mozilla/sccache

sccache - Shared Compilation Cache

sccache is a ccache-like compiler caching tool. It is used as a compiler wrapper and avoids compilation when possible, storing cached results either on local disk or in one of several cloud storage backends. Multi-level caching with automatic backfill is supported for hierarchical cache architectures (see Multi-Level Cache).

sccache includes support for caching the compilation of Assembler, C/C++ code, Rust, as well as NVIDIA's CUDA using nvcc, and clang, AMD's ROCm HIP.

sccache also provides 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 guide for more information.

sccache is also available as a GitHub Actions to facilitate the deployment using GitHub Actions cache.

---

Table of Contents (ToC)

• Installation
• Usage
• Build Requirements
• Build
• Separating caches between invocations
• Overwriting the cache
• Debugging
• Interaction with GNU make jobserver
• Known Caveats
• Storage Options
  • Multi-Level (Hierarchical Caching)
  • Local
  • S3
  • R2
  • Redis
  • Memcached
  • Google Cloud Storage
  • Azure
  • GitHub Actions
  • WebDAV (Ccache/Bazel/Gradle compatible)
  • Alibaba OSS
  • Tencent Cloud Object Storage

---

Installation

There are prebuilt x86-64 binaries available for Windows, Linux (a portable binary compiled against musl), and macOS on the releases page. 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:

brew install sccache

or via MacPorts:

sudo port install sccache

Windows

On Windows, sccache can be installed via scoop:

scoop install sccache

or winget:

winget install Mozilla.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.

cargo install sccache --locked

Or, using cargo-binstall to install a prebuilt binary:

cargo binstall sccache

With Nix

Sccache is available in nixpkgs, so if you don't need the latest version you can use that:

buildInputs = [ pkgs.sccache ];

We also provide a flake with an overlay for getting the latest version:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
    sccache = {
      url = "github:mozilla/sccache";
      inputs.nixpkgs.follows = "nixpkgs";
    };
  };

  outputs = { self, nixpkgs, sccache, ... }:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        overlays = [ sccache.overlays.default ];
      };
    in {
      devShells.${system}.default = pkgs.mkShell {
        buildInputs = [ pkgs.sccache ];
      };
    };
}

Or use it directly from the flake without the overlay:

nix run github:mozilla/sccache -- --help
nix shell github:mozilla/sccache

---

Usage

Running sccache is like running ccache: prefix your compilation commands with it, like so:

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. For example, you can set it globally in $HOME/.cargo/config.toml by adding:

[build]
rustc-wrapper = "/path/to/sccache"

Note that you need to use cargo 1.40 or newer for this to work.

Alternatively you can use the environment variable RUSTC_WRAPPER:

export RUSTC_WRAPPER=/path/to/sccache
cargo build

sccache supports gcc, clang, MSVC, rustc, NVCC, NVC++, hipcc, and Wind River's diab compiler. Both gcc and msvc support Response Files, read more about their implementation here.

If you don't specify otherwise, sccache will use a local disk cache.

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.

By default sccache server will listen on 127.0.0.1:4226, you can specify environment variable SCCACHE_SERVER_PORT to use a different port or SCCACHE_SERVER_UDS to listen on unix domain socket. Abstract unix socket is also supported as long as the path is escaped following the format. For example:

% env SCCACHE_SERVER_UDS=$HOME/sccache.sock sccache --start-server # unix socket
% env SCCACHE_SERVER_UDS=\\x00sccache.sock sccache --start-server # abstract unix socket

You can run sccache --stop-server to terminate the server. It will also terminate after (by default) 10 minutes of inactivity.

Running sccache --show-stats will print a summary of cache statistics.

Some notes about using sccache with Jenkins are here.

To use sccache with cmake, provide the following command line arguments to cmake 3.4 or newer:

-DCMAKE_C_COMPILER_LAUNCHER=sccache
-DCMAKE_CXX_COMPILER_LAUNCHER=sccache

The process for using sccache with MSVC and cmake, depends on which version of cmake you're using. For versions of cmake 3.24 and earlier, to generate PDB files for debugging with MSVC, you can use the /Z7 option. 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:

if(CMAKE_BUILD_TYPE STREQUAL "Debug")
  string(REPLACE "/Zi" "/Z7" CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")
  string(REPLACE "/Zi" "/Z7" CMAKE_C_FLAGS_DEBUG "${CMAKE_C_FLAGS_DEBUG}")
elseif(CMAKE_BUILD_TYPE STREQUAL "Release")
  string(REPLACE "/Zi" "/Z7" CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE}")
  string(REPLACE "/Zi" "/Z7" CMAKE_C_FLAGS_RELEASE "${CMAKE_C_FLAGS_RELEASE}")
elseif(CMAKE_BUILD_TYPE STREQUAL "RelWithDebInfo")
  string(REPLACE "/Zi" "/Z7" CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO}")
  string(REPLACE "/Zi" "/Z7" CMAKE_C_FLAGS_RELWITHDEBINFO "${CMAKE_C_FLAGS_RELWITHDEBINFO}")
endif()

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.

