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 log()                   Calculate Natural Logarithm

 #include   <math.h>

 double     log(x);
 double     x;                           Floating-point value

    log() returns the natural logarithm of 'x'.

    Returns:    Natural logarithm of 'x'.  If 'x' is negative, matherr()
                is called with a DOMAIN error, a DOMAIN error message is
                printed to 'stderr', 'errno' (defined in <stdlib.h>) is
                set to EDOM (defined in <math.h>), and the value negative
                HUGE_VAL (defined in <math.h>) is returned.  If 'x' is
                0.0, matherr() is called with a SING (singularity) error,
                a SING error message is printed to 'stderr', 'errno' is
                set to ERANGE, and the value negative HUGE_VAL is
                returned.

      Notes:    Error handling can be modified with the matherr()
                routine.

 Compatibility: MSC version 4.0 set 'errno' to EDOM if 'x' was either 0.0
                or negative.

   -------------------------------- Example ---------------------------------

    The following statements calculate the natural logarithm of several
    values in an array and report any DOMAIN (negative arguments) or SING
    (0.0 argument) errors.


           #include <math.h> /* for log(), SING, DOMAIN and struct
                                exception*/
           #include <stdio.h> /* for printf() */

                              /* 0.0 and -15.0 cause errors */
           double value[5] = {47.3, 1.0e300, 0.0, 0.1e-20, -15.0};

           main()
           {
               double nat_log;
               int i;

               for (i = 0;  i < 5;  i++) {
                   nat_log = log(value[i]);
                    printf("natural logarithm of %e = %e\n", value[i],
                            nat_log);
               }
           }