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+# Contributing to `libc`
+
+Welcome! If you are reading this document, it means you are interested in contributing
+to the `libc` crate.
+
+## Adding an API
+
+Want to use an API which currently isn't bound in `libc`? It's quite easy to add
+one!
+
+The internal structure of this crate is designed to minimize the number of
+`#[cfg]` attributes in order to easily be able to add new items which apply
+to all platforms in the future. As a result, the crate is organized
+hierarchically based on platform. Each module has a number of `#[cfg]`'d
+children, but only one is ever actually compiled. Each module then reexports all
+the contents of its children.
+
+This means that for each platform that libc supports, the path from a
+leaf module to the root will contain all bindings for the platform in question.
+Consequently, this indicates where an API should be added! Adding an API at a
+particular level in the hierarchy means that it is supported on all the child
+platforms of that level. For example, when adding a Unix API it should be added
+to `src/unix/mod.rs`, but when adding a Linux-only API it should be added to
+`src/unix/linux_like/linux/mod.rs`.
+
+If you're not 100% sure at what level of the hierarchy an API should be added
+at, fear not! This crate has CI support which tests any binding against all
+platforms supported, so you'll see failures if an API is added at the wrong
+level or has different signatures across platforms.
+
+New symbol(s) (i.e. functions, constants etc.) should also be added to the
+symbols list(s) found in the `libc-test/semver` directory. These lists keep
+track of what symbols are public in the libc crate and ensures they remain
+available between changes to the crate. If the new symbol(s) are available on
+all supported Unixes it should be added to `unix.txt` list<sup>1</sup>,
+otherwise they should be added to the OS specific list(s).
+
+With that in mind, the steps for adding a new API are:
+
+1. Determine where in the module hierarchy your API should be added.
+2. Add the API, including adding new symbol(s) to the semver lists.
+3. Send a PR to this repo.
+4. Wait for CI to pass, fixing errors.
+5. Wait for a merge!
+
+<sup>1</sup>: Note that this list has nothing to do with any Unix or Posix
+standard, it's just a list shared between all OSs that declare `#[cfg(unix)]`.
+
+## Test before you commit
+
+We have two automated tests running on [GitHub Actions](https://github.com/rust-lang/libc/actions):
+
+1. [`libc-test`](https://github.com/gnzlbg/ctest)
+  - `cd libc-test && cargo test`
+  - Use the `skip_*()` functions in `build.rs` if you really need a workaround.
+2. Style checker
+  - `rustc ci/style.rs && ./style src`
+
+## Breaking change policy
+
+Sometimes an upstream adds a breaking change to their API e.g. removing outdated items,
+changing the type signature, etc. And we probably should follow that change to build the
+`libc` crate successfully. It's annoying to do the equivalent of semver-major versioning
+for each such change. Instead, we mark the item as deprecated and do the actual change
+after a certain period. The steps are:
+
+1. Add `#[deprecated(since = "", note="")]` attribute to the item.
+  - The `since` field should have a next version of `libc`
+    (e.g., if the current version is `0.2.1`, it should be `0.2.2`).
+  - The `note` field should have a reason to deprecate and a tracking issue to call for comments
+    (e.g., "We consider removing this as the upstream removed it.
+    If you're using it, please comment on #XXX").
+2. If we don't see any concerns for a while, do the change actually.
+
+## Releasing your change to crates.io
+
+Now that you've done the amazing job of landing your new API or your new
+platform in this crate, the next step is to get that sweet, sweet usage from
+crates.io! The only next step is to bump the version of libc and then publish
+it. If you'd like to get a release out ASAP you can follow these steps:
+
+1. Increment the patch version number in `Cargo.toml` and `libc-test/Cargo.toml`.
+1. Send a PR to this repository. It should [look like this][example-pr], but it'd
+   also be nice to fill out the description with a small rationale for the
+   release (any rationale is ok though!)
+1. Once merged, the release will be tagged and published by one of the libc crate
+   maintainers.
+
+[example-pr]: https://github.com/rust-lang/libc/pull/2120