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c99 [ flag... ] file... -lm [ library... ] #include <math.h> double log(double x);
float logf(float x);
long double logl(long double x);
These functions compute the natural logarithm of their argument x, log(e)(x).
Upon successful completion, log() returns the natural logarithm of x.
If x is \(+-0, a pole error occurs and log(), logf(), and logl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
For finite values of x that are less than 0, or if x is -Inf, a domain error occurs and a NaN is returned.
If x is NaN, a NaN is returned.
If x is 1, +0 is returned.
If x is +Inf, x is returned.
For exceptional cases, matherr(3M) tabulates the values to be returned by log() as specified by SVID3 and XPG3.
These functions will fail if:
Domain Error
The finite value of x is negative, or x is -Inf. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised. The log() function sets errno to EDOM if the value of x is negative.
Pole Error
The value of x is 0. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised.
An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.
An application can also set errno to 0 before calling log(). On return, if errno is non-zero, an error has occurred. The logf() and logl() functions do not set errno.
See attributes(7) for descriptions of the following attributes:
| ATTRIBUTE TYPE ATTRIBUTE VALUE |
| Interface Stability Standard |
| MT-Level MT-Safe |
math.h (3HEAD), exp (3M), feclearexcept (3M), fetestexcept (3M), isnan (3M), log10 (3M), log1p (3M), matherr (3M), attributes (7), standards (7)