## table of contents

LOG(3) | Linux Programmer's Manual | LOG(3) |

# NAME¶

log, logf, logl - natural logarithmic function

# SYNOPSIS¶

#include <math.h>

double log(doublex);float logf(floatx);long double logl(long doublex);

Link with *-lm*.

**logf**(), **logl**():

|| /* Since glibc 2.19: */ _DEFAULT_SOURCE

|| /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE

# DESCRIPTION¶

These functions return the natural logarithm of *x*.

# RETURN VALUE¶

On success, these functions return the natural logarithm of
*x*.

If *x* is a NaN, a NaN is returned.

If *x* is 1, the result is +0.

If *x* is positive infinity, positive infinity is
returned.

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

If *x* is negative (including negative infinity), then 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 negative *errno*is set to**EDOM**. An invalid floating-point exception (**FE_INVALID**) is raised.- Pole error:
*x*is zero *errno*is set to**ERANGE**. 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 |

log (), logf (), logl () | Thread safety | MT-Safe |

# CONFORMING TO¶

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

The variant returning *double* also conforms to SVr4, 4.3BSD,
C89.

# BUGS¶

In glibc 2.5 and earlier, taking the **log**() of a NaN
produces a bogus invalid floating-point (**FE_INVALID**) exception.

# SEE ALSO¶

# COLOPHON¶

This page is part of release 5.10 of the Linux *man-pages*
project. A description of the project, information about reporting bugs, and
the latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.

2017-09-15 |