Add some C code.

.gitignore

@@ -1,3 +1,5 @@
 /target
 main
 .idea
+*.o
+*.so

build

@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+cc -c src/test.c
+cc -shared test.o -o libtest.so
+rustc -l test -L . src/main.rs

src/main.rs

@@ -18,6 +18,10 @@ impl Greeter {
     }
 }
 
+extern "C" {
+    fn print_num(num: i32);
+}
+
 fn main() {
     Greeter::new("Hello, Jesse!").greet();
 
@@ -42,4 +46,8 @@ fn main() {
     bst.insert(0);
     println!("{:?}", bst);
     println!("find result: {:?}", bst.find(0));
+
+    // calling some C
+    let find_result = bst.find(3).unwrap_or(-1);
+    unsafe { print_num(find_result); }
 }

src/test.c

@@ -0,0 +1,5 @@
+#include <stdio.h>
+
+void print_num(int num) {
+    printf("[from c]: num is %d\n", num);
+}