sccache is a [ccache](https://ccache.dev/)-like compiler caching tool. It is used as a compiler wrapper and avoids compilation when possible, storing cached results either on [local disk](#local) or in one of [several cloud storage backends](#storage-options).
sccache includes support for caching the compilation of C/C++ code, [Rust](docs/Rust.md), as well as NVIDIA's CUDA using [nvcc](https://docs.nvidia.com/cuda/cuda-compiler-driver-nvcc/index.html).
sccache also provides [icecream](https://github.com/icecc/icecream)-style distributed compilation (automatic packaging of local toolchains) for all supported compilers (including Rust). The distributed compilation system includes several security features that icecream lacks such as authentication, transport layer encryption, and sandboxed compiler execution on build servers. See [the distributed quickstart](docs/DistributedQuickstart.md) guide for more information.
There are prebuilt x86-64 binaries available for Windows, Linux (a portable binary compiled against musl), and macOS [on the releases page](https://github.com/mozilla/sccache/releases/latest). Several package managers also include sccache packages, you can install the latest release from source using cargo, or build directly from a source checkout.
### macOS
On macOS sccache can be installed via [Homebrew](https://brew.sh/):
```bash
brew install sccache
```
### Windows
On Windows, sccache can be installed via [scoop](https://scoop.sh/):
```
scoop install sccache
```
### Via cargo
If you have a Rust toolchain installed you can install sccache using cargo. **Note that this will compile sccache from source which is fairly resource-intensive. For CI purposes you should use prebuilt binary packages.**
```bash
cargo install sccache
```
---
Usage
-----
Running sccache is like running ccache: prefix your compilation commands with it, like so:
```bash
sccache gcc -o foo.o -c foo.c
```
If you want to use sccache for caching Rust builds you can define `build.rustc-wrapper` in the
[cargo configuration file](https://doc.rust-lang.org/cargo/reference/config.html). For example, you can set it globally
sccache works using a client-server model, where the server runs locally on the same machine as the client. The client-server model allows the server to be more efficient by keeping some state in memory. The sccache command will spawn a server process if one is not already running, or you can run `sccache --start-server` to start the background server process without performing any compilation.
To generate PDB files for debugging with MSVC, you can use the [`/Z7` option](https://docs.microsoft.com/en-us/cpp/build/reference/z7-zi-zi-debug-information-format?view=msvc-160). Alternatively, the `/Zi` option together with `/Fd` can work if `/Fd` names a different PDB file name for each object file created. Note that CMake sets `/Zi` by default, so if you use CMake, you can use `/Z7` by adding code like this in your CMakeLists.txt:
By default, sccache will fail your build if it fails to successfully communicate with its associated server. To have sccache instead gracefully failover to the local compiler without stopping, set the environment variable `SCCACHE_IGNORE_SERVER_IO_ERROR=1`.
sccache is a [Rust](https://www.rust-lang.org/) program. Building it requires `cargo` (and thus `rustc`). sccache currently requires **Rust 1.60.0**. We recommend you install Rust via [Rustup](https://rustup.rs/).
By default, `sccache` builds with support for all storage backends, but individual backends may be disabled by resetting the list of features and enabling all the other backends. Refer the [Cargo Documentation](http://doc.crates.io/manifest.html#the-features-section) for details on how to select features with Cargo.
When building with the `dist-server` feature, `sccache` will depend on OpenSSL, which can be an annoyance if you want to distribute portable binaries. It is possible to statically link against OpenSSL using the `openssl/vendored` feature.
sccache defaults to using local disk storage. You can set the `SCCACHE_DIR` environment variable to change the disk cache location. By default it will use a sensible location for the current platform: `~/.cache/sccache` on Linux, `%LOCALAPPDATA%\Mozilla\sccache` on Windows, and `~/Library/Caches/Mozilla.sccache` on MacOS.
The default cache size is 10 gigabytes. To change this, set `SCCACHE_CACHE_SIZE`, for example `SCCACHE_CACHE_SIZE="1G"`.
To use Azure Blob Storage, you'll need your Azure connection string and an _existing_ Blob Storage container name. Set the `SCCACHE_AZURE_CONNECTION_STRING`
environment variable to your connection string, and `SCCACHE_AZURE_BLOB_CONTAINER` to the name of the container to use. Note that sccache will not create
the container for you - you'll need to do that yourself.
