inline-c

inline-c is a small crate that allows a user to write C (including C++) code inside Rust. Both environments are strictly sandboxed: it is non-obvious for a value to cross the boundary. The C code is transformed into a string which is written in a temporary file. This file is then compiled into an object file, that is finally executed. It is possible to run assertions about the execution of the C program.

The primary goal of inline-c is to ease the testing of a C API of a Rust program (generated with cbindgen for example). Note that it's not tied to a Rust program exclusively, it's just its initial reason to live.

Install

Add the following lines to your Cargo.toml file:

[dev-dependencies]
inline-c = "0.1"

Documentation

The assert_c and assert_cxx macros live in the inline-c-macro crate, but are re-exported in this crate for the sake of simplicity.

Being able to write C code directly in Rust offers nice opportunities, like having C examples inside the Rust documentation that are executable and thus tested (with cargo test --doc). Let's dig into some examples.

Basic usage

The following example is super basic: C prints Hello, World! on the standard output, and Rust asserts that.

use inline_c::assert_c;

fn test_stdout() {
    (assert_c! {
        #include <stdio.h>

        int main() {
            printf("Hello, World!");

            return 0;
        }
    })
    .success()
    .stdout("Hello, World!");
}

Or with a C++ program:

use inline_c::assert_cxx;

fn test_cxx() {
    (assert_cxx! {
        #include <iostream>

        using namespace std;

        int main() {
            cout << "Hello, World!";

            return 0;
        }
    })
    .success()
    .stdout("Hello, World!");
}

The assert_c and assert_cxx macros return a Result<Assert, Box<dyn Error>>. See Assert to learn more about the possible assertions.

The following example tests the returned value:

use inline_c::assert_c;

fn test_result() {
    (assert_c! {
        int main() {
            int x = 1;
            int y = 2;

            return x + y;
        }
    })
    .failure()
    .code(3);
}

Environment variables

It is possible to define environment variables for the execution of the given C program. The syntax is using the special #inline_c_rs C directive with the following syntax:

#inline_c_rs <variable_name>: "<variable_value>"

Please note the double quotes around the variable value.

use inline_c::assert_c;

fn test_environment_variable() {
    (assert_c! {
        #inline_c_rs FOO: "bar baz qux"

        #include <stdio.h>
        #include <stdlib.h>

        int main() {
            const char* foo = getenv("FOO");

            if (NULL == foo) {
                return 1;
            }

            printf("FOO is set to `%s`", foo);

            return 0;
        }
    })
    .success()
    .stdout("FOO is set to `bar baz qux`");
}

Meta environment variables

Using the #inline_c_rs C directive can be repetitive if one needs to define the same environment variable again and again. That's why meta environment variables exist. They have the following syntax:

INLINE_C_RS_<variable_name>=<variable_value>

It is usually best to define them in a build.rs script for example. Let's see it in action with a tiny example:

use inline_c::assert_c;
use std::env::{set_var, remove_var};

fn test_meta_environment_variable() {
    set_var("INLINE_C_RS_FOO", "bar baz qux");

    (assert_c! {
        #include <stdio.h>
        #include <stdlib.h>

        int main() {
            const char* foo = getenv("FOO");

            if (NULL == foo) {
                return 1;
            }

            printf("FOO is set to `%s`", foo);

            return 0;
        }
    })
    .success()
    .stdout("FOO is set to `bar baz qux`");

    remove_var("INLINE_C_RS_FOO");
}

Note: If you have multiple inline C tests that use the same environment variables, you may see flakiness in test runs because by default cargo test runs tests parallely. This problem can occur even if your tests are removing the environment variables during teardown. To fix this, you can do either of the following:

• Use unique environment variable names for each test

• (Not recommended for Production) Run the tests serially instead of parallely. You can use the following command: cargo test -- --test-threads=1

CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS

Some classical Makefile variables like CFLAGS, CPPFLAGS, CXXFLAGS and LDFLAGS are understood by inline-c and consequently have a special treatment. Their values are added to the appropriate compilers when the C code is compiled and linked into an object file.

Pro tip: Let's say we have a Rust crate named foo, and it exports a C API. It is possible to define CFLAGS and LDFLAGS as follow to correctly compile and link all the C codes to the Rust libfoo shared object by writing this in a build.rs script (it is assumed that libfoo lands in the target/<profile>/ directory, and that foo.h lands in the root directory):

use std::{env, path::PathBuf};

fn main() {
    let include_dir = env::var("CARGO_MANIFEST_DIR").unwrap();

    let mut shared_object_dir = PathBuf::from(env::var("CARGO_MANIFEST_DIR").unwrap());
    shared_object_dir.push("target");
    shared_object_dir.push(env::var("PROFILE").unwrap());
    let shared_object_dir = shared_object_dir.as_path().to_string_lossy();

    // The following options mean:
    //
    // * `-I`, add `include_dir` to include search path,
    // * `-L`, add `shared_object_dir` to library search path,
    // * `-D_DEBUG`, enable debug mode to enable `assert.h`.
    println!(
        "cargo:rustc-env=INLINE_C_RS_CFLAGS=-I{I} -L{L} -D_DEBUG",
        I = include_dir,
        L = shared_object_dir.clone(),
    );

    // Here, we pass the fullpath to the shared object with
    // `LDFLAGS`.
    println!(
        "cargo:rustc-env=INLINE_C_RS_LDFLAGS={shared_object_dir}/{lib}",
        shared_object_dir = shared_object_dir,
        lib = if cfg!(target_os = "windows") {
            "foo.dll".to_string()
        } else if cfg!(target_os = "macos") {
            "libfoo.dylib".to_string()
        } else {
            "libfoo.so".to_string()
        }
    );
}

Et voilà ! Now run cargo build --release (to generate the shared objects) and then cargo test --release to see it in action.

Using inline-c inside Rust documentation

Since it is now possible to write C code inside Rust, it is consequently possible to write C examples, that are:

1. Part of the Rust documentation with cargo doc, and
2. Tested with all the other Rust examples with cargo test --doc.

Yes. Testing C code with cargo test --doc. How fun is that? No trick needed. One can write:

/// Blah blah blah.
///
/// # Example
///
/// ```rust
/// # use inline_c::assert_c;
/// #
/// # fn main() {
/// #     (assert_c! {
/// #include <stdio.h>
///
/// int main() {
///     printf("Hello, World!");
///
///     return 0;
/// }
/// #    })
/// #    .success()
/// #    .stdout("Hello, World!");
/// # }
/// ```
pub extern "C" fn some_function() {}

which will compile down into something like this:

int main() {
    printf("Hello, World!");

    return 0;
}

Notice that this example above is actually Rust code, with C code inside. Only the C code is printed, due to the # hack of rustdoc, but this example is a valid Rust example, and is fully tested!

There is one minor caveat though: the highlighting. The Rust set of rules are applied, rather than the C ruleset. See this issue on rustdoc to follow the fix.

C macros

C macros with the #define directive is supported only with Rust nightly. One can write:

use inline_c::assert_c;

fn test_c_macro() {
    (assert_c! {
        #define sum(a, b) ((a) + (b))

        int main() {
            return !(sum(1, 2) == 3);
        }
    })
    .success();
}

Note that multi-lines macros don't work! That's because the \ symbol is consumed by the Rust lexer. The best workaround is to define the macro in another .h file, and to include it with the #include directive.

License

BSD-3-Clause, see LICENSE.md.