Name
log1p, log1pf, log1pl - logarithm of 1 plus argumentLibrary
Math library ( libm ", " -lm )Synopsis
#include <math.h> double log1p(double x );Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
float log1pf(float x );
long double log1pl(long double x );
log1p() _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| _XOPEN_SOURCE >= 500
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
log1pf() log1pl()
_ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L
|| /* Since glibc 2.19: */ _DEFAULT_SOURCE
|| /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
Description
These functions return a value equivalent to log (1 + x)
The result 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 errno is set to EDOM (but see BUGS). An invalid floating-point exception ( FE_INVALID ) is raised.
Pole error: x is -1 errno is set to ERANGE (but see BUGS). A divide-by-zero floating-point exception ( FE_DIVBYZERO ) is raised.
Attributes
For an explanation of the terms used in this section, see attributes(7).| Interface | Attribute | Value |
| T} | Thread safety | MT-Safe |
Standards
C11, POSIX.1-2008.History
C99, POSIX.1-2001.Bugs
Before glibc 2.22, the glibc implementation did not set errno to EDOM when a domain error occurred.Before glibc 2.22, the glibc implementation did not set errno to ERANGE when a range error occurred.