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src/unsafe/extern-functions.md

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+# Calling External Code
+
+Functions from other languages might violate the guarantees of Rust. Calling
+them is thus unsafe:
+
+```rust,editable
+extern "C" {
+    fn abs(input: i32) -> i32;
+}
+
+fn main() {
+    unsafe {
+        // Undefined behavior if abs misbehaves.
+        println!("Absolute value of -3 according to C: {}", abs(-3));
+    }
+}
+```