#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)):
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L || /* Since glibc 2.19: */ _DEFAULT_SOURCE || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
These functions return the natural logarithm of x.
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.
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.
For an explanation of the terms used in this section, see attributes(7).
|log(), logf(), logl()||Thread safety||MT-Safe|
C99, POSIX.1-2001, POSIX.1-2008.
The variant returning double also conforms to SVr4, 4.3BSD, C89.
In glibc 2.5 and earlier, taking the log() of a NaN produces a bogus invalid floating-point (FE_INVALID) exception.
cbrt(3), clog(3), log10(3), log1p(3), log2(3), sqrt(3)
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expm1(3), ilogb(3), log10(3), log1p(3), log2(3), logb(3), matherr(3), math_error(7).
The man pages logf(3) and logl(3) are aliases of log(3).