Add tutorial about non-system libraries (#2318)

* add tutorial about non-system libraries

* fix broken comment

* ignore code snippet

Author: pvdrz

book/src/SUMMARY.md

@@ -10,6 +10,7 @@
         - [Include the Generated Bindings in `src/lib.rs`](./tutorial-4.md)
         - [Write a Sanity Test](./tutorial-5.md)
         - [Publish Your Crate!](./tutorial-6.md)
+    - [Bindings for non-system libraries](./non-system-libraries.md)
 - [Command Line Usage](./command-line-usage.md)
 - [Customizing the Generated Bindings](./customizing-generated-bindings.md)
     - [Allowlisting](./allowlisting.md)

book/src/non-system-libraries.md

@@ -0,0 +1,106 @@
+Now let's suppose we want to generate bindings for a non-system library. We
+will be the same crate setup as the previous tutorial. First let's create a new
+directory `hello` with two files inside it. A `c` source file `hello.c`
+containing
+```c
+int hello() {
+    return 42;
+}
+```
+and a `c` header file `hello.h` containing
+```c
+int hello();
+```
+given that the library has not been compiled yet, we need to modify the
+`build.rs` build script to compile the `hello.c` source file into a static
+libary:
+
+```rust,ignore
+extern crate bindgen;
+
+use std::env;
+use std::path::PathBuf;
+
+use bindgen::CargoCallbacks;
+
+fn main() {
+    // This is the directory where the `c` library is located.
+    let libdir_path = PathBuf::from("hello")
+        // Canonicalize the path as `rustc-link-search` requires an absolute
+        // path.
+        .canonicalize()
+        .expect("cannot canonicalize path");
+
+    // This is the path to the `c` headers file.
+    let headers_path = libdir_path.join("hello.h");
+    let headers_path_str = headers_path.to_str().expect("Path is not a valid string");
+
+    // This is the path to the intermediate object file for our library.
+    let obj_path = libdir_path.join("hello.o");
+    // This is the path to the static library file.
+    let lib_path = libdir_path.join("libhello.a");
+
+    // Tell cargo to look for shared libraries in the specified directory
+    println!("cargo:rustc-link-search={}", libdir_path.to_str().unwrap());
+
+    // Tell cargo to tell rustc to link our `hello` library. Cargo will
+    // automatically know it must look for a `libhello.a` file.
+    println!("cargo:rustc-link-lib=hello");
+
+    // Tell cargo to invalidate the built crate whenever the header changes.
+    println!("cargo:rerun-if-changed={}", headers_path_str);
+
+    // Run `clang` to compile the `hello.c` file into a `hello.o` object file.
+    // Unwrap if it is not possible to spawn the process.
+    if !std::process::Command::new("clang")
+        .arg("-c")
+        .arg("-o")
+        .arg(&obj_path)
+        .arg(libdir_path.join("hello.c"))
+        .output()
+        .expect("could not spawn `clang`")
+        .status
+        .success()
+    {
+        // Panic if the command was not successful.
+        panic!("could not compile object file");
+    }
+
+    // Run `ar` to generate the `libhello.a` file from the `hello.o` file.
+    // Unwrap if it is not possible to spawn the process.
+    if !std::process::Command::new("ar")
+        .arg("rcs")
+        .arg(lib_path)
+        .arg(obj_path)
+        .output()
+        .expect("could not spawn `ar`")
+        .status
+        .success()
+    {
+        // Panic if the command was not successful.
+        panic!("could not emit library file");
+    }
+
+    // The bindgen::Builder is the main entry point
+    // to bindgen, and lets you build up options for
+    // the resulting bindings.
+    let bindings = bindgen::Builder::default()
+        // The input header we would like to generate
+        // bindings for.
+        .header(headers_path_str)
+        // Tell cargo to invalidate the built crate whenever any of the
+        // included header files changed.
+        .parse_callbacks(Box::new(CargoCallbacks))
+        // Finish the builder and generate the bindings.
+        .generate()
+        // Unwrap the Result and panic on failure.
+        .expect("Unable to generate bindings");
+
+    // Write the bindings to the $OUT_DIR/bindings.rs file.
+    let out_path = PathBuf::from(env::var("OUT_DIR").unwrap()).join("bindings.rs");
+    bindings
+        .write_to_file(out_path)
+        .expect("Couldn't write bindings!");
+}
+```
+