log(), logf(), logl()

Synopsis:

#include <math.h>

double log( double x );

float logf( float x );

long double logl( long double x );

Arguments:

x: The number that you want to compute the natural log of.

Library:

libm

Use the -l m option to qcc to link against this library.

Description:

The log(), logf(), and logl() functions compute the natural logarithm (base e) of x:

log_e x

To check for error situations, use feclearexcept() and fetestexcept(). For example:

Call feclearexcept(FE_ALL_EXCEPT) before calling log(), logf(), or logl().
On return, if fetestexcept(FE_ALL_EXCEPT) is nonzero, then an error has occurred.

Returns:

The natural logarithm of x.

If `x` is:	These functions return:	Errors:
`x` is ±0.0	-Inf	FE_DIVBYZERO
1.0	0.0	—
Less than 0.0 and finite, or -Inf	NaN	FE_INVALID
Inf	Inf	—
NaN	NaN	—

These functions raise FE_INEXACT if the FPU reports that the result can't be exactly represented as a floating-point number.

Examples:

#include <stdio.h>
#include <math.h>
#include <fenv.h>
#include <stdlib.h>

int main( void )
{
    int except_flags;

    feclearexcept(FE_ALL_EXCEPT);

    printf( "%f\n", log(.5) );

    except_flags = fetestexcept(FE_ALL_EXCEPT);
    if(except_flags) {
        /* An error occurred; handle it appropriately. */
    }

    return EXIT_SUCCESS;
}

produces the output:

-0.693147

Classification:

ANSI, POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	No
Thread	Yes