Build Time Tooling

This section covers "build-time" tooling, or code that is run prior to compiling a crate's source code. Conventionally, build-time code lives in a build.rs file and is commonly referred to as a "build script". Common use cases include rust code generation and compilation of bundled C/C++/asm code. See crates.io's documentation on the matter for more information.

Compile and link statically to a bundled C library

To accommodate scenarios where additional C, C++, or assembly is required in a project, the cc crate offers a simple api for compiling bundled C/C++/asm code into static libraries (.a) that can be statically linked to by rustc.

The following example has some bundled C code (src/hello.c) that will be used from rust. Before compiling rust source code, the "build" file (build.rs) specified in Cargo.toml runs. Using the cc crate, a static library file will be produced (in this case, libhello.a, see compile docs) which can then be used from rust by declaring the external function signatures in an extern block.

Since the bundled C is very simple, only a single source file needs to be passed to cc::Build. For more complex build requirements, cc::Build offers a full suite of builder methods for specifying include paths and extra compiler flags.

`Cargo.toml`

[package]
...
build = "build.rs"

[build-dependencies]
cc = "1"

[dependencies]
anyhow = "1"

`build.rs`

fn main() {
    cc::Build::new()
        .file("src/hello.c")
        .compile("hello");   // outputs `libhello.a`
}

`src/hello.c`

#include <stdio.h>


void hello() {
    printf("Hello from C!\n");
}

void greet(const char* name) {
    printf("Hello, %s!\n", name);
}

`src/main.rs`

use anyhow::Result;
use std::ffi::CString;
use std::os::raw::c_char;

fn prompt(s: &str) -> Result<String> {
    use std::io::Write;
    print!("{}", s);
    std::io::stdout().flush()?;
    let mut input = String::new();
    std::io::stdin().read_line(&mut input)?;
    Ok(input.trim().to_string())
}

extern {
    fn hello();
    fn greet(name: *const c_char);
}

fn main() -> Result<()> {
    unsafe { hello() }
    let name = prompt("What's your name? ")?;
    let c_name = CString::new(name)?;
    unsafe { greet(c_name.as_ptr()) }
    Ok(())
}

Compile and link statically to a bundled C++ library

Linking a bundled C++ library is very similar to linking a bundled C library. The two core differences when compiling and statically linking a bundled C++ library are specifying a C++ compiler via the builder method cpp(true) and preventing name mangling by the C++ compiler by adding the extern "C" section at the top of our C++ source file.

`Cargo.toml`

[package]
...
build = "build.rs"

[build-dependencies]
cc = "1"

`build.rs`

fn main() {
    cc::Build::new()
        .cpp(true)
        .file("src/foo.cpp")
        .compile("foo");   
}

`src/foo.cpp`

extern "C" {
    int multiply(int x, int y);
}

int multiply(int x, int y) {
    return x*y;
}

`src/main.rs`

extern {
    fn multiply(x : i32, y : i32) -> i32;
}

fn main(){
    unsafe {
        println!("{}", multiply(5,7));
    }   
}

Compile a C library while setting custom defines

It is simple to build bundled C code with custom defines using cc::Build::define. The method takes an Option value, so it is possible to create defines such as #define APP_NAME "foo" as well as #define WELCOME (pass None as the value for a value-less define). This example builds a bundled C file with dynamic defines set in build.rs and prints "Welcome to foo - version 1.0.2" when run. Cargo sets some environment variables which may be useful for some custom defines.

`Cargo.toml`

[package]
...
version = "1.0.2"
build = "build.rs"

[build-dependencies]
cc = "1"

`build.rs`

fn main() {
    cc::Build::new()
        .define("APP_NAME", "\"foo\"")
        .define("VERSION", format!("\"{}\"", env!("CARGO_PKG_VERSION")).as_str())
        .define("WELCOME", None)
        .file("src/foo.c")
        .compile("foo");
}

`src/foo.c`

#include <stdio.h>

void print_app_info() {
#ifdef WELCOME
    printf("Welcome to ");
#endif
    printf("%s - version %s\n", APP_NAME, VERSION);
}

`src/main.rs`

extern {
    fn print_app_info();
}

fn main(){
    unsafe {
        print_app_info();
    }   
}

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