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Name

log1p, log1pf, log1pl - logarithm of 1 plus argument

Synopsis


#include <math.h>
double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);
Link with -lm.

Feature Test Macro Requirements for glibc (see feature_test_macros(7) ):

log1p():

_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

log1pf(), log1pl():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

Description

log1p(x) returns a value equivalent to 
    log (1 + x)
It is computed in a way that is accurate even if the value of x is near zero.

Return Value

On success, these functions return the natural logarithm of (1 + x).

If x is a NaN, a NaN is returned.

If x is positive infinity, positive infinity is returned.

If x is -1, a pole error occurs, and the functions return -HUGE_VAL, -HUGE_VALF, or -HUGE_VALL, respectively.

If x is less than -1 (including negative infinity), a domain error occurs, and a NaN (not a number) is returned.

Errors

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 less than -1
An invalid floating-point exception (FE_INVALID) is raised.
Pole error: x is -1
A divide-by-zero floating-point exception (FE_DIVBYZERO) is raised.

These functions do not set errno.

Attributes

Multithreading (see pthreads(7) )

The log1p(), log1pf(), and log1pl() functions are thread-safe.

Conforming to

C99, POSIX.1-2001.

See Also

exp(3) , expm1(3) , log(3)

Colophon

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