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63 lines
3.4 KiB
XML
63 lines
3.4 KiB
XML
<sect1 id="ch05-whystatic">
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<title>Why do we use static linking?</title>
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<?dbhtml filename="whystatic.html" dir="chapter05"?>
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<para>(Thanks to Plasmatic for posting the text on which this is mainly
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based to one of the LFS mailing lists.)</para>
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<para>When making (compiling) a program, rather than having to rewrite all the
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functions for dealing with the kernel, hardware, files, etc. every time you
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write a new program, all these basic functions are instead kept in libraries.
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glibc, which you install later, is one of these major libraries, which
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contains code for all the basic functions programs use, like opening files,
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printing information on the screen, and getting feedback from the user. When
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the program is compiled, these libraries of code are linked together with the
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new program, so that it can use any of the functions that the library
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has.</para>
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<para>However, these libraries can be very large (for example, libc.a
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can often be around 2.5 MB), so you may not want a separate copy of each
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library attached to the program. Just imagine if you had a simple command
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like ls with an extra 2.5 MB attached to it! Instead of making the library
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an actual part of the program, or statically linked, the library is stored
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as a separate file, which is loaded only when the program needs it. This
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is what we call dynamically linked, as the library is loaded and unloaded
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dynamically, as the program needs it.</para>
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<para>So now we have a 1 KB file and a 2.5 MB file, but we still haven't
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saved any space (except maybe RAM until the library is needed). The
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<emphasis>real</emphasis> advantage of dynamically linked libraries is
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that we only need one copy of the library. If <filename>ls</filename> and
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<filename>rm</filename> both use the same library, then we don't need two
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copies of the library, as they can both get the code from the same file.
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Even when in memory, the two programs share the same code, rather than loading
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duplicates into memory. So not only are we saving hard disk space, but also
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precious RAM.</para>
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<para>If dynamic linking saves so much room, then why are we making everything
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statically linked? Well, that's because when you chroot into your brand new
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(but very incomplete) LFS environment, these dynamic libraries won't be
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available because they are somewhere else in your old directory tree
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(<filename>/usr/lib</filename> for example) which won't be accessible
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from within your LFS root (<filename>$LFS</filename>).</para>
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<para>So in order for your new programs to run inside the chroot environment
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you need to make sure that the libraries are statically linked when you build
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them, hence the <userinput>--enable-static-link</userinput>,
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<userinput>--disable-shared</userinput>, and
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<userinput>-static</userinput> flags used
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through Chapter 5. Once in Chapter 6, the first thing we do is build the
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main set of system libraries, glibc. Once this is made we start rebuilding
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all the programs we just did in Chapter 5, but this time dynamically linked,
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so that we can take advantage of the space saving opportunities.</para>
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<para>And there you have it, that's why you need to use those weird
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<userinput>-static</userinput> flags. If you try building everything
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without them, you'll see very quickly what
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happens when you chroot into your newly crippled LFS system.</para>
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<para>If you want to know more about Dynamically Linked Libraries, consult
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a book or website on programming, especially a Linux-related site.</para>
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</sect1>
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