<sect2>

	<title>Automatic Detection Causes Problems</title>

	<para>Dependencies must be declared either explicitly or by

	  using the <link

	    linkend="makefile-options">OPTIONS framework</link>.

	  Using other methods like automatic detection complicates

	  indexing, which causes problems for port and package

	  management.</para>

	<example>

	  <title>Wrong Declaration of an Optional Dependency</title>

	  <programlisting>.include &lt;bsd.port.pre.mk&gt;

.if exists(${LOCALBASE}/bin/foo)

LIB_DEPENDS=	bar:${PORTSDIR}/foo/bar

.endif</programlisting>

	</example>

	<para>The problem with the above method is that it declares a

	  dependency based on the existence of a file that may have

	  been installed by another port.  If all options were

	  recursively defined for a batch of ports and an index of

	  these ports was built, then the index would become

	  inconsistent during the installation of this batch if one of

	  its ports installed that file.  A new index would need to be

	  created, but it too may be inconsistent if there are other

	  ports that use this same method.</para>

	<example>

	  <title>Correct Declaration of an Optional Dependency</title>

	  <programlisting>OPTIONS=		BAR	"Enable bar support" on

.include <bsd.port.pre.mk>

.if defined(WITH_BAR) && !defined(WITHOUT_BAR)

LIB_DEPENDS=	bar:${PORTSDIR}/foo/bar

.endif</programlisting>

	</example>

	<para>This second method is correct because it would not cause

	  inconsistencies in the index of a batch of ports, provided

	  the batch's options were defined prior to the index's build.

	  This makes it possible to use simple scripts to automate the

	  building, installation, and updating of these ports and

	  their packages.</para>

      </sect2>