log(3) — Linux manual page


log(3)                  Library Functions Manual                  log(3)

NAME         top

       log, logf, logl - natural logarithmic function

LIBRARY         top

       Math library (libm, -lm)

SYNOPSIS         top

       #include <math.h>

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

   Feature Test Macro Requirements for glibc (see

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

DESCRIPTION         top

       These functions return the natural logarithm of x.

RETURN VALUE         top

       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         top

       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
              exception (FE_DIVBYZERO) is raised.

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       │ Interface                           Attribute     Value   │
       │ log(), logf(), logl()               │ Thread safety │ MT-Safe │

STANDARDS         top

       C11, POSIX.1-2008.

HISTORY         top

       C99, POSIX.1-2001.

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

BUGS         top

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

SEE ALSO         top

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

Linux man-pages (unreleased)     (date)                           log(3)

Pages that refer to this page: expm1(3)ilogb(3)log10(3)log1p(3)log2(3)logb(3)matherr(3)math_error(7)