You can also define a prefix that will be prepended to the keys of all cache objects created and read within the container, effectively creating a scope. To do that use the `SCCACHE_AZURE_KEY_PREFIX` environment variable. This can be useful when sharing a bucket with another application.
To use the [GitHub Actions cache](https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows), you need to set the `SCCACHE_GHA_CACHE_URL`/`ACTIONS_CACHE_URL` and `SCCACHE_GHA_RUNTIME_TOKEN`/`ACTIONS_RUNTIME_TOKEN` environmental variables. The `SCCACHE_` prefixed environmental variables override the variables without the prefix.
In a GitHub Actions workflow, you can set these environmental variables using the following step.
In contrast to the [`@actions/cache`](https://github.com/actions/cache) action, which saves a single large archive per cache key, `sccache` with GHA cache storage saves each cache entry separately.
GHA cache storage will create many small caches with the same cache key, e.g. `SCCACHE_GHA_CACHE_TO` and `SCCACHE_GHA_CACHE_FROM`. These GHA caches are differentiated by their [_version_](https://github.com/actions/cache#cache-version). The GHA cache implementation in `sccache` calculates the cache version from the [`sccache` entry key](docs/Caching.md), e.g. the source file path.
For example, if a cache entry has the version `main.rs` and has GHA cache entries for the `sccache-1` and `sccache-2` keys, then `SCCACHE_GHA_CACHE_FROM=sccache-` will match both and [return the most recent entry](https://docs.github.com/en/actions/using-workflows/caching-dependencies-to-speed-up-workflows#matching-a-cache-key).
This behavior is useful for scoping caches from different versions of Rust or for cross-platform builds (`rust-sdk-{RUST_TOOLKIT}-{TARGET_TRIPLE}-`), and to allow newer commits to override older caches by adding the Git SHA as a suffix (`-{GITHUB_SHA}`), as in the following screenshot.
In situations where the cache contains broken build artifacts, it can be necessary to overwrite the contents in the cache. That can be achieved by setting the `SCCACHE_RECACHE` environment variable.
You can set the `SCCACHE_ERROR_LOG` environment variable to a path and set `SCCACHE_LOG` to get the server process to redirect its logging there (including the output of unhandled panics, since the server sets `RUST_BACKTRACE=1` internally).
Alternatively, if you are compiling locally, you can run the server manually in foreground mode by running `SCCACHE_START_SERVER=1 SCCACHE_NO_DAEMON=1 sccache`, and send logging to stderr by setting the [`SCCACHE_LOG` environment variable](https://docs.rs/env_logger/0.7.1/env_logger/#enabling-logging) for example. This method is not suitable for CI services because you need to compile in another shell at the same time.
sccache provides support for a [GNU make jobserver](https://www.gnu.org/software/make/manual/html_node/Job-Slots.html). When the server is started from a process that provides a jobserver, sccache will use that jobserver and provide it to any processes it spawns. (If you are running sccache from a GNU make recipe, you will need to prefix the command with `+` to get this behavior.) If the sccache server is started without a jobserver present it will create its own with the number of slots equal to the number of available CPU cores.
This is most useful when using sccache for Rust compilation, as rustc supports using a jobserver for parallel codegen, so this ensures that rustc will not overwhelm the system with codegen tasks. Cargo implements its own jobserver ([see the information on `NUM_JOBS` in the cargo documentation](https://doc.rust-lang.org/stable/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts)) for rustc to use, so using sccache for Rust compilation in cargo via `RUSTC_WRAPPER` should do the right thing automatically.
* Absolute paths to files must match to get a cache hit. This means that even if you are using a shared cache, everyone will have to build at the same absolute path (i.e. not in `$HOME`) in order to benefit each other. In Rust this includes the source for third party crates which are stored in `$HOME/.cargo/registry/cache` by default.
* Crates that invoke the system linker cannot be cached. This includes `bin`, `dylib`, `cdylib`, and `proc-macro` crates. You may be able to improve compilation time of large `bin` crates by converting them to a `lib` crate with a thin `bin` wrapper.
* Incrementally compiled crates cannot be cached. By default, in the debug profile Cargo will use incremental compilation for workspace members and path dependencies. [You can disable incremental compilation.](https://doc.rust-lang.org/cargo/reference/profiles.html#incremental)
