Add example of binding C

Cargo.toml

@@ -21,6 +21,9 @@ x86_64 = "0.1.2"
 once = "0.3.3"
 linked_list_allocator = "0.5.0"
 
+[build-dependencies]
+cc = "1.0"
+
 [dependencies.lazy_static]
 version = "1.0.0"
 features = ["spin_no_std"]

build.rs

@@ -0,0 +1,16 @@
+extern crate cc;
+use std::process::Command;
+
+fn main() {
+	let output = Command::new("uname").output()
+						 .expect("failed to get uname");
+	let compiler = match output.stdout.as_slice() {
+		b"Darwin\n" => "x86_64-elf-gcc",
+		b"Linux\n" => "gcc",
+		_ => panic!("unknown os")
+	};
+    cc::Build::new()
+		.compiler(compiler)
+        .file("src/test.c")
+        .compile("cobj");
+}

src/lib.rs

@@ -46,6 +46,7 @@ mod arch;
 #[no_mangle]
 pub extern "C" fn rust_main(multiboot_information_address: usize) {
     // ATTENTION: we have a very small stack and no guard page
+    test!(extern_fn);
     test!(find_mp);
 
     arch::driver::acpi::init();
@@ -80,6 +81,14 @@ pub const HEAP_SIZE: usize = 100 * 1024; // 100 KiB
 static HEAP_ALLOCATOR: LockedHeap = LockedHeap::empty();
 
 mod test {
+    extern {
+        fn square(x: u32) -> u32;
+    }
+
+    pub fn extern_fn() {
+        assert_eq!(unsafe{square(2)}, 4);
+    }
+
     pub fn global_allocator() {
         for i in 0..10000 {
             format!("Some String");

src/test.c

@@ -0,0 +1,3 @@
+int square(int x) {
+	return x * x;
+}