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