Add example of linking against a C++ shared library

Author: domfarolino

src/development_tools/build_tools.md

@@ -11,6 +11,8 @@ See crates.io's [documentation on the matter][build-script-docs] for more inform
 
 {{#include build_tools/cc-defines.md}}
 
+{{#include build_tools/cc-shared-library.md}}
+
 {{#include ../links.md}}
 
 [build-script-docs]: http://doc.crates.io/build-script.html

src/development_tools/build_tools/cc-shared-library.md

@@ -0,0 +1,156 @@
+## Compile and link statically to a bundled C library
+
+[![cc-badge]][cc] [![cat-development-tools-badge]][cat-development-tools]
+
+To accommodate more complex scenarios where your rust project needs to integrate
+with a shared library that will be dynamically linked at runtime, we can use a
+mixture of build scripts (**build.rs**) and compiler directives to teach the
+compiler and the final binary how to find and link to the shared library. This
+is useful when embedding a system-level library that aren't intended to be
+compiled into their client applications directly, or a library that gets updated
+regularly outside the scope of your project.
+
+This example does not use the [**cc**][cc] crate to compile the shared library,
+since you often don't have access to a shared library's source or build
+process. Rather, you're just consuming the library's binary directly.
+
+For the purpose of this example, the shared library's source is in
+**src/mylibrary.h** and **src/mylibrary.cc** so we can transparently see its API
+surface. But you can build and distribute the resulting shared library binary
+independent of your rust project's build process. This example also provides a
+simple **src/Makefile** to show how you might build this simple library outside
+of our our rust build process, to produce a suitable binary that our project can
+consume.
+
+Before compiling rust source code, the "build" file (**build.rs**) specified in
+**Cargo.toml** runs. It enumerates the shared libraries that our project depends
+on with the [`cargo:rustc-link-lib`][rustc-link-lib] directive, and tells
+**rustc** how to find those libraries at compile time with the
+[`cargo:rustc-link-search`][rustc-link-search] directive.
+
+The [`cargo:rustc-link-lib`][rustc-link-lib] directive also bakes dynamic
+dependency metadata into the final binary, which tells the linker the names of
+the libraries to find at runtime. At runtime, it will search for these libraries
+in its default places (usually system defaults like `/usr/lib`), but you can
+also include specific runtime path information in the final binary with the
+[`cargo:rustc-link-arg][rustc-link-arg] directive, which informs the linker
+*where* to search for shared library dependencies. This can make your binary
+more portable, but we do not do this in the simple example below.
+
+### `Cargo.toml`
+
+```toml
+[package]
+...
+build = "build.rs"
+```
+
+### `build.rs`
+
+```rust,edition2021,no_run
+fn main() {
+    // At compile time, we specify dynamic library dependency entries to bake
+    // into the binary for the linker to consult at runtime.
+    println!("cargo:rustc-link-search=native=.");
+    println!("cargo:rustc-link-lib=dylib=mylibrary");
+}
+```
+
+### `src/library.c`
+
+```cpp
+#include "api.h"
+
+#include <iostream>
+
+extern "C" {
+
+int add_numbers(int a, int b) {
+    return a + b;
+}
+
+void greet(const char* string) {
+  std::cout << "Hello from a C shared library, " << string << std::endl;
+}
+
+}  // extern "C"
+```
+
+### `src/library.h`
+
+```cpp
+// Use C linkage for ABI stability.
+extern "C" {
+
+int add_numbers(int a, int b);
+void greet(const char* string);
+
+}  // extern "C"
+```
+
+### `src/Makefile`
+
+```Makefile
+CC=g++
+CFLAGS=--std=c++11 -Wall -Werror -I.
+
+all: lib
+
+lib: src/mylibrary.c src/mylibrary.h
+        # Position-independent code (`-fpic`) is required for shared libraries.
+        $(CC) -c -fpic src/mylibrary.c -o mylibrary.o $(CFLAGS)
+        $(CC) -shared -o libmylibrary.so mylibrary.o
+        rm mylibrary.o
+```
+
+### `src/main.rs`
+
+```rust,edition2021,ignore
+use std::ffi::CString;
+use std::os::raw::c_char;
+
+// Compiler directive to tell the Rust which library contains the external
+// symbols we enumerate in this specific `extern` block. Here we specify a C
+// shared library called `libmylibrary`.
+#[link(name = "mylibrary")]
+extern "C" {
+    fn add_numbers(a: i32, b: i32) -> i32;
+    fn greet(name: *const c_char);
+}
+
+fn prompt(s: &str) -> String {
+    use std::io::Write;
+    print!("{}", s);
+    std::io::stdout().flush().unwrap();
+    let mut input = String::new();
+    std::io::stdin().read_line(&mut input).unwrap();
+    input.trim().to_string()
+}
+
+// Safe wrapper around the unsafe C add_numbers function.
+pub fn library_add_numbers(a: i32, b: i32) -> i32 {
+    unsafe {
+        add_numbers(a, b)
+    }
+}
+
+// Safe wrapper around the unsafe C add_numbers function.
+pub fn library_greet(name: CString) {
+    unsafe {
+        greet(name.as_ptr())
+    }
+}
+
+fn main() {
+    let name = prompt("What's your name? ");
+    let c_name = CString::new(name).unwrap();
+    library_greet(c_name);
+    let result = library_add_numbers(10, 12);
+    println!("Addition result: {}", result);
+}
+```
+
+[cc]: https://docs.rs/cc/latest/cc/
+[rustc-link-arg]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-arg
+[rustc-link-lib]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-lib
+[rustc-link-search]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#rustc-link-search