LOG1P(3M) Mathematical Library Functions LOG1P(3M)

# NAME

log1p, log1pf, log1pl - compute natural logarithm

# SYNOPSIS

```c99 [ flag... ] file... -lm [ library... ]
#include <math.h>
double log1p(double x);```

`float log1pf(float x);`

`long double log1pl(long double x);`

# DESCRIPTION

These functions compute log(e)(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.

# ERRORS

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.

# USAGE

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.

# ATTRIBUTES

See attributes(7) for descriptions of the following attributes:

 ATTRIBUTE TYPE ATTRIBUTE VALUE Interface Stability Standard MT-Level MT-Safe