LOG(3) | Library Functions Manual | LOG(3)

# NAME

`log`, `logf`, `log10`, `log10f`, `log1p`, `log1pf`, `log2`, `log2f` —
logarithm functions

# LIBRARY

Math Library (libm, -lm)

# SYNOPSIS

`#include <math.h>`

double
`log`(double x);

float
`logf`(float x);

double
`log10`(double x);

float
`log10f`(float x);

double
`log1p`(double x);

float
`log1pf`(float x);

double
`log2`(double x);

float
`log2f`(float x);

# DESCRIPTION

The following functions compute logarithms:

- The `log`() and `logf`() functions return the natural logarithm.
- The `log10`() and `log10f`() functions return the base 10 logarithm.
- The `log1p`() and `log1pf`() functions return the natural logarithm of (1.0 + x) accurately even for very small values of x.
- The `log2`() and `log2f`() functions return the base 2 logarithm.

# RETURN VALUES

Upon successful completion, the functions return the logarithm of x as described above. Otherwise the following may occur:

1. If x is NaN, all functions return NaN.
2. If x is positive infinity, all functions return x. If x is negative infinity, all functions return NaN.
3. If x is +0.0 or -0.0, the `log`(), `log10`(), and `log2`() families return either `-HUGE_VAL`, `-HUGE_VALF`, or `-HUGE_VALL`, whereas the `log1p`() family returns x.
4. If x is +1.0, the `log`(), `log10`(), and `log2`() families return +0.0. If x is -1.0, the `log1p`() family returns `-HUGE_VAL`, `-HUGE_VALF`, or `-HUGE_VALL`.

In addition, on a VAX, errno is set to `EDOM` and the reserved operand is returned by `log`() unless x > 0, by `log1p`() unless x > -1.

# SEE ALSO

exp(3), ilogb(3), math(3)

# STANDARDS

The described functions conform to ISO/IEC 9899:1999 (“ISO C99”).

# HISTORY

A `log`() function appeared in Version 1 AT&T UNIX.

September 2, 2019 | NetBSD 9.3