doc: drafted hacking/overview.md
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Hacking Jehanne
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===============
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Jehanne is a work in progress that still needs a lot of effort to become
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useful.
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Here you find a quick tour of the project organization.
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Coding Styles
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=============
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Jehanne is a small operating system, but it's composed of a kernel, a
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few libraries and a minimal set of commands coded in C.
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Most of libraries and commands come either from Bell Labs' Plan 9 or
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from 9front, and thus they loosely follow the coding conventions
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described in the [style(6)] manual page there.
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It's wise to stick to that convention for such code, in the hope to
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share improvements between the projects in a friendly manner.
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Other libraries and commands may come from the Unix ecosystem:
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we stick with the existing conventions there too.
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Here I describe the **arbitrary** conventions that are followed for
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original C components **and for the kernel**.
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This unfortunately means that, depending on the age of a kernel file,
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you will find either the loosely followed Pike' style or my ugly
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conventions.
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Hacking Tools
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=============
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To fasten development, a set of simple tools have been created.
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As tools to a greater goal, they are **disposable** and can only evolve
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as required by Jehanne's development.
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This document is a brief overview of such tools and it will follow
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their fate.
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Further help can be obtained from the [mailing list].
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Development Environment
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-----------------------
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Currently, Jehanne is coded and cross compiled from Linux (I work on a
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stable Debian GNU/Linux).
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To build it you need `bash`, `git`, `golang`, `build-essential` `flex`,
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`bison` and `qemu-system`.
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Inside the root of the repository you can enter the development
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environment with `./hacking/devshell.sh` that will start a new Bash:
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* your `$PS1` will be prepended with "JehanneDEV "
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* the environment variable `$JEHANNE` will hold the path of the root
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of the respository
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* the environment variable `$ARCH` will be "amd64" (aka x86_64, the
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only supported architecture so far)
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* `$PATH` will include `$JEHANNE/hacking/bin` and
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`$JEHANNE/hacking/cross/toolchain/bin`
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* the environment variable `$TOOLPREFIX` will contain the prefix of
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the cross compiling toolchain
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To build the cross compiler you "only" need to run
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`(cd $JEHANNE/hacking/cross/; ./init.sh; git clean -xdf src/)`.
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It will automatically download and compile Binutils and GCC
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(and their obsolete build dependencies).
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The process requires around 30 minutes (depending on your hardware)
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and 4 GB of free disk space, but fortunately it's seldom required
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during development.
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All other development tools can be built with the command
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`./hacking/buildtools.sh`.
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You can also invoke it with `--no-drawterm` or `--no-tools`: since
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building drawterm is slower than all the other tools together, I
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usually build it alone.
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The build system
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----------------
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Jehanne's build system is an evolution of the Harvey's original one
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based on Go and Json. It violates the [principle of least surprise],
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so that [I was originally pretty skeptic about it], but it turns out
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that [Aki was right]: a general purpose language provide both power
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and painless evolution to a build system.
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Thus, to build Jehanne you use the `build` commands.
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Its source code is at `./hacking/src/jehanne/cmd/` and its documantation
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can be obtained with `build --help`.
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It consumes small JSON files (usually named `build.json`) describing the
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build process. Some example to get started are [the commands one] and
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[the libc one].
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When I need to rebuild the entire system (for example after a
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change to libc) I simply run `cd $JEHANNE; git clean -xdf . && build`.
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However you can always build just a specific component
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(or component set), for example `build sys/src/cmd/rc/`
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or `build sys/src/cmds.json Cmds` or even
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`cd sys/src/cmd/hmi/console/ && build screenconsole.json && cd -`.
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Note that the Jehanne's build system does not track or enforce
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dependencies or file changes: it always run the **entire build**
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described in the provided JSON **and nothing more**. Thus it's simple,
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fast enough and fully **predictable**.
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The workhorse
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-------------
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A simplified kernel is built before the others: [the workhorse].
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It's used during the compilation, whenever we need to run a Jehanne's
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program that is impractical to port to Linux. For example it's run in a
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qemu instance to create the initial ram disk for the other kernels.
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Custom Go tools
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---------------
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Here is a brief summary of the other custom tools in
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`./hacking/src/jehanne/cmd/`:
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* `runqemu` runs Jehanne in a qemu instance and send commands to it.
