<html lang="en">
<head>
<title>Hierarchy Conventions - The GNU C Library</title>
<meta http-equiv="Content-Type" content="text/html">
<meta name="description" content="The GNU C Library">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="start" href="index.html#Top">
<link rel="up" href="Porting.html#Porting" title="Porting">
<link rel="next" href="Porting-to-Unix.html#Porting-to-Unix" title="Porting to Unix">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<!--
This file documents the GNU C library.

This is Edition 0.12, last updated 2007-10-27,
of `The GNU C Library Reference Manual', for version
2.8 (Sourcery G++ Lite 2011.03-41).

Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002,
2003, 2007, 2008, 2010 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ``Free Software Needs Free Documentation''
and ``GNU Lesser General Public License'', the Front-Cover texts being
``A GNU Manual'', and with the Back-Cover Texts as in (a) below.  A
copy of the license is included in the section entitled "GNU Free
Documentation License".

(a) The FSF's Back-Cover Text is: ``You have the freedom to
copy and modify this GNU manual.  Buying copies from the FSF
supports it in developing GNU and promoting software freedom.''-->
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
  pre.display { font-family:inherit }
  pre.format  { font-family:inherit }
  pre.smalldisplay { font-family:inherit; font-size:smaller }
  pre.smallformat  { font-family:inherit; font-size:smaller }
  pre.smallexample { font-size:smaller }
  pre.smalllisp    { font-size:smaller }
  span.sc    { font-variant:small-caps }
  span.roman { font-family:serif; font-weight:normal; } 
  span.sansserif { font-family:sans-serif; font-weight:normal; } 
--></style>
<link rel="stylesheet" type="text/css" href="../cs.css">
</head>
<body>
<div class="node">
<a name="Hierarchy-Conventions"></a>
<p>
Next:&nbsp;<a rel="next" accesskey="n" href="Porting-to-Unix.html#Porting-to-Unix">Porting to Unix</a>,
Up:&nbsp;<a rel="up" accesskey="u" href="Porting.html#Porting">Porting</a>
<hr>
</div>

<h4 class="appendixsubsec">D.2.1 Layout of the <samp><span class="file">sysdeps</span></samp> Directory Hierarchy</h4>

<p>A GNU configuration name has three parts: the CPU type, the
manufacturer's name, and the operating system.  <samp><span class="file">configure</span></samp> uses
these to pick the list of system-dependent directories to look for.  If
the &lsquo;<samp><span class="samp">--nfp</span></samp>&rsquo; option is <em>not</em> passed to <samp><span class="file">configure</span></samp>, the
directory <samp><var>machine</var><span class="file">/fpu</span></samp> is also used.  The operating system
often has a <dfn>base operating system</dfn>; for example, if the operating
system is &lsquo;<samp><span class="samp">Linux</span></samp>&rsquo;, the base operating system is &lsquo;<samp><span class="samp">unix/sysv</span></samp>&rsquo;. 
The algorithm used to pick the list of directories is simple:
<samp><span class="file">configure</span></samp> makes a list of the base operating system,
manufacturer, CPU type, and operating system, in that order.  It then
concatenates all these together with slashes in between, to produce a
directory name; for example, the configuration &lsquo;<samp><span class="samp">i686-linux-gnu</span></samp>&rsquo;<!-- /@w -->
results in <samp><span class="file">unix/sysv/linux/i386/i686</span></samp>.  <samp><span class="file">configure</span></samp> then
tries removing each element of the list in turn, so
<samp><span class="file">unix/sysv/linux</span></samp> and <samp><span class="file">unix/sysv</span></samp> are also tried, among others. 
Since the precise version number of the operating system is often not
important, and it would be very inconvenient, for example, to have
identical <samp><span class="file">irix6.2</span></samp> and <samp><span class="file">irix6.3</span></samp> directories,
<samp><span class="file">configure</span></samp> tries successively less specific operating system names
by removing trailing suffixes starting with a period.

   <p>As an example, here is the complete list of directories that would be
tried for the configuration &lsquo;<samp><span class="samp">i686-linux-gnu</span></samp>&rsquo;<!-- /@w --> (with the
<samp><span class="file">crypt</span></samp> and <samp><span class="file">linuxthreads</span></samp> add-on):

