Chapter 6 System Initialization
1. GLIBC
(libc-x.x.x.so) contains the standard C library functions such as
printf() and many others that most application programs depend on.
2. The
Linux dynamic loader (ld-x.x.x.so) is responsible for loading the
binary executable into memory and performing the dynamic linking
required by the application’s reference to shared library functions.
Tips: There is a project Library Optimizer Tool used
to reduce the size of shared libraries for an embedded system or other
size-contrained environment.You can find it in:
[url]http://libraryopt.sourceforge.net/[/url]
3. Most
processes have two categories of dependencies: those that are needed to
resolve unresolved references within a dynamically linked executable,
and external confinguration or data files that an application might
need.
4. The developer can control which initial process is executed at startup by a kernel command line parameter.
5. init
process provides the default set of environment parameters for all
other processes to inherit, including such things as PATH and CONSOLE.
Its primary role is to spawn additional processes under the direction
of a special configuration file that is usually stored as /etc/inittab.
6. The
runlevel scripts are commonly found under a directory called
/etc/rc.d/init.d. A runlevel is defined by the services that are
enabled at that runlevel. Most Linux distributions contain a directory
structure under /etc that contains symbolic links to the service
scripts in /etc/rc.d/init.d. Each of runlevels is defined by the
scripts contained in the rcN.d, where N is the runlevel. Inside each
rcN.d directory, you will find numerous symlinks arranged in a specific
order. These symbolic links start with either a K or a S.
Those beginning with S point to service scripts, which are invoked with
startup instructions; those starting with a K point to service scripts
that are invoked with shutdown instructions.
7. The
Linux kernel contains a mechanism to mount an early root file system to
perform certain startup-related system initialization and
configuration. This mechanism is known as the initial RAM disk, or simply initrd.
The initial RAM disk is a small self-contained root file system that
usually contains directives to load specific device drivers before the
completion of the boot cycle.
8. When the kernel mounts the initial ramdisk, it looks for a specific file called linuxrc.
It treats this file as a script file and proceeds to execute the
commands contained therein. This mechanism enables the system designer
to specify the behavior of initrd.