Crate cc

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A library for Cargo build scripts to compile a set of C/C++/assembly/CUDA files into a static archive for Cargo to link into the crate being built. This crate does not compile code itself; it calls out to the default compiler for the platform. This crate will automatically detect situations such as cross compilation and various environment variables and will build code appropriately.


First, you’ll want to both add a build script for your crate ( and also add this crate to your Cargo.toml via:

cc = "1.0"

Next up, you’ll want to write a build script like so:


And that’s it! Running cargo build should take care of the rest and your Rust application will now have the C files foo.c and bar.c compiled into a file named libfoo.a. If the C files contain

void foo_function(void) { ... }


int32_t bar_function(int32_t x) { ... }

you can call them from Rust by declaring them in your Rust code like so:

extern "C" {
    fn foo_function();
    fn bar_function(x: i32) -> i32;

pub fn call() {
    unsafe {

fn main() {

See the Rustonomicon for more details.

§External configuration via environment variables

To control the programs and flags used for building, the builder can set a number of different environment variables.

  • CFLAGS - a series of space separated flags passed to compilers. Note that individual flags cannot currently contain spaces, so doing something like: -L=foo\ bar is not possible.
  • CC - the actual C compiler used. Note that this is used as an exact executable name, so (for example) no extra flags can be passed inside this variable, and the builder must ensure that there aren’t any trailing spaces. This compiler must understand the -c flag. For certain TARGETs, it also is assumed to know about other flags (most common is -fPIC).
  • AR - the ar (archiver) executable to use to build the static library.
  • CRATE_CC_NO_DEFAULTS - the default compiler flags may cause conflicts in some cross compiling scenarios. Setting this variable will disable the generation of default compiler flags.
  • CC_ENABLE_DEBUG_OUTPUT - if set, compiler command invocations and exit codes will be logged to stdout. This is useful for debugging build script issues, but can be overly verbose for normal use.
  • CXX... - see C++ Support.

Furthermore, projects using this crate may specify custom environment variables to be inspected, for example via the Build::try_flags_from_environment function. Consult the project’s own documentation or its use of the cc crate for any additional variables it may use.

Each of these variables can also be supplied with certain prefixes and suffixes, in the following prioritized order:

  1. <var>_<target> - for example, CC_x86_64-unknown-linux-gnu
  2. <var>_<target_with_underscores> - for example, CC_x86_64_unknown_linux_gnu
  3. <build-kind>_<var> - for example, HOST_CC or TARGET_CFLAGS
  4. <var> - a plain CC, AR as above.

If none of these variables exist, cc-rs uses built-in defaults.

In addition to the above optional environment variables, cc-rs has some functions with hard requirements on some variables supplied by cargo’s build-script driver that it has the TARGET, OUT_DIR, OPT_LEVEL, and HOST variables.

§Optional features


Currently cc-rs supports parallel compilation (think make -jN) but this feature is turned off by default. To enable cc-rs to compile C/C++ in parallel, you can change your dependency to:

cc = { version = "1.0", features = ["parallel"] }

By default cc-rs will limit parallelism to $NUM_JOBS, or if not present it will limit it to the number of cpus on the machine. If you are using cargo, use -jN option of build, test and run commands as $NUM_JOBS is supplied by cargo.

§Compile-time Requirements

To work properly this crate needs access to a C compiler when the build script is being run. This crate does not ship a C compiler with it. The compiler required varies per platform, but there are three broad categories:

  • Unix platforms require cc to be the C compiler. This can be found by installing cc/clang on Linux distributions and Xcode on macOS, for example.
  • Windows platforms targeting MSVC (e.g. your target triple ends in -msvc) require Visual Studio to be installed. cc-rs attempts to locate it, and if it fails, cl.exe is expected to be available in PATH. This can be set up by running the appropriate developer tools shell.
  • Windows platforms targeting MinGW (e.g. your target triple ends in -gnu) require cc to be available in PATH. We recommend the MinGW-w64 distribution, which is using the Win-builds installation system. You may also acquire it via MSYS2, as explained here. Make sure to install the appropriate architecture corresponding to your installation of rustc. GCC from older MinGW project is compatible only with 32-bit rust compiler.

§C++ support

cc-rs supports C++ libraries compilation by using the cpp method on Build:

    .cpp(true) // Switch to C++ library compilation.

For C++ libraries, the CXX and CXXFLAGS environment variables are used instead of CC and CFLAGS.

The C++ standard library may be linked to the crate target. By default it’s libc++ for macOS, FreeBSD, and OpenBSD, libc++_shared for Android, nothing for MSVC, and libstdc++ for anything else. It can be changed in one of two ways:

  1. by using the cpp_link_stdlib method on Build:
    .cpp_link_stdlib("stdc++") // use libstdc++
  1. by setting the CXXSTDLIB environment variable.

In particular, for Android you may want to use c++_static if you have at most one shared library.

Remember that C++ does name mangling so extern "C" might be required to enable Rust linker to find your functions.

§CUDA C++ support

cc-rs also supports compiling CUDA C++ libraries by using the cuda method on Build:

    // Switch to CUDA C++ library compilation using NVCC.
    // Generate code for Maxwell (GTX 970, 980, 980 Ti, Titan X).
    // Generate code for Maxwell (Jetson TX1).
    // Generate code for Pascal (GTX 1070, 1080, 1080 Ti, Titan Xp).
    // Generate code for Pascal (Tesla P100).
    // Generate code for Pascal (Jetson TX2).
    // Generate code in parallel



  • A builder for compilation of a native library.
  • Represents an internal error that occurred, with an explanation.
  • Configuration used to represent an invocation of a C compiler.