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c99 [ flag... ] file... -lm [ library... ] #include <math.h> double log(double x);

float logf(float x);

long double logl(long double x);

These functions compute the natural logarithm of their argument x, log(e)(x).

Upon successful completion, log() returns the natural logarithm of x.

If x is \(+-0, a pole error occurs and log(), logf(), and logl() return -HUGE_VAL, -HUGE_VALF, and -HUGE_VALL, respectively.

For finite values of x that are less than 0, or if x is -Inf, a domain error occurs and a NaN is returned.

If x is NaN, a NaN is returned.

If x is 1, +0 is returned.

If x is +Inf, x is returned.

For exceptional cases, matherr(3M) tabulates the values to be returned by log() as specified by SVID3 and XPG3.

These functions will fail if:

Domain Error

The finite value of x is negative, or x is -Inf. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the invalid floating-point exception is raised. The log() function sets errno to EDOM if the value of x is negative.

Pole Error

The value of x is 0. If the integer expression (math_errhandling & MATH_ERREXCEPT) is non-zero, the divide-by-zero floating-point exception is raised.

An application wanting to check for exceptions should call feclearexcept(FE_ALL_EXCEPT) before calling these functions. On return, if fetestexcept(FE_INVALID | FE_DIVBYZERO | FE_OVERFLOW | FE_UNDERFLOW) is non-zero, an exception has been raised. An application should either examine the return value or check the floating point exception flags to detect exceptions.

An application can also set errno to 0 before calling log(). On return, if errno is non-zero, an error has occurred. The logf() and logl() functions do not set errno.

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE ATTRIBUTE VALUE |

Interface Stability Standard |

MT-Level MT-Safe |

exp(3M), feclearexcept(3M), fetestexcept(3M), isnan(3M), log10(3M), log1p(3M), math.h(3HEAD), matherr(3M), attributes(5), standards(5)