ysv</code> (for System V shared memory allocated via
        <code class="function">shmget</code>) and <code class="literal">windows</code> (for Windows
        shared memory).  Not all values are supported on all platforms; the
        first supported option is the default for that platform.  The use of
        the <code class="literal">sysv</code> option, which is not the default on any
        platform, is generally discouraged because it typically requires
        non-default kernel settings to allow fo