1 | <sect1 id="ch05-whystatic">
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2 | <title>Why we use static linking</title>
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3 | <?dbhtml filename="whystatic.html" dir="chapter05"?>
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4 |
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5 | <para>Most programs have to perform, beside their specific task, many rather
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6 | common and trivial operations, such as allocating memory, searching
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7 | directories, opening and closing files, reading and writing them, string
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8 | handling, pattern matching, arithmetic, and so on. Instead of obliging each
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9 | program to reinvent the wheel, the GNU system provides all these basic
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10 | functions ready-made in libraries. The major library on any Linux system is
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11 | <filename>glibc</filename>. To get an idea of what it contains, have a look at
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12 | <filename>glibc/index.html</filename> somewhere on your host system.</para>
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13 |
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14 | <para>There are two ways of linking the functions from a library to a program
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15 | that uses them: statically or dynamically. When a program is linked
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16 | statically, the code of the used functions is included in the executable,
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17 | resulting in a rather bulky program. When a program is dynamically linked,
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18 | what is included is a reference to the linker, the name of the library, and
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19 | the name of the function, resulting in a much smaller executable. This
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20 | executable has the disadvantage of being somewhat slower than a statically
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21 | linked one, as the linking at run time takes a few moments.</para>
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22 |
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23 | <para>Aside from this small drawback, dynamic linking has two major advantages
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24 | over static linking. First, you need only one copy of the executable library
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25 | code on your hard disk, instead of having many copies of the same code included
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26 | into a whole bunch of programs -- thus saving disk space. Second, when several
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27 | programs use the same library function at the same time, only one copy of the
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28 | function's code is required in core -- thus saving memory space.</para>
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29 |
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30 | <para>Nowadays saving a few megabytes of space may not seem like much, but
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31 | many moons ago, when disks were measured in megabytes and core in kilobytes,
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32 | such savings were essential. It meant being able to keep several programs in
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33 | core at the same time and to contain an entire Unix system on just a few disk
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34 | volumes.</para>
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35 |
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36 | <para>A third but minor advantage of dynamic linking is that when a library
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37 | function gets a bug fixed, or is otherwise improved, you only need to recompile
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38 | this one library, instead of having to recompile all the programs that make use
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39 | of the improved function.</para>
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40 |
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41 | <para>In summary we can say that dynamic linking trades run time against
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42 | memory space, disk space, and recompile time.</para>
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43 |
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44 | <para>But if dynamic linking saves so much space, why then are we linking
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45 | all programs in this chapter statically? The reason is that we won't be
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46 | compiling a temporary <filename>glibc</filename> here. And we avoid doing this
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47 | simply to save some time -- around 14 SBUs. Another reason is that the
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48 | Glibc version on the LFS system might not be compatible with the Glibc on
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49 | the host system. Applications compiled against your host system's Glibc
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50 | version may not run properly (or at all) on the LFS system.</para>
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51 |
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52 | <para>This means that the tools compiled in this chapter will have to be
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53 | self-contained, because when later on we chroot to the LFS partition the
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54 | GNU library won't be available. That is why we use the
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55 | <userinput>-static</userinput>, <userinput>--enable-static-link</userinput>,
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56 | and <userinput>--disable-shared</userinput> flags throughout this chapter, to
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57 | ensure that all executables are statically linked. When we come to the next
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58 | chapter, almost the first thing we do is build <filename>glibc</filename>, the
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59 | main set of system libraries. Once this is done, we can link all other programs
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60 | dynamically (including the ones installed statically in this chapter) and
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61 | take advantage of the space saving opportunities.</para>
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62 |
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63 | </sect1>
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64 |
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