log, logf, logl, log10, log10f, log10l

Calculates logarithms.


double log( double x );
float logf( float x );
long double logl( double x );
double log10( double x );
float log10f ( float x );
long double log10l( double x );
float log( float x );  // C++ only
long double log( long double x );  // C++ only
float log10( float x );  // C++ only
long double log10( long double x );  // C++ only


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 (IND), by default. If x is 0, they return infinity (INF).

Input SEH Exception Matherr Exception

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


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


Routine Required header
log, logf, logl, log10, log10f, log10l <math.h>

For additional compatibility information, see Compatibility.


// 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 );
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);
Log base 2 of 65536.000000 is 16.000000

See also

Floating-Point Support
exp, expf, expl
pow, powf, powl