sys/stat: add a safe wrapper for mknodat(2)

This introduces a new `mknodat` helper.

Ref: https://pubs.opengroup.org/onlinepubs/9699919799/functions/mknod.html

CHANGELOG.md

@@ -8,6 +8,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 
 - Added `IPV6_V6ONLY` sockopt.
   (#[1470](https://github.com/nix-rust/nix/pull/1470))
+- Added `mknodat`.
+  (#[1473](https://github.com/nix-rust/nix/pull/1473))
 
 ### Changed
 

src/sys/stat.rs

@@ -9,6 +9,7 @@ use std::os::unix::io::RawFd;
 use crate::sys::time::{TimeSpec, TimeVal};
 
 libc_bitflags!(
+    /// "File type" flags for `mknod` and related functions.
     pub struct SFlag: mode_t {
         S_IFIFO;
         S_IFCHR;
@@ -22,6 +23,7 @@ libc_bitflags!(
 );
 
 libc_bitflags! {
+    /// "File mode / permissions" flags.
     pub struct Mode: mode_t {
         S_IRWXU;
         S_IRUSR;
@@ -41,11 +43,26 @@ libc_bitflags! {
     }
 }
 
+/// Create a special or ordinary file, by pathname.
 pub fn mknod<P: ?Sized + NixPath>(path: &P, kind: SFlag, perm: Mode, dev: dev_t) -> Result<()> {
-    let res = path.with_nix_path(|cstr| {
-        unsafe {
-            libc::mknod(cstr.as_ptr(), kind.bits | perm.bits() as mode_t, dev)
-        }
+    let res = path.with_nix_path(|cstr| unsafe {
+        libc::mknod(cstr.as_ptr(), kind.bits | perm.bits() as mode_t, dev)
+    })?;
+
+    Errno::result(res).map(drop)
+}
+
+/// Create a special or ordinary file, relative to a given directory.
+#[cfg(not(any(target_os = "ios", target_os = "macos", target_os = "redox")))]
+pub fn mknodat<P: ?Sized + NixPath>(
+    dirfd: RawFd,
+    path: &P,
+    kind: SFlag,
+    perm: Mode,
+    dev: dev_t,
+) -> Result<()> {
+    let res = path.with_nix_path(|cstr| unsafe {
+        libc::mknodat(dirfd, cstr.as_ptr(), kind.bits | perm.bits() as mode_t, dev)
     })?;
 
     Errno::result(res).map(drop)