| 1 | <sect1 id="ch02-aboutdependencies">
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| 2 | <title>About dependencies</title>
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| 3 | <?dbhtml filename="aboutdependencies.html" dir="chapter02"?>
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| 4 |
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| 5 | <para>There are a few ways to compile a list of a package's installation
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| 6 | dependencies. What we consider the best way is using the
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| 7 | <command>strace</command> program available at <ulink
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| 8 | url="http://www.wi.leidenuniv.nl/~wichert/strace/"/>.</para>
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| 9 |
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| 10 | <para><command>strace</command> is a program that provides a trace of all
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| 11 | system calls made by another program. One of the most useful system calls
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| 12 | to trace when figuring out dependencies is the <emphasis>execve(2)</emphasis>
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| 13 | system call, which is used to execute programs (see its man page for
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| 14 | all the details). Whenever you run a program, be it from a shell or via a
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| 15 | configure script or Makefile file, the execve call is made. If you trace
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| 16 | these calls, you will know what programs were executed behind the
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| 17 | scenes.</para>
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| 18 |
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| 19 | <para>Here is a line of output from running a configure script:</para>
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| 20 |
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| 21 | <screen>19580 execve("/bin/rm", ["rm", "-f", "conf19538", "conf19538.exe", "conf19538.file"], [/* 26 vars */]) = 0</screen>
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| 22 |
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| 23 | <para>This line tells us that the <command>/bin/rm</command> program was
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| 24 | run with a PID of 19580, which command line parameters it was given (rm -f
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| 25 | conf195838 conf19538.exe conf19538.file) and its exit value (0).</para>
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| 26 |
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| 27 | <para>For dependency purposes all we care about is that
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| 28 | <command>/bin/rm</command> was run during the configure script, so this is
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| 29 | an installation dependency. Without <command>rm</command>, the script
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| 30 | wouldn't be able to run properly.</para>
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| 31 |
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| 32 | <para>Unfortunately, this method is not foolproof. Configure scripts check
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| 33 | for the presense of many programs, but not all of them are considered real
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| 34 | dependencies. For instance, configure scripts may check for the presence of
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| 35 | the <command>autoconf</command> program. It will be listed in the strace
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| 36 | output, but it's not a real installation dependency. A package will in most
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| 37 | if not all cases install just fine without that program. There are other
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| 38 | such false positives.</para>
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| 39 |
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| 40 | <para>This means automatic dependency gathering is never accurate. You will
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| 41 | always need to validate the list and figure out the false positives. In
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| 42 | some (rare) cases autoconf might be a real dependency, so you
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| 43 | can't simply ignore all autoconf entries. A manual validation really is a
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| 44 | requirement for an accurate list.</para>
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| 45 |
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| 46 | <para>This book is not so verbose as to list exactly which program from which
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| 47 | package is required for a successfull installation (we used to, but it had
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| 48 | become too much work to maintain it). The book will contain simply the
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| 49 | names of packages you need to have installed. If you need the verbosity
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| 50 | in the form of "package a needs file b and c from package d", have a look
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| 51 | at <enter URL when it's available>.</para>
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| 52 |
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| 53 | </sect1>
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| 54 |
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