functions return the natural logarithm of x.
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RETURN VALUE

29       On success, these functions return the natural logarithm of x.
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31       If x is a NaN, a NaN is returned.
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33       If x is 1, the result is +0.
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35       If x is positive infinity, positive infinity is returned.
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37       If  x  is  zero,  then  a  pole  error occurs, and the functions return
38       -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.
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40       If x is negative (including negative infinity), then a domain error oc‐
41       curs, and a NaN (not a number) is returned.
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ERRORS

44       See  math_error(7) for information on how to determine whether an error
45       has occurred when calling these functions.
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47       The following errors can occur:
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49       Domain error: x is negative
50              errno is set  to  EDOM.   An  invalid  floating-point  exception
51              (FE_INVALID) is raised.
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53       Pole error: x is zero
54              errno  is set to ERANGE.  A divide-by-zero floating-point excep‐
55              tion (FE_DIVBYZERO) is raised.
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ATTRIBUTES

58       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
59       tributes(7).
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61       ┌────────────────────────────────────────────┬───────────────┬─────────┐
62       │Interface                                   │ Attribute     │ Value   │
63       ├────────────────────────────────────────────┼───────────────┼─────────┤
64       │log(), logf(), logl()                       │ Thread safety │ MT-Safe │
65       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

68       C11, POSIX.1-2008.
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HISTORY

71       C99, POSIX.1-2001.
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73       The variant returning double also conforms to SVr4, 4.3BSD, C89.
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BUGS

76       In  glibc  2.5  and earlier, taking the log() of a NaN produces a bogus
77       invalid floating-point (FE_INVALID) exception.
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