Header File 

math.h  

Category 

Math Routines 

Prototype 

double log(double x); 

long double logl(long double x); 

Description 

Calculates the natural logarithm of x. 

log calculates the natural logarithm of x. 

logl is the long double version; it takes a long double argument and returns a long double result. 

Return Value 

On success, log and logl return the value calculated, ln(x). 

errno

If x is 0, the functions return the value negative HUGE_VAL (log) or negative _LHUGE_VAL (logl), and set errno to ERANGE. Error handling for these routines can be modified through the functions _matherr and _matherrl. 

Example  

#include <math.h>
#include <stdio.h>
int main(void)
{
   double result;
   double x = 8.6872;
   result = log(x);
   printf("The natural log of %lf is %lf\n", x, result);
   return 0;
}

Portability