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(doublex);float log1pf(floatx);long double log1pl(long doublex);

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 |

log1p (), log1pf (), log1pl () | Thread safety | MT-Safe |

# STANDARDS

C99, POSIX.1-2001, POSIX.1-2008.

# 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.

# SEE ALSO

2023-02-05 | Linux man-pages 6.03 |