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This chapter describes where in the Linux kernel sources you should start looking for particular kernel functions.
This book does not depend on a knowledge of the 'C' programming language or require that you have the Linux kernel sources available in order to understand how the Linux kernel works. That said, it is a fruitful exercise to look at the kernel sources to get an in-depth understanding of the Linux operating system. This chapter gives an overview of the kernel sources; how they are arranged and where you might start to look for particular code.
Where to Get The Linux Kernel SourcesAll of the major Linux distributions ( Craftworks, Debian, Slackware, Red Hat etcetera) include the kernel sources in them. Usually the Linux kernel that got installed on your Linux system was built from those sources. By their very nature these sources tend to be a little out of date so you may want to get the latest sources from one of the web sites mentioned in chapter www-appendix. They are kept on
ftp://ftp.cs.helsinki.fiand all of the other web sites shadow them. This makes the Helsinki web site the most up to date, but sites like MIT and Sunsite are never very far behind.
If you do not have access to the web, there are many CD ROM vendors who offer snapshots of the world's major web sites at a very reasonable cost. Some even offer a subscription service with quarterly or even monthly updates. Your local Linux User Group is also a good source of sources.
The Linux kernel sources have a very simple numbering system.
Any even number kernel (for example
2.0.30) is a stable, released,
kernel and any odd numbered kernel (for example
2.1.42 is a
This book is based on the stable
2.0.30 source tree.
Development kernels have all of the latest features and support
all of the latest devices.
Although they can be unstable, which may not be exactly what you want it,
is important that the Linux community tries the latest kernels.
That way they are tested for the whole community.
Remember that it is always worth backing up your system thoroughly if you do
try out non-production kernels.
Changes to the kernel sources are distributed as patch files. The patch utility is used to apply a series of edits to a set of source files. So, for example, if you have the 2.0.29 kernel source tree and you wanted to move to the 2.0.30 source tree, you would obtain the 2.0.30 patch file and apply the patches (edits) to that source tree:
$ cd /usr/src/linux $ patch -p1 < patch-2.0.30
This saves copying whole source trees, perhaps over slow serial connections.
A good source of kernel patches (official and unofficial) is
http://www.linuxhq.com web site.
How The Kernel Sources Are ArrangedAt the very top level of the source tree
/usr/src/linuxyou will see a number of directories:
archsubdirectory contains all of the architecture specific kernel code. It has further subdirectories, one per supported architecture, for example
includesubdirectory contains most of the include files needed to build the kernel code. It too has further subdirectories including one for every architecture supported. The
include/asmsubdirectory is a soft link to the real include directory needed for this architecture, for example
include/asm-i386. To change architectures you need to edit the kernel makefile and rerun the Linux kernel configuration program.
- This directory contains the initialization code for the kernel and it is a very good place to start looking at how the kernel works.
- This directory contains all of the memory management
code. The architecture specific memory management code lives
arch/*/mm/, for example
- All of the system's device drivers live in this
directory. They are further sub-divided into classes of
device driver, for example
- This directory contains the kernels inter-process communications code.
- This is simply a directory used to hold built modules.
- All of the file system code. This is further sub-divided
into directories, one per supported file system, for example
- The main kernel code. Again, the architecture
specific kernel code is in
- The kernel's networking code.
- This directory contains the kernel's library code.
The architecture specific library code can be found in
- This directory contains the scripts (for example awk and tk scripts) that are used when the kernel is configured.
Where to Start LookingA large complex program like the Linux kernel can be rather daunting to look at. It is rather like a large ball of string with no end showing. Looking at one part of the kernel often leads to looking at several other related files and before long you have forgotten what you were looking for. The next subsections give you a hint as to where in the source tree the best place to look is for a given subject.
System Startup and InitializationOn an Intel based system, the kernel starts when either loadlin.exe or LILO has loaded the kernel into memory and passed control to it. Look in
arch/i386/kernel/head.Sfor this part.
Head.Sdoes some architecture specific setup and then jumps to the
Memory ManagementThis code is mostly in
mmbut the architecture specific code is in
arch/*/mm. The page fault handling code is in
mm/memory.cand the memory mapping and page cache code is in
mm/filemap.c. The buffer cache is implemented in
mm/buffer.cand the swap cache in
KernelMost of the relevent generic code is in
kernelwith the architecture specific code in
arch/*/kernel. The scheduler is in
kernel/sched.cand the fork code is in
kernel/fork.c. The bottom half handling code is in
task_structdata structure can be found in
PCIThe PCI pseudo driver is in
drivers/pci/pci.cwith the system wide definitions in
include/linux/pci.h. Each architecture has some specific PCI BIOS code, Alpha AXP's is in
Interprocess CommunicationThis is all in
ipc. All System V IPC objects include an
ipc_permdata structure and this can be found in
include/linux/ipc.h. System V messages are implemented in
ipc/msg.c, shared memory in
ipc/shm.cand semaphores in
ipc/sem.c. Pipes are implemented in
Interrupt HandlingThe kernel's interrupt handling code is almost all microprocessor (and often platform) specific. The Intel interrupt handling code is in
arch/i386/kernel/irq.cand its definitions in
Device DriversMost of the lines of the Linux kernel's source code are in its device drivers. All of Linux's device driver sources are held in
driversbut these are further broken out by type:
- block device drivers such as ide (in
ide.c). If you want to look at how all of the devices that could possibly contain file systems are initialized then you should look at
drivers/block/genhd.c. It not only initializes the hard disks but also the network as you need a network to mount
nfsfile systems. Block devices include both IDE and SCSI based devices.
- This the place to look for character based devices such
ttys, serial ports and mice.
- All of the CDROM code for Linux. It is here that the
special CDROM devices (such as Soundblaster CDROM) can be found. Note
that the ide CD driver is
drivers/blockand that the SCSI CD driver is in
- This are the sources for the PCI pseudo-driver. A good place
to look at how the PCI subsystem is mapped and initialized. The Alpha AXP
PCI fixup code is also worth looking at in
- This is where to find all of the SCSI code as well as all of the
drivers for the scsi devices supported by Linux.
- This is where to look to find the network device drivers such as
the DECChip 21040 PCI ethernet driver which is in
- This is where all of the sound card drivers are.
File SystemsThe sources for the
EXT2file system are all in the
fs/ext2/directory with data structure definitions in
ext2_fs_sb.h. The Virtual File System data structures are described in
include/linux/fs.hand the code is in
fs/*. The buffer cache is implemented in
fs/buffer.calong with the
NetworkThe networking code is kept in
netwith most of the include files in
include/net. The BSD socket code is in
net/socket.cand the IP version 4 INET socket code is in
net/ipv4/af_inet.c. The generic protocol support code (including the
sk_buffhandling routines) is in
net/corewith the TCP/IP networking code in
net/ipv4. The network device drivers are in
ModulesThe kernel module code is partially in the kernel and partially in the
modulespackage. The kernel code is all in
kernel/modules.cwith the data structures and kernel demon
include/linux/kerneld.hrespectively. You may want to look at the structure of an
ELFobject file in
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╘ 1996-1999 David A Rusling copyright notice.
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