log(), logf(), logl()

Compute the natural logarithm of a number

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
The general-purpose math library.
libm-sve
(QNX Neutrino 7.1 or later) A library that optimizes the code for ARMv8.2 chips that have Scalable Vector Extension hardware.

Your system requirements will determine how you should work with these libraries:

Note: Compile your program with the -fno-builtin option to prevent the compiler from using a built-in version of the function.

Description:

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

loge x

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

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:

C11, POSIX 1003.1

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