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 log10()                 Calculate Base 10 Logarithm

 #include   <math.h>

 double     log10(x);
 double     x;                           Floating-point value

    log10() returns the base 10 logarithm of 'x'.

       Returns:     Base 10 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 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 base 10 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 log10(), SING, DOMAIN, exception*/
           #include <stdio.h>  /* for printf() */

                              /* 0.0 and -1.1e-5 cause errors */
           double value[5] = {100.1, 1.0e300, 0.0, 5.1e-99, -1.1e-5};
           int merrno;    /* holds type of last math error */

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

           main()
           {
               double base10_log;
               int i;

               for (i = 0;  i < 5;  i++) {
                   merrno = 0;
                   base10_log = log10(value[i]);
               /* on an error, an error message is also printed to stderr*/
                   if (merrno == SING)
                       printf("SING error in log10()\n");
                   else if (merrno == DOMAIN)
                       printf("DOMAIN error in log10()\n");
                   printf("base 10 logarithm of %e = %e\n",
                          value[i],base10_log);
               }
           }