rust/rust-error.md

@@ -373,6 +373,52 @@ fn main() -> Result<(), std::io::Error> {
 
 ## `Box<dyn std::error::Error>`周りでlifetime関連のエラーが発生した場合、思い切って、`thiserror`ですべてエラーを統一した方が良い
 
+## libcのerronoの値に対応するエラーメッセージを返したい
+* libcのerrnoに応じたメッセージの取得方法: [rust - How does one get the error message as provided by the system without the "os error n" suffix? - Stack Overflow]( https://stackoverflow.com/questions/40710115/how-does-one-get-the-error-message-as-provided-by-the-system-without-the-os-err )
+* strerrorの参考link: [rust - How does one get the error message as provided by the system without the "os error n" suffix? - Stack Overflow]( https://stackoverflow.com/questions/40710115/how-does-one-get-the-error-message-as-provided-by-the-system-without-the-os-err )
+
+``` rust
+use thiserror::Error;
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;
+
+#[derive(Error, Debug)]
+pub enum Error {
+    #[error("libc error({errno}): {message}")]
+    LibcError { errno: i32, message: String },
+}
+
+use std::ffi::CStr;
+use std::os::raw::{c_char, c_int};
+use std::str;
+
+const TMPBUF_SZ: usize = 128;
+
+pub fn error_string(errno: i32) -> String {
+    extern "C" {
+        #[cfg_attr(
+            any(target_os = "linux", target_env = "newlib"),
+            link_name = "__xpg_strerror_r"
+        )]
+        fn strerror_r(errnum: c_int, buf: *mut c_char, buflen: libc::size_t) -> c_int;
+    }
+
+    let mut buf = [0 as c_char; TMPBUF_SZ];
+
+    let p = buf.as_mut_ptr();
+    unsafe {
+        if strerror_r(errno as c_int, p, buf.len()) < 0 {
+            panic!("strerror_r failure");
+        }
+
+        let p = p as *const _;
+        str::from_utf8(CStr::from_ptr(p).to_bytes())
+            .unwrap()
+            .to_owned()
+    }
+}
+```
+
 ## 型変換
 
 ### エラーの自動型変換