<pre class="smallexample">     sysdeps/i386/elf
     crypt/sysdeps/unix
     linuxthreads/sysdeps/unix/sysv/linux
     linuxthreads/sysdeps/pthread
     linuxthreads/sysdeps/unix/sysv
     linuxthreads/sysdeps/unix
     linuxthreads/sysdeps/i386/i686
     linuxthreads/sysdeps/i386
     linuxthreads/sysdeps/pthread/no-cmpxchg
     sysdeps/unix/sysv/linux/i386
     sysdeps/unix/sysv/linux
     sysdeps/gnu
     sysdeps/unix/common
     sysdeps/unix/mman
     sysdeps/unix/inet
     sysdeps/unix/sysv/i386/i686
     sysdeps/unix/sysv/i386
     sysdeps/unix/sysv
     sysdeps/unix/i386
     sysdeps/unix
     sysdeps/posix
     sysdeps/i386/i686
     sysdeps/i386/i486
     sysdeps/libm-i387/i686
     sysdeps/i386/fpu
     sysdeps/libm-i387
     sysdeps/i386
     sysdeps/wordsize-32
     sysdeps/ieee754
     sysdeps/libm-ieee754
     sysdeps/generic
</pre>
   <p>Different machine architectures are conventionally subdirectories at the
top level of the <samp><span class="file">sysdeps</span></samp> directory tree.  For example,
<samp><span class="file">sysdeps/sparc</span></samp><!-- /@w --> and <samp><span class="file">sysdeps/m68k</span></samp><!-- /@w -->.  These contain
files specific to those machine architectures, but not specific to any
particular operating system.  There might be subdirectories for
specializations of those architectures, such as
<samp><span class="file">sysdeps/m68k/68020</span></samp><!-- /@w -->. Code which is specific to the
floating-point coprocessor used with a particular machine should go in
<samp><span class="file">sysdeps/</span><var>machine</var><span class="file">/fpu</span></samp><!-- /@w -->.

   <p>There are a few directories at the top level of the <samp><span class="file">sysdeps</span></samp>
hierarchy that are not for particular machine architectures.

     <dl>
<dt><samp><span class="file">generic</span></samp><dd>As described above (see <a href="Porting.html#Porting">Porting</a>), this is the subdirectory
that every configuration implicitly uses after all others.

     <br><dt><samp><span class="file">ieee754</span></samp><dd>This directory is for code using the IEEE 754 floating-point format,
where the C type <code>float</code> is IEEE 754 single-precision format, and
<code>double</code> is IEEE 754 double-precision format.  Usually this
directory is referred to in the <samp><span class="file">Implies</span></samp> file in a machine
architecture-specific directory, such as <samp><span class="file">m68k/Implies</span></samp>.

     <br><dt><samp><span class="file">libm-ieee754</span></samp><dd>This directory contains an implementation of a mathematical library
usable on platforms which use IEEE&nbsp;754<!-- /@w --> conformant floating-point
arithmetic.

     <br><dt><samp><span class="file">libm-i387</span></samp><dd>This is a special case.  Ideally the code should be in
<samp><span class="file">sysdeps/i386/fpu</span></samp> but for various reasons it is kept aside.

     <br><dt><samp><span class="file">posix</span></samp><dd>This directory contains implementations of things in the library in
terms of <span class="sc">POSIX.1</span> functions.  This includes some of the <span class="sc">POSIX.1</span>
functions themselves.  Of course, <span class="sc">POSIX.1</span> cannot be completely
implemented in terms of itself, so a configuration using just
<samp><span class="file">posix</span></samp> cannot be complete.

     <br><dt><samp><span class="file">unix</span></samp><dd>This is the directory for Unix-like things.  See <a href="Porting-to-Unix.html#Porting-to-Unix">Porting to Unix</a>. 
<samp><span class="file">unix</span></samp> implies <samp><span class="file">posix</span></samp>.  There are some special-purpose
subdirectories of <samp><span class="file">unix</span></samp>:

          <dl>
<dt><samp><span class="file">unix/common</span></samp><dd>This directory is for things common to both BSD and System V release 4. 
Both <samp><span class="file">unix/bsd</span></samp> and <samp><span class="file">unix/sysv/sysv4</span></samp> imply <samp><span class="file">unix/common</span></samp>.

          <br><dt><samp><span class="file">unix/inet</span></samp><dd>This directory is for <code>socket</code> and related functions on Unix systems. 
<samp><span class="file">unix/inet/Subdirs</span></samp> enables the <samp><span class="file">inet</span></samp> top-level subdirectory. 
<samp><span class="file">unix/common</span></samp> implies <samp><span class="file">unix/inet</span></samp>. 
</dl>

     <br><dt><samp><span class="file">mach</span></samp><dd>This is the directory for things based on the Mach microkernel from CMU
(including the GNU operating system).  Other basic operating systems
(VMS, for example) would have their own directories at the top level of
the <samp><span class="file">sysdeps</span></samp> hierarchy, parallel to <samp><span class="file">unix</span></samp> and <samp><span class="file">mach</span></samp>. 
</dl>

   </body></html>