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✏️ Editing: build_system.md

# Build System

There are many ways to build prism, which means the build system is a bit more complicated than usual.

## Requirements

* It must work to build prism for all 6 uses-cases below.
* It must be possible to build prism without needing ruby/rake/etc.
  Because once prism is the single parser in TruffleRuby, JRuby or CRuby there won't be another Ruby parser around to parse such Ruby code.
  Most/every Ruby implementations want to avoid depending on another Ruby during the build process as that is very brittle.
* It is desirable to compile prism with the same or very similar compiler flags for all use-cases (e.g. optimization level, warning flags, etc).
  Otherwise, there is the risk prism does not work correctly with those different compiler flags.

The main solution for the second point seems a Makefile, otherwise many of the usages would have to duplicate the logic to build prism.

## General Design

1. Templates are generated by `templates/template.rb`
2. The `Makefile` compiles both `libprism.a` and `libprism.{so,dylib,dll}` from the `src/**/*.c` and `include/**/*.h` files
3. The `Rakefile` `:compile` task ensures the above prerequisites are done, then calls `make`,
   and uses `Rake::ExtensionTask` to compile the C extension (using its `extconf.rb`)

This way there is minimal duplication, and each layer builds on the previous one and has its own responsibilities.

The static library exports no symbols, to avoid any conflict.
The shared library exports some symbols, and this is fine since there should only be one libprism shared library
loaded per process (i.e., at most one version of the prism *gem* loaded in a process, only the gem uses the shared library).

## The various ways to build prism

### Building from ruby/prism repository with `bundle exec rake`

`rake` calls `make` and then uses `Rake::ExtensionTask` to compile the C extension (see above).

### Building the prism gem by `gem install/bundle install`

The gem contains the pre-generated templates.

When installing the gem on CRuby, `extconf.rb` is used and that compiles the C extension with mkmf, including both the extension files and the sources of prism itself.

When installing the gem on JRuby and TruffleRuby, no C extension is built, so instead the `extconf.rb` runs `make build/libprism.{so,dylib,dll}`.
There is Ruby code using FFI which uses `libprism.{so,dylib,dll}` to implement the same methods as the C extension, but using serialization instead of many native calls/accesses (JRuby does not support C extensions, serialization is faster on TruffleRuby than the C extension).

### Building the prism gem from git, e.g. `gem "prism", github: "ruby/prism"`

The same as above, except the `extconf.rb` additionally runs first:
* `templates/template.rb` to generate the templates

Because of course those files are not part of the git repository.

### Building prism as part of CRuby

[This script](https://github.com/ruby/ruby/blob/5124f9ac7513eb590c37717337c430cb93caa151/tool/sync_default_gems.rb#L399-L422) imports prism sources in CRuby.

The script generates the templates when importing.

prism's `Makefile` is not used at all in CRuby. Instead, CRuby's `Makefile` is used.

### Building prism as part of TruffleRuby

[This script](https://github.com/truffleruby/truffleruby/blob/master/tool/import-prism.sh) imports prism sources in TruffleRuby.
The script generates the templates when importing.

Then when `mx build` builds TruffleRuby and the `prism` mx project inside, it runs `make`.

Then the `prism bindings` mx project is built, which contains the [bindings](https://github.com/truffleruby/truffleruby/blob/vm-24.1.1/src/main/c/yarp_bindings/src/yarp_bindings.c)
and links to `libprism.a` (to avoid exporting symbols, so no conflict when installing the prism gem).

### Building prism as part of JRuby

TODO, similar to TruffleRuby.

### Building prism for embedded system

For instance, you can build a static library `libprism.a` targeting the Arm Cortex-M0+ embedded system by the commands below:

* `templates/template.rb`
* `CFLAGS="-mcpu=cortex-m0plus" make static CC=arm-none-eabi-gcc`

The build process internally looks up `_POSIX_MAPPED_FILES` and `_WIN32` macros to determine whether the functions of the memory map are available on the target platform.

### Building prism with custom memory allocator

If you need to use memory allocation functions implemented outside of the standard library, follow these steps:

* Add `-D PRISM_XALLOCATOR` to the build options
* Additionally, include `-I [path/to/custom_allocator]` where your `prism_xallocator.h` is located
* Link the implementation of `prism_xallocator.c` that contains functions declared in `prism_xallocator.h`

For further clarity, refer to `include/prism/defines.h`.

### Building prism from source as a C library

All of the source files match `src/**/*.c` and all of the headers match `include/**/*.h`.

If you want to build prism as a shared library and link against it, you should compile with:

* `-fPIC -shared` - Compile as a shared library
* `-DPRISM_EXPORT_SYMBOLS` - Export the symbols (by default nothing is exported)

#### Flags

`make` respects the `MAKEFLAGS` environment variable. As such, to speed up the build you can run:

```
MAKEFLAGS="-j10" bundle exec rake compile
```

## Build options

* `PRISM_BUILD_DEBUG` - Will cause all file reading to copy into its own allocation to allow easier tracking of reading off the end of the buffer. By default this is off.
* `PRISM_BUILD_MINIMAL` - Define all of the `PRISM_EXCLUDE_*` flags at once.
* `PRISM_ENCODING_EXCLUDE_FULL` - Will cause the library to exclude the full encoding API, and only include the minimal number of encodings to support parsing Ruby code without encoding comments. By default this is off.
* `PRISM_EXPORT_SYMBOLS` - Will cause the shared library to export symbols. By default this is off.
* `PRISM_EXCLUDE_JSON` - Will cause the library to exclude the JSON API. By default this is off.
* `PRISM_EXCLUDE_PACK` - Will cause the library to exclude the pack API. By default this is off.
* `PRISM_EXCLUDE_PRETTYPRINT` - Will cause the library to exclude the prettyprint API. By default this is off.
* `PRISM_EXCLUDE_SERIALIZATION` - Will cause the library to exclude the serialization API. By default this is off.
* `PRISM_XALLOCATOR` - Will cause the library to use the custom memory allocator. By default this is off.

Sid Gifari From Gifari Industries - BD Cyber Security Team