LOG(3)                     Linux Programmer's Manual                    LOG(3)



NAME
       log, logf, logl - natural logarithmic function

SYNOPSIS
       #include <math.h>

       double log(double x);
       float logf(float x);
       long double logl(long double x);

       Link with -lm.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       logf(), logl():
           _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       These functions return the natural logarithm of x.

RETURN VALUE
       On success, these functions return the natural logarithm of x.

       If x is a NaN, a NaN is returned.

       If x is 1, the result is +0.

       If x is positive infinity, positive infinity is returned.

       If  x  is  zero,  then  a  pole  error occurs, and the functions return
       -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

       If x is negative (including negative infinity),  then  a  domain  error
       occurs, and a NaN (not a number) is returned.

ERRORS
       See  math_error(7) for information on how to determine whether an error
       has occurred when calling these functions.

       The following errors can occur:

       Domain error: x is negative
              errno is set  to  EDOM.   An  invalid  floating-point  exception
              (FE_INVALID) is raised.

       Pole error: x is zero
              errno  is set to ERANGE.  A divide-by-zero floating-point excep-
              tion (FE_DIVBYZERO) is raised.

ATTRIBUTES
       For  an  explanation  of  the  terms  used   in   this   section,   see
       attributes(7).

       +----------------------+---------------+---------+
       |Interface             | Attribute     | Value   |
       +----------------------+---------------+---------+
       |log(), logf(), logl() | Thread safety | MT-Safe |
       +----------------------+---------------+---------+
CONFORMING TO
       C99, POSIX.1-2001, POSIX.1-2008.

       The variant returning double also conforms to SVr4, 4.3BSD, C89.

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

SEE ALSO
       cbrt(3), clog(3), log10(3), log1p(3), log2(3), sqrt(3)

COLOPHON
       This  page  is  part of release 4.16 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.



                                  2017-09-15                            LOG(3)