log1p(3) Library Functions Manual log1p(3)

# NAME¶

log1p, log1pf, log1pl - logarithm of 1 plus argument

# LIBRARY¶

Math library (libm, -lm)

# SYNOPSIS¶

`#include <math.h>`
```double log1p(double x);
float log1pf(float x);
long double log1pl(long double x);```

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

```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:

errno is set to EDOM (but see BUGS). An invalid floating-point exception (FE_INVALID) is raised.
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 log1p (), log1pf (), log1pl () 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.