Click above to get retro games delivered to your door ever month!

 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 <errno.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 EDOM, and the value negative HUGE_VAL is returned.

         Notes:     You can modify error handling with 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, because the same errno value is returned
    for both errors.

           #include <math.h>    /* for log(), SING, and 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);
               }
           }