Return_Values

   0                  Indicates an error. When the specified name
                      value:

                      o  Is invalid, errno is set to EINVAL.

                      o  Does not have a system-defined value, errno
                         is not set.

   n                  The size of the buffer needed to hold the
                      value.

                      o  When the value of the name argument is
                         system-defined, confstr returns the size of
                         the buffer needed to hold the entire value.
                         If this return value is greater than the
                         len value, the string returned as the buf
                         value is truncated.

                      o  When the value of the len argument is set
                         to 0 or the buf value is NULL, confstr
                         returns the size of the buffer needed to
                         hold the entire system-defined value. The
                         string value is not copied.