# log, logf, log10, log10f

Calculates logarithms.

``````double log(
double x
);
float log(
float x
);  // C++ only
long double log(
long double x
);  // C++ only
float logf(
float x
);
double log10(
double x
);
float log10(
float x
);  // C++ only
long double log10(
long double x
);  // C++ only
float log10f (
float x
);
``````

## Parameters

• x
Value whose logarithm is to be found.

## Return Value

The log functions return the natural logarithm (base e) of x if successful. The log10 functions return the base-10 logarithm. If x is negative, these functions return an indefinite, by default. If x is 0, they return INF (infinite).

Input

SEH Exception

Matherr Exception

± QNAN,IND

none

_DOMAIN

± 0

ZERODIVIDE

_SING

x < 0

INVALID

_DOMAIN

log and log10 has an implementation that uses Streaming SIMD Extensions 2 (SSE2). See _set_SSE2_enable for information and restrictions on using the SSE2 implementation.

## Remarks

C++ allows overloading, so you can call overloads of log and log10. In a C program, log and log10 always take and return a double.

## Requirements

Routine

log, logf, log10, log10f

<math.h>

For additional compatibility information, see Compatibility in the Introduction.

### Libraries

All versions of the C run-time libraries.

## Example

``````// crt_log.c
/* This program uses log and log10
* to calculate the natural logarithm and
* the base-10 logarithm of 9,000.
*/

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

int main( void )
{
double x = 9000.0;
double y;

y = log( x );
printf( "log( %.2f ) = %f\n", x, y );
y = log10( x );
printf( "log10( %.2f ) = %f\n", x, y );
}
``````

## Output

``````log( 9000.00 ) = 9.104980
log10( 9000.00 ) = 3.954243
``````

To generate logarithms for other bases, use the mathematical relation: log base b of a == natural log (a) / natural log (b).

``````// logbase.cpp
#include <math.h>
#include <stdio.h>

double logbase(double a, double base)
{
return log(a) / log(base);
}

int main()
{
double x = 65536;
double result;

result = logbase(x, 2);
printf("Log base 2 of %lf is %lf\n", x, result);
}
``````

## Output

``````Log base 2 of 65536.000000 is 16.000000
``````

## .NET Framework Equivalent

#### Reference

Floating-Point Support

exp, expf

_matherr

pow, powf

_CIlog

_CIlog10