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<H2>Linux System Administrator's Survival Guide lsg25.htm</H2>



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<A HREF="#E68E135" >Upgrading and Installing New Kernel Software</A>



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<A HREF="#E68E136" >Compiling the Kernel from Source Code</A>



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<A HREF="#E69E146" >Where to Find Kernel Sources</A>



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<A HREF="#E69E147" >Using New Kernel Sources</A></UL>



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<A HREF="#E68E137" >Adding Drivers to the Kernel</A>



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<A HREF="#E68E138" >Upgrading Libraries</A>



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<A HREF="#E68E139" >Using Linux's C Compiler</A>



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<A HREF="#E69E148" >Compiler Options</A>



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<A HREF="#E69E149" >Debugging and Profiling Options</A>



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<A HREF="#E69E150" >Debugging gcc Programs with gdb</A></UL>



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<A HREF="#E68E140" >Summary</A></UL></UL></UL>



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<H1 ALIGN=CENTER>



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<FONT SIZE=6 COLOR="#FF0000"><B>Chapter 25</B></FONT></CENTER></H1>



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<FONT SIZE=6 COLOR="#FF0000"><B>Modifying the Kernel</B></FONT></CENTER></H2>



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<P>Usually you will want to leave the Linux kernel alone except when performing a major upgrade, installing a new networking component (such as NFS or NIS), or installing a new device driver that has special kernel requirements. The details of the process used to install the kernel drivers are usually supplied with the software. Because this isn't always the case, though, this chapter should give you a good idea of the general process for working with the kernel.



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<NOTE>Don't modify the kernel without knowing what you are doing. If you damage the source code or configuration information, your kernel may be unusable, and in the worst cases, your filesystem may be affected. Take care and follow instructions carefully. keep in mind that this chapter only covers the basics of kernel manipulation.</NOTE>



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<P>The several versions of Linux in common use have a few inconsistencies between them. For that reason, the exact instructions supplied in the following sections may not work with your version of Linux. The general approach is the same, however, and only the directory or utility names may be different. Most versions of Linux supply documentation that lists the recompilation process and the locations of the source code and compiled programs.



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<NOTE>Before doing anything with the kernel or utilities, make sure you have a good set of emergency boot disks and a complete backup on tape or floppy disk. Although the process of modifying the kernel is not difficult, it does cause problems every now and again that can leave you stranded without a working system. Boot disks are the best way to recover, so make at least one extra set.</NOTE>



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</BLOCKQUOTE></BLOCKQUOTE>



<P>Because the kernel is compiled with the C compiler supplied as part of Linux, the latter part of this chapter looks at the C compiler and its flags and how you can use it to your advantage. This information isn't meant to be a complete reference to the C system, of course, but it should be useful for some basic manipulations you may require when modifying the kernel (or any other source code compiled by C).



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<H3 ALIGN=CENTER>



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<FONT SIZE=5 COLOR="#FF0000"><B>Upgrading and Installing New Kernel Software</B></FONT></CENTER></H3>



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<P>Linux is a dynamic operating system. New releases of the kernel or parts of the operating system that can be linked into the kernel are made available at regular intervals to users. Whether you want to upgrade to the new releases usually depends on the features or bug fixes that the new release offers. You will probably have to relink the kernel when you add new software, unless the software is loaded as a utility or device driver.



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<P>Avoid upgrading your system with every new release, for a couple of reasons. The most common problem with constant upgrades is that you may be stuck with a new software package that causes backward compatibility problems with your existing system or that has a major problem with it. Most new releases of software wipe out existing configuration information, so you will have to reconfigure the packages that are being installed from scratch. Also, the frequency with which new releases are made available is so high that you can probably spend more time loading and recompiling kernels and utilities than using the system. Read the release notes carefully to ensure that the release is worth the installation time and trouble. Remember that few installations proceed smoothly!



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<P>The best advice is to upgrade only once or twice a year, and only when there is a new feature or enhancement that will make a significant difference to the way you use Linux. It's tempting to always have the latest and newest versions of the operating system, but there is a lot to be said for having a stable, functioning operating system, too.



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<P>If you do upgrade to a new release, bear in mind that you don't have to upgrade everything. The last few Linux releases have changed only about five percent of the operating system with each new major package upgrade. Instead of replacing the entire system, just install those parts that will have a definite effect, such as the kernel, compilers and their libraries, and frequently used utilities. This method saves time and reconfiguration.



