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<h4 class="appendixsubsec">D.2.2 Porting the GNU C Library to Unix Systems</h4>

<p>Most Unix systems are fundamentally very similar.  There are variations
between different machines, and variations in what facilities are
provided by the kernel.  But the interface to the operating system
facilities is, for the most part, pretty uniform and simple.

   <p>The code for Unix systems is in the directory <samp><span class="file">unix</span></samp>, at the top
level of the <samp><span class="file">sysdeps</span></samp> hierarchy.  This directory contains
subdirectories (and subdirectory trees) for various Unix variants.

   <p>The functions which are system calls in most Unix systems are
implemented in assembly code, which is generated automatically from
specifications in files named <samp><span class="file">syscalls.list</span></samp>.  There are several
such files, one in <samp><span class="file">sysdeps/unix</span></samp> and others in its subdirectories. 
Some special system calls are implemented in files that are named with a
suffix of &lsquo;<samp><span class="samp">.S</span></samp>&rsquo;; for example, <samp><span class="file">_exit.S</span></samp>.  Files ending in
&lsquo;<samp><span class="samp">.S</span></samp>&rsquo; are run through the C preprocessor before being fed to the
assembler.

   <p>These files all use a set of macros that should be defined in
<samp><span class="file">sysdep.h</span></samp>.  The <samp><span class="file">sysdep.h</span></samp> file in <samp><span class="file">sysdeps/unix</span></samp>
partially defines them; a <samp><span class="file">sysdep.h</span></samp> file in another directory must
finish defining them for the particular machine and operating system
variant.  See <samp><span class="file">sysdeps/unix/sysdep.h</span></samp> and the machine-specific
<samp><span class="file">sysdep.h</span></samp> implementations to see what these macros are and what
they should do.

   <p>The system-specific makefile for the <samp><span class="file">unix</span></samp> directory
(<samp><span class="file">sysdeps/unix/Makefile</span></samp>) gives rules to generate several files
from the Unix system you are building the library on (which is assumed
to be the target system you are building the library <em>for</em>).  All
the generated files are put in the directory where the object files are
kept; they should not affect the source tree itself.  The files
generated are <samp><span class="file">ioctls.h</span></samp>, <samp><span class="file">errnos.h</span></samp>, <samp><span class="file">sys/param.h</span></samp>, and
<samp><span class="file">errlist.c</span></samp> (for the <samp><span class="file">stdio</span></samp> section of the library).

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