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man pages section 3: Extended Library Functions, Volume 2

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Updated: July 2017



log1p, log1pf, log1pl - 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).

Return Values

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:

Domain Error

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.

Pole Error

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:

Interface Stability

See Also

feclearexcept(3M), fetestexcept(3M), log(3M), math.h(3HEAD), matherr(3M), attributes(5), standards(5)