For versions of cmake 3.25 and later, to compile with MSVC, you have to use the new CMAKE_MSVC_DEBUG_INFORMATION_FORMAT option, meant to configure the -Z7 flag. Additionally, you must set the cmake policy number 0141 to the NEW setting:

set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT "$<$<CONFIG:Debug,RelWithDebInfo>:Embedded>")
cmake_policy(SET CMP0141 NEW)

Example configuration where we automatically look for sccache in the PATH:

find_program(SCCACHE sccache REQUIRED)

set(CMAKE_C_COMPILER_LAUNCHER ${SCCACHE})
set(CMAKE_CXX_COMPILER_LAUNCHER ${SCCACHE})
set(CMAKE_MSVC_DEBUG_INFORMATION_FORMAT "$<$<CONFIG:Debug,RelWithDebInfo>:Embedded>")
cmake_policy(SET CMP0141 NEW)

Alternatively, if configuring cmake with MSVC on the command line, assuming that sccache is on the default search path:

cmake -DCMAKE_C_COMPILER_LAUNCHER=sccache -DCMAKE_CXX_COMPILER_LAUNCHER=sccache -DCMAKE_MSVC_DEBUG_INFORMATION_FORMAT=Embedded -DCMAKE_POLICY_CMP0141=NEW [...]

And you can build code as usual without any additional flags in the command line, which is useful for IDEs.

---

Build Requirements

sccache is a Rust program. Building it requires cargo (and thusrustc). sccache currently requires Rust 1.85.0. We recommend you install Rust via Rustup.

Build

If you are building sccache for non-development purposes make sure you use cargo build --release to get optimized binaries:

cargo build --release [--no-default-features --features=s3|redis|gcs|memcached|azure|gha|webdav|oss|cos]

The list of features can be found in the Cargo.toml file, [features] section.

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 for details on how to select features with Cargo.

Building portable binaries

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.

Linux

Build with cargo and use ldd to check that the resulting binary does not depend on OpenSSL anymore.

macOS

Build with cargo and use otool -L to check that the resulting binary does not depend on OpenSSL anymore.

Windows

On Windows, the binary might also depend on a few MSVC CRT DLLs that are not available on older Windows versions.

It is possible to statically link against the CRT using a .cargo/config.toml file with the following contents.

[target.x86_64-pc-windows-msvc]
rustflags = ["-Ctarget-feature=+crt-static"]

Build with cargo and use dumpbin /dependents to check that the resulting binary does not depend on MSVC CRT DLLs anymore.

When statically linking with OpenSSL, you will need Perl available in your $PATH.

---

Separating caches between invocations

In situations where several different compilation invocations should not reuse the cached results from each other, one can set SCCACHE_C_CUSTOM_CACHE_BUSTER to a unique value that'll be mixed into the hash. MACOSX_DEPLOYMENT_TARGET and IPHONEOS_DEPLOYMENT_TARGET variables already exhibit such reuse-suppression behaviour. There are currently no such variables for compiling Rust.

---

Overwriting the cache

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.

---

Debugging

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).

SCCACHE_ERROR_LOG=/tmp/sccache_log.txt SCCACHE_LOG=debug sccache

You can also set these environment variables for your build system, for example

SCCACHE_ERROR_LOG=/tmp/sccache_log.txt SCCACHE_LOG=debug cmake --build /path/to/cmake/build/directory

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 for example. This method is not suitable for CI services because you need to compile in another shell at the same time.

SCCACHE_LOG=debug SCCACHE_START_SERVER=1 SCCACHE_NO_DAEMON=1 sccache

---

Interaction with GNU make jobserver

sccache provides support for a GNU make jobserver. 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) for rustc to use, so using sccache for Rust compilation in cargo via RUSTC_WRAPPER should do the right thing automatically.

---

Normalizing Paths with SCCACHE_BASEDIRS

By default, sccache requires absolute paths to match for cache hits. To enable cache sharing across different build directories, you can set SCCACHE_BASEDIRS to strip a base directory from paths before hashing:

export SCCACHE_BASEDIRS=/home/user/project

You can also specify multiple base directories by separating them by ; on Windows hosts and by : on any other operating system. When multiple directories are provided, the longest matching prefix is used:

export SCCACHE_BASEDIRS="/home/user/project:/home/user/workspace"

Path matching is case-insensitive on Windows and case-sensitive on other operating systems.

This is similar to ccache's CCACHE_BASEDIR and helps when:

• Building the same project from different directories
• Sharing cache between CI jobs with different checkout paths
• Multiple developers working with different username paths
• Working with multiple project checkouts simultaneously

Note: Only absolute paths are supported. Relative paths will prevent server from starting.

You can also configure this in the sccache config file:

# Single directory
basedirs = ["/home/user/project"]

# Or multiple directories
basedirs = ["/home/user/project", "/home/user/workspace"]

---

Known Caveats

General

• By default, absolute paths to files must match to get a cache hit. To work around this, use SCCACHE_BASEDIRS (see above) to normalize paths before hashing.

Rust

• 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.

More details on Rust caveats

C++20 Modules

sccache has partial support for C++20 named modules when using Clang. The following flags are supported:

• -fmodule-file=<path> and -fmodule-file=<name>=<path> - importing precompiled module interfaces
• -fmodule-output=<path> - generating module interface output alongside object files
• --precompile - compiling module interface units
• -fmodules-reduced-bmi - generating reduced BMI files

The following module-related flags are not supported and will bypass the cache:

• -fmodules and -fcxx-modules - Clang header modules (not C++20 named modules)
• -fprebuilt-implicit-modules and -fprebuilt-module-path - implicit module discovery

GCC and MSVC C++20 modules are not yet supported. Compilations using -fmodules-ts (GCC) or /interface, /ifcOutput, etc. (MSVC) will bypass the cache.

User Agent

• Requests sent to your storage option of choice will have a user agent header indicating the current sccache version, e.g. sccache/0.8.2.

Storage Options

• Local
• S3
• R2
• Redis
• Memcached
• Google Cloud Storage
• Azure
• GitHub Actions
• WebDAV (Ccache/Bazel/Gradle compatible)
• Alibaba OSS
• Tencent Cloud Object Storage