<sect3 xml:id="makefile-options-implies-prevents">

	<title>Options Dependencies/Conflicts</title>

	<para>Where options have dependencies or conflicts between

	  each other, these should be registered using the

	  <varname><replaceable>OPT</replaceable>_IMPLIES</varname>

	  and <varname><replaceable>OPT</replaceable>_PREVENTS</varname>

	  macros.</para>

	<sect4>

	  <title><varname><replaceable>OPT</replaceable>_IMPLIES</varname></title>

	  <para>If an option depends on other options, these should be

	    registered by a space separated list using the

	    <replaceable>OPT</replaceable>_IMPLIES macro.  If the

	    option with dependencies is enabled, the dependent options

	    named in <replaceable>OPT</replaceable>_IMPLIES will get

	    enabled automatically.</para>

	  <para>For example, if <replaceable>OPT1</replaceable>

	    depends on <replaceable>OPT2</replaceable> and

	    <replaceable>OPT3</replaceable>:</para>

	  <programlisting>OPT1_IMPLIES=	OPT2 OPT3</programlisting>

	  <note>

	    <para>No notification or warning is provided in the build

	      output or the options configuration dialog to alert the

	      user that an option has been automatically

	      enabled.</para>

	  </note>

	</sect4>

	<sect4>

	  <title><varname><replaceable>OPT</replaceable>_PREVENTS</varname></title>

	  <para>If options conflict with each other, these should be

	    registered by a space separated list using the

	    <replaceable>OPT</replaceable>_PREVENTS macro.  When an

	    option is enabled, if any of the conflicting options named

	    in <replaceable>OPT</replaceable>_PREVENTS are also

	    enabled, the port build will produce an error which

	    explains the conflicting options.</para>

	  <para>For example, if <replaceable>OPT1</replaceable>

	    conflicts with <replaceable>OPT2</replaceable> and

	    <replaceable>OPT3</replaceable>:</para>

	  <programlisting>OPT1_PREVENTS=	OPT2 OPT3</programlisting>

	  <note>

	    <para>No notification is provided in the options

	      configuration dialog to alert the user that conflicting

	      options have been enabled, however an error will be

	      generated during the port build explaining the

	      conflict.</para>

	  </note>

	  <para>If options are conflicting because a maximum of one

	    option should be enabled in a list, then the

	    <literal>OPTIONS_SINGLE</literal> or

	    <literal>OPTIONS_RADIO</literal> macros should be used

	    in preference to the <replaceable>OPT</replaceable>_PREVENTS

	    macro.  The <replaceable>OPT</replaceable>_PREVENTS macro

	    should be used for more complicated conflict scenarios,

	    for example:</para>

	  <programlisting>OPTIONS_DEFINE= PORTS_SSL

OPTIONS_SINGLE= GSSAPI

OPTIONS_SINGLE_GSSAPI=	GSSAPI_BASE GSSAPI_HEIMDAL GSSAPI_MIT GSSAPI_NONE

# Ensure port does not link with libcrypto.so from both base and ports

GSSAPI_BASE_PREVENTS= PORTS_SSL</programlisting>

	</sect4>

      </sect3>