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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
);

Parameter

x
Value whose logarithm is to be found.

Return Value

The log functions return the logarithm of x if successful. 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 Required header Compatibility
log, logf, log10, log10f <math.h> ANSI, Win 98, Win Me, Win NT, Win 2000, Win XP

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

See Also

Floating-Point Support Routines | exp | _matherr | pow | Run-Time Routines and .NET Framework Equivalents

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