Added if_indextoname function. (nix-rust#2340)

* Added if_indextoname function.

* Added changelog.

* Fixed pointer cast.

* Applied suggestions.

changelog/2340.added.md

@@ -0,0 +1 @@
+Add if_indextoname function.

src/net/if_.rs

@@ -3,9 +3,9 @@
 //! Uses Linux and/or POSIX functions to resolve interface names like "eth0"
 //! or "socan1" into device numbers.
 
-use std::fmt;
-use crate::{Error, NixPath, Result};
-use libc::c_uint;
+use std::{ffi::{CStr, CString}, fmt};
+use crate::{errno::Errno, Error, NixPath, Result};
+use libc::{c_uint, IF_NAMESIZE};
 
 #[cfg(not(solarish))]
 /// type alias for InterfaceFlags
@@ -14,7 +14,7 @@ pub type IflagsType = libc::c_int;
 /// type alias for InterfaceFlags
 pub type IflagsType = libc::c_longlong;
 
-/// Resolve an interface into a interface number.
+/// Resolve an interface into an interface number.
 pub fn if_nametoindex<P: ?Sized + NixPath>(name: &P) -> Result<c_uint> {
     let if_index = name
         .with_nix_path(|name| unsafe { libc::if_nametoindex(name.as_ptr()) })?;
@@ -26,6 +26,19 @@ pub fn if_nametoindex<P: ?Sized + NixPath>(name: &P) -> Result<c_uint> {
     }
 }
 
+/// Resolve an interface number into an interface.
+pub fn if_indextoname(index: c_uint) -> Result<CString> {
+    // We need to allocate this anyway, so doing it directly is faster.
+    let mut buf = vec![0u8; IF_NAMESIZE];
+
+    let return_buf = unsafe {
+        libc::if_indextoname(index, buf.as_mut_ptr().cast())
+    };
+
+    Errno::result(return_buf.cast())?;
+    Ok(CStr::from_bytes_until_nul(buf.as_slice()).unwrap().to_owned())
+}
+
 libc_bitflags!(
     /// Standard interface flags, used by `getifaddrs`
     pub struct InterfaceFlags: IflagsType {

test/test_net.rs

@@ -13,3 +13,14 @@ const LOOPBACK: &[u8] = b"loop";
 fn test_if_nametoindex() {
     if_nametoindex(LOOPBACK).expect("assertion failed");
 }
+
+#[test]
+fn test_if_indextoname() {
+    let loopback_index = if_nametoindex(LOOPBACK).expect("assertion failed");
+    assert_eq!(
+        if_indextoname(loopback_index)
+            .expect("assertion failed")
+            .as_bytes(),
+        LOOPBACK
+    );
+}