log1p(), log1pf(), log1pl() — Natural log of x+1

Standards

Standards / Extensions	C or C++	Dependencies
XPG4.2 C99 Single UNIX Specification, Version 3 C++ TR1 C99	both

Format

#define _XOPEN_SOURCE_EXTENDED 1
#include <math.h>

double log1p(double x);

C99:

#define _ISOC99_SOURCE
#include <math.h>

float log1pf(float x);
long double log1pl(long double x);

C++ TR1 C99:

#define _TR1_C99
#include <math.h>

float log1p(float x); 
long double log1p(long double x);

General description

Computes

The value of x must be greater than -1.0.

Note: The following table shows the viable formats for these functions. See IEEE binary floating-point for more information about IEEE Binary Floating-Point.

Function	Hex	IEEE
log1p	X	X
log1pf	X	X
log1pl	X	X

Returned value

If successful, log1p() returns the value of the above function of x.

log1p() will fail under the following conditions:

If x is less than -1.0, log1p() will return -HUGE_VAL and set errno to EDOM.
If x is equal to -1.0, log1p() will return -HUGE_VAL and set errno to ERANGE.

Special behavior for IEEE: If successful, log1p() returns the