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72 lines
2.5 KiB
Markdown
72 lines
2.5 KiB
Markdown
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# System Bootup Process
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## Initial Ram Disk to Rootfs transition
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After mounting the root filesystem, either the kernel or the initial ram disk
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startup process is expected to exec the init program from the root filesystem.
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At the current time, there is no support for re-scanning the service files
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*yet*, so when init is started, the final configuration in `/etc/init.d` has to
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be present. As a result, we currently cannot perform mounting of `/etc/` or
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packing init into the initial ram disk and doing the rootfs transition.
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Also, as a result of this, changing the service configuration requires a system
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reboot to be effective.
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This _will_ change in the future.
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## Processing Service Descriptions
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The init process reads service description files from `/etc/init.d` which are
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usually symlinks to actual files in `/usr/share/init`.
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The exact locations may be changed through configure flags when compiling init.
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Service files specify a *target* which is basically like a SystemV runlevel and
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can be one of the following:
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* boot
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* reboot
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* shutdown
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* ctrlaltdel
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After parsing the configuration files, the init process starts running the
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services for the `boot` target in a topological order as determined by their
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*before* and *after* dependencies.
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Services can be of one of the following *types*:
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* wait
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* once
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* respawn
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Services of type `wait` are started exactly once and the init process waits
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until they terminate before continuing with other services.
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The type `once` also only runs services once, but immediately continues
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starting other services in the mean time without waiting.
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Services of type `respawn` also don't stall the init process and are re-started
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whenever they terminate.
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## Service Process Setup
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If a service description contains only a single `exec` line, the init process
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forks and then execs the command directly in the child process.
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If the service description contains a `tty` field, the specified device file
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is opened in the child process and standard I/O is redirected to it before
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calling exec. Also, a new session is created.
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If a service description contains multiple `exec` lines, the init process forks
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off to a single child process that does the same setup as above, and then runs
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the command lines sequentially by forking a second time for each one, followed
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by an exec in the grand child and a wait in the original child.
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If a single command line returns something other than `EXIT_SUCCESS`,
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processing of multiple command lines is immediately stopped and the offending
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exit status is returned to init.
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