From edb350a3aa63e7c92ced84ee3fd76a6d65fcff73 Mon Sep 17 00:00:00 2001
From: mqy <meng.qingyou@gmail.com>
Date: Mon, 21 Feb 2022 15:52:00 +0800
Subject: [PATCH] ffi: explicitly declare hello_from_rust for C99

---
 src/ffi.md | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/ffi.md b/src/ffi.md
index 9e02eee8..018f4f35 100644
--- a/src/ffi.md
+++ b/src/ffi.md
@@ -281,6 +281,10 @@ We'll create a C file to call the `hello_from_rust` function and compile it by `
 C file should look like:
 
 ```c
+// Explicitly declare to avoid error like this:
+// implicit declaration of function 'hello_from_rust' is invalid in C99 ...
+extern void hello_from_rust();
+
 int main(void) {
     hello_from_rust();
     return 0;