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It is used both during compilation (to create the initial ram disk,
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for example) and to run [quality checks].
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* `ksyscalls` and `usyscalls` produce the boring code for system calls,
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in kernel and in libc respectively. It reads [sysconf.json].
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* `mksys` produces several headers reading from the [sysconf.json] too
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* `data2c` and `elf2c` embeed in programs in kernels (mainly in the
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workhorse).
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* `fetch` downloads external resources listed in [a fetch.json file]
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* `telnet` can connect a running instance of Jehanne.
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It was used before drawterm was available.
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* `preen` pretty print the JSON files used by `build`
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Inside the development shell, these tools are available in `$PATH`.
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Miscellaneous utilities
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-----------------------
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Among [devtools] you can also find several shell scripts and files
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that are designed to be used only from the repository root.
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To get a bootable usb stick (or a disk image to be used with Bochs
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or Qemu) you can use:
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* `./hacking/disk-create.sh` creates a raw disk image at `$DISK`
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(default `./hacking/sample-disk.img`). It uses syslinux, its bios
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files (looked up at `$SYSLINUXBIOS`) and fdisk, but it can be run as
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a user **without** `sudo`. The image will contains two separate
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partitions, one for syslinux, the kernel and the initial ram disk
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(the `dos` partition) and one for the rest of the system
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(the `plan9` partition) in a [hjfs] file system.
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* `./hacking/disk-boot-update.sh` updates syslinux, the kernel and
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the initial ram disk in the appropriate partition of `$DISK`
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* `./hacking/disk-update.sh file1 file2 ...` copy the files provided
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as arguments **to** the [hjfs] partition of `$DISK`.
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* `./hacking/disk-get.sh file1 files2 ...` copy the files provided as
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arguments **from** the [hjfs] partition of `$DISK`
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to `$JEHANNE/usr/glenda/tmp`.
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To run the system in Qemu you can use:
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* `./hacking/runOver9P.sh` that uses a 9P2000 file server running on
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the host as the root file system
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* `./hacking/runDisk.sh [path/to/disk/image]` that uses the disk image
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provided (or `$DISK`) to as the root file system.
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Moreover `./hacking/QA.sh` is used by `runqemu` to start the workhorse
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and execute the QA checks.
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In the default configuration (see [cfg/startup]), Jehanne is started as
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a cpu server owned by glenda. You can connect it with the
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`./hacking/drawterm.sh` script. The password is "demodemo".
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Finally `./hacking/continuous-build.sh` and
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`./hacking/coverity-scan.sh` are used during continuous build.
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Third parties
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-------------
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In the hacking/third_party directory you can file the
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[Go 9P2000 server] used during development and [drawterm], both
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downloaded as git submodules and compiled by `./hacking/buildtools.sh`.
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[mailing list]: https://groups.google.com/forum/#!forum/jehanneos
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[principle of least surprise]: https://en.wikipedia.org/wiki/Principle_of_least_astonishment
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[I was originally pretty skeptic about it]: https://groups.google.com/d/msg/harvey/IwK8-gebgyw/vxCPQVaGBAAJ
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[Aki was right]: https://groups.google.com/d/msg/harvey/IwK8-gebgyw/vxCPQVaGBAAJ
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[the commands one]: https://github.com/JehanneOS/jehanne/blob/master/sys/src/cmd/cmds.json
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[the libc one]: https://github.com/JehanneOS/jehanne/blob/master/sys/src/lib/c/build.json
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[sysconf.json]: https://github.com/JehanneOS/jehanne/blob/master/sys/src/sysconf.json
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[a fetch.json file]: https://github.com/JehanneOS/devtools/blob/master/cross/src/fetch.json
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[quality checks]: https://github.com/JehanneOS/jehanne/tree/master/qa
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[the workhorse]: https://github.com/JehanneOS/jehanne/blob/master/sys/src/kern/amd64/workhorse.json
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[devtools]: https://github.com/JehanneOS/devtools/
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[hjfs]: http://man2.aiju.de/4/hjfs
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[cfg/startup]: https://github.com/JehanneOS/jehanne/tree/master/cfg
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[Go 9P2000 server]: https://github.com/lionkov/ninep
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[drawterm]: https://github.com/0intro/drawterm
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[style(6)]: http://man.cat-v.org/9front/6/style
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