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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 ‘<samp><span class="samp">.S</span></samp>’; for example, <samp><span class="file">_exit.S</span></samp>. Files ending in |
| ‘<samp><span class="samp">.S</span></samp>’ 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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