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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 (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 EDOM, and the value negative HUGE is returned.

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

  -------------------------------- 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.  A matherr() function is used to distinguish
    between the two error types, since the same errno value is returned
    for both 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};
         int merrno;    /* holds type of last math error */

         int matherr(e)
         struct exception *e;
         {
             merrno = e->type;
             return (0);
         }

         main()
         {
             double nat_log;
             int i;

             for (i = 0;  i < 5;  i++) {
                 merrno = 0;
                 nat_log = log(value[i]);
                 /* on an error, an error message is also printed to stderr*/
                 if (merrno == SING)
                     printf("SING error in log()\n");
                 else if (merrno == DOMAIN)
                     printf("DOMAIN error in log()\n");
                 printf("natural logarithm of %e = %e\n", value[i], nat_log);
             }
         }