- compute natural logarithm
c99 [ flag... ] file... -lm [ library... ] #include <math.h> double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
These functions compute loge(1.0 + x).
Upon successful completion, these functions return the natural logarithm of 1.0 + x.
If x is -1, a pole error occurs and log1p(), log1pf(), and log1pl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.
For finite values of x that are less than -1, 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 ±0 or +Inf, x is returned.
For exceptional cases, matherr(3M) tabulates the values to be returned by log1p() as specified by SVID3 and XPG3.
These functions will fail if:
The finite value of x is less than -1, or x is -Inf.
If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised.
The log1p() function sets errno to EDOM if the value of x is less than -1.
The value of x is -1.
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 log1p(). On return, if errno is non-zero, an error has occurred. The log1pf() and log1pl() functions do not set errno.
See attributes(5) for descriptions of the following attributes: