add support readlink|readlinkat

Author: Mic92

CHANGELOG.md

@@ -6,7 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 <!--### Added-->
-- Added `openat`, `fstatat` in `::nix::unistd`
+- Added `nix::unistd::{openat, fstatat, readlink, readlinkat}`
   ([#497](https://github.com/nix-rust/nix/pull/551))
 
 ### Changed

src/fcntl.rs

@@ -1,7 +1,8 @@
 use {Errno, Result, NixPath};
-use libc::{self, c_int, c_uint};
+use libc::{self, c_int, c_uint, c_char, size_t};
 use sys::stat::Mode;
 use std::os::unix::io::RawFd;
+use std::ffi::CStr;
 
 #[cfg(any(target_os = "linux", target_os = "android"))]
 use sys::uio::IoVec;  // For vmsplice
@@ -34,6 +35,31 @@ pub fn openat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P, oflag: OFlag, mode: M
     Errno::result(fd)
 }
 
+pub fn readlink<'a, P: ?Sized + NixPath>(path: &P, buffer: &'a mut [u8]) -> Result<&'a CStr> {
+    let res = try!(path.with_nix_path(|cstr| {
+        unsafe { libc::readlink(cstr.as_ptr(), buffer.as_mut_ptr() as *mut c_char, buffer.len() as size_t) }
+    }));
+
+    Errno::result(res).map(|_| {                     
+        let len = buffer.len();
+        buffer[len - 1] = 0; // ensure always null-terminated
+        unsafe { CStr::from_ptr(buffer.as_ptr() as *const c_char) }
+    })
+}
+
+
+pub fn readlinkat<'a, P: ?Sized + NixPath>(dirfd: RawFd, path: &P, buffer: &'a mut [u8]) -> Result<&'a CStr> {
+    let res = try!(path.with_nix_path(|cstr| {
+        unsafe { libc::readlinkat(dirfd, cstr.as_ptr(), buffer.as_mut_ptr() as *mut c_char, buffer.len() as size_t) }
+    }));
+
+    Errno::result(res).map(|_| {                     
+        let len = buffer.len();
+        buffer[len - 1] = 0; // ensure always null-terminated
+        unsafe { CStr::from_ptr(buffer.as_ptr() as *const c_char) }
+    })
+}
+
 pub enum FcntlArg<'a> {
     F_DUPFD(RawFd),
     F_DUPFD_CLOEXEC(RawFd),

test/test_fcntl.rs

@@ -1,8 +1,10 @@
-use nix::fcntl::{openat, open, O_PATH, O_RDONLY};
+use nix::fcntl::{openat, open, O_PATH, O_RDONLY, readlink, readlinkat};
 use nix::sys::stat::Mode;
 use nix::unistd::{close, read};
+use tempdir::TempDir;
 use tempfile::NamedTempFile;
 use std::io::prelude::*;
+use std::os::unix::fs;
 
 #[test]
 fn test_openat() {
@@ -26,6 +28,25 @@ fn test_openat() {
     close(dirfd).unwrap();
 }
 
+#[test]
+fn test_readlink() {
+    let tempdir = TempDir::new("nix-test_readdir")
+        .unwrap_or_else(|e| panic!("tempdir failed: {}", e));
+    let src = tempdir.path().join("a");
+    let dst = tempdir.path().join("b");
+    println!("a: {:?}, b: {:?}", &src, &dst);
+    fs::symlink(&src.as_path(), &dst.as_path()).unwrap();
+    let dirfd = open(tempdir.path(),
+                     O_PATH,
+                     Mode::empty()).unwrap();
+
+    let mut buf = vec![0; src.to_str().unwrap().len() + 1];
+    assert_eq!(readlink(&dst, &mut buf).unwrap().to_str().unwrap(),
+               src.to_str().unwrap());
+    assert_eq!(readlinkat(dirfd, "b", &mut buf).unwrap().to_str().unwrap(),
+               src.to_str().unwrap());
+}
+
 #[cfg(any(target_os = "linux", target_os = "android"))]
 mod linux_android {
     use std::io::prelude::*;