Added some notes how to create dependency lists

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@3026 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter02/aboutdependencies.xml

@@ -0,0 +1,54 @@
+<sect1 id="ch02-aboutdependencies">
+<title>About dependencies</title>
+<?dbhtml filename="aboutdependencies.html" dir="chapter02"?>
+
+<para>There are a few ways to compile a list of a package's installation
+dependencies. What we consider the best way is using the
+<command>strace</command> program available at <ulink
+url="http://www.wi.leidenuniv.nl/~wichert/strace/"/>.</para>
+
+<para><command>strace</command> is a program that provides a trace of all
+system calls made by another program. One of the most useful system calls
+to trace when figuring out dependencies is the <emphasis>execve(2)</emphasis>
+system call, which is used to execute programs (see its man page for
+all the details). Whenever you run a program, be it from a shell or via a
+configure script or Makefile file, the execve call is made. If you trace
+these calls, you will know what programs were executed behind the
+scenes.</para>
+
+<para>Here is a line of output from running a configure script:</para>
+
+<screen>19580 execve("/bin/rm", ["rm", "-f", "conf19538", "conf19538.exe", "conf19538.file"], [/* 26 vars */]) = 0</screen>
+
+<para>This line tells us that the <command>/bin/rm</command> program was
+run with a PID of 19580, which command line parameters it was given (rm -f
+conf195838 conf19538.exe conf19538.file) and its exit value (0).</para>
+
+<para>For dependency purposes all we care about is that
+<command>/bin/rm</command> was run during the configure script, so this is
+an installation dependency. Without <command>rm</command>, the script
+wouldn't be able to run properly.</para>
+
+<para>Unfortunately, this method is not foolproof. Configure scripts check
+for the presense of many programs, but not all of them are considered real
+dependencies. For instance, configure scripts may check for the presence of
+the <command>autoconf</command> program. It will be listed in the strace
+output, but it's not a real installation dependency. A package will in most
+if not all cases install just fine without that program. There are other
+such false positives.</para>
+
+<para>This means automatic dependency gathering is never accurate. You will
+always need to validate the list and figure out the false positives. In
+some (rare) cases autoconf might be a real dependency, so you
+can't simply ignore all autoconf entries. A manual validation really is a
+requirement for an accurate list.</para>
+
+<para>This book is not as verbose to list exactly which program from which
+package is required for a successfull installation (we used to, but it had
+become too much work to maintain it). The book will contain simply the
+names of packages you need to have installed. If you need the verbosity
+in the form of "package a needs file b and c from package d", have a look
+at &lt;enter URL when it's available&gt;.</para>
+
+</sect1>
+

chapter02/chapter02.xml

@@ -5,6 +5,7 @@
 &c2-aboutlfs;
 &c2-aboutsbus;
 &c2-abouttestsuites;
+&c2-aboutdependencies;
 &c2-askforhelp;
 
 </chapter>

entities/book-structure.xml

@@ -16,6 +16,7 @@
 <!ENTITY c2-aboutlfs SYSTEM "../chapter02/aboutlfs.xml">
 <!ENTITY c2-aboutsbus SYSTEM "../chapter02/aboutsbus.xml">
 <!ENTITY c2-abouttestsuites SYSTEM "../chapter02/abouttestsuites.xml">
+<!ENTITY c2-aboutdependencies SYSTEM "../chapter02/aboutdependencies.xml">
 <!ENTITY c2-askforhelp SYSTEM "../chapter02/askforhelp.xml">
 
 <!ENTITY chapter03 SYSTEM "../chapter03/chapter03.xml">