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<H3 ALIGN=CENTER>



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<FONT SIZE=5 COLOR="#FF0000"><B>Compiling the Kernel from Source Code</B></FONT></CENTER></H3>



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<P>Upgrading, replacing, or adding new code to the kernel is usually a simple process. You obtain the source for the kernel, make any configuration changes, compile it, and then place it in the proper location on the filesystem to run the system properly. The process is often automated for you by a shell script or installation program, and some upgrades are completely automated with no need to do anything more than start the upgrade utility.



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<H4 ALIGN=CENTER>



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<FONT SIZE=4 COLOR="#FF0000"><B>Where to Find Kernel Sources</B></FONT></CENTER></H4>



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<P>Kernel sources for new releases of Linux are available from CD-ROM distributions, FTP sites, user groups, and many other locations. Most kernel versions are numbered with a version and a patch level, so you see kernel names like 1.12.123 where 1 is the major release, 12 is the minor version release, and 123 is the patch number. Most kernel source sites maintain several versions simultaneously, so check through the source directories for the latest version of the kernel.



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<P>Patch releases are sometimes numbered differently, and do not require the entire source of the kernel to install. In most cases, the patch overlays a section of existing source code, and you only need to recompile the kernel to install the patch. Patches are released quite frequently.



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<P>Most kernel source programs are maintained as a gzipped tar file. Unpack the files into a subdirectory of /usr/src, which is where most of the source code is kept for Linux. Some versions of Linux keep other directories for the kernel source, so you may want to check any documentation supplied with the system or look for a README file in the /usr/src directory for more instructions.



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<A NAME="E69E147"></A>



<H4 ALIGN=CENTER>



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<FONT SIZE=4 COLOR="#FF0000"><B>Using New Kernel Sources</B></FONT></CENTER></H4>



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<P>Often, unpacking the gzipped tar file in /usr/src creates a subdirectory called /usr/src/linux, which can overwrite your last version of the kernel source. Before starting the unpacking process, rename or copy any existing /usr/src/linux (or whatever name is used with the new kernel) file so you have a backup version in case of problems.



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<P>After unpacking the kernel source, you need to create two symbolic links to the /usr/include directory (if they are not created already or set by the installation procedure). Usually, the link commands required are the following:



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<PRE>



<FONT COLOR="#000080">ln -sf /usr/src/linux/include/linux /usr/include/linux



ln -sf /usr/src/linux/include/asm /usr/include/asm</FONT></PRE>



<P>If the directory names are different with your version of Linux, substitute them for /usr/src/linux. Without these links, the upgrade or installation of a new kernel cannot proceed.



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<P>After ungzipping and untarring the source code and establishing the links, you can begin the compilation process. You must have a version of gcc or g++ (the GNU C and C++ compilers) or some other compatible compiler available for the compilation. You may have to check with the source code documentation to make sure you have the correct versions of the compilers; occasionally new kernel features are added that are not supported by older versions of gcc or g++.



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<P>Check the file /usr/src/linux/Makefile (or whatever path Makefile is in with your source distribution). This file has a line that defines the ROOT_DEV, the device that is used as the root filesystem when Linux boots. Usually the line looks like the following:



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<FONT COLOR="#000080">ROOT_DEV = CURRENT</FONT></PRE>



<P>If you have any other value, make sure it is correct for your filesystem configuration. If the Makefile has no value, set it as shown in the preceding line.



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<P>The compilation process begins with you changing to the /usr/src/linux directory and issuing the command



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<PRE>



<FONT COLOR="#000080">make config</FONT></PRE>



<P>which invokes the make utility for the C compiler. The process may be slightly different for some versions of Linux, so check any release or installation notes supplied with the source code.



<BR>



<P>The config program issues a series of questions and prompts you to answer to indicate any configuration issues that need to be completed before the compilation begins. These questions may be about the type of disk drive you are using, the CPU, any partitions, or other devices like CD-ROMs. Answer the questions as well as you can. If you are unsure, choose the default values or the one that makes the most sense. The worst case is that you will have to redo the process if the system doesn't run properly. (You do have an emergency boot disk ready, don't you?)



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<P>Next, you have to set all the source dependencies. This step is commonly skipped and can cause a lot of problems if is not performed for each software release. Issue the following command:



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<PRE>



<FONT COLOR="#000080">make dep</FONT></PRE>



<P>If the software you are installing does not have a dep file, check the release or installation notes to ensure that the dependencies are correctly handled by the other steps.



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<P>Now you can finally compile the new kernel. The command to start the process is



<BR>