Merge #1745

1745: Change gethostname to use a buffer of MaybeUninit values r=asomers a=nathaniel-daniel

Changing `gethostname` to accept a buffer of `MaybeUninit` bytes allows the user to avoid needlessly initializing a buffer. This is a breaking API change.

Co-authored-by: Nathaniel Daniel <nathaniel.daniel12@gmail.com>

3 files changed, 19 insertions, 17 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b7c3df5f..424b9d89 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -32,6 +32,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
   (#[1713](https://github.com/nix-rust/nix/pull/1713))
 - `nix::poll::ppoll`: `sigmask` parameter is now optional.
   (#[1739](https://github.com/nix-rust/nix/pull/1739))
+- Changed `gethostname` to return an owned `OsString`.
+  (#[1745](https://github.com/nix-rust/nix/pull/1745))
 
 ### Fixed
 
diff --git a/README.md b/README.md
index 44e620cc..7c13cf20 100644
--- a/README.md
+++ b/README.md
@@ -24,8 +24,8 @@ call:
 // libc api (unsafe, requires handling return code/errno)
 pub unsafe extern fn gethostname(name: *mut c_char, len: size_t) -> c_int;
 
-// nix api (returns a nix::Result<CStr>)
-pub fn gethostname<'a>(buffer: &'a mut [u8]) -> Result<&'a CStr>;
+// nix api (returns a nix::Result<OsString>)
+pub fn gethostname() -> Result<OsString>;
 ```
 
 ## Supported Platforms
diff --git a/src/unistd.rs b/src/unistd.rs
index 4c9b67ff..056b5529 100644
--- a/src/unistd.rs
+++ b/src/unistd.rs
@@ -991,34 +991,34 @@ pub fn sethostname<S: AsRef<OsStr>>(name: S) -> Result<()> {
     Errno::result(res).map(drop)
 }
 
-/// Get the host name and store it in the provided buffer, returning a pointer
-/// the `CStr` in that buffer on success (see
+/// Get the host name and store it in an internally allocated buffer, returning an
+/// `OsString` on success (see
 /// [gethostname(2)](https://pubs.opengroup.org/onlinepubs/9699919799/functions/gethostname.html)).
 ///
 /// This function call attempts to get the host name for the running system and
-/// store it in a provided buffer.  The buffer will be populated with bytes up
-/// to the length of the provided slice including a NUL terminating byte.  If
-/// the hostname is longer than the length provided, no error will be provided.
-/// The posix specification does not specify whether implementations will
-/// null-terminate in this case, but the nix implementation will ensure that the
-/// buffer is null terminated in this case.
+/// store it in an internal buffer, returning it as an `OsString` if successful.
 ///
 /// ```no_run
 /// use nix::unistd;
 ///
-/// let mut buf = [0u8; 64];
-/// let hostname_cstr = unistd::gethostname(&mut buf).expect("Failed getting hostname");
-/// let hostname = hostname_cstr.to_str().expect("Hostname wasn't valid UTF-8");
+/// let hostname = unistd::gethostname().expect("Failed getting hostname");
+/// let hostname = hostname.into_string().expect("Hostname wasn't valid UTF-8");
 /// println!("Hostname: {}", hostname);
 /// ```
-pub fn gethostname(buffer: &mut [u8]) -> Result<&CStr> {
+pub fn gethostname() -> Result<OsString> {
+    // The capacity is the max length of a hostname plus the NUL terminator.
+    let mut buffer: Vec<u8> = Vec::with_capacity(256);
     let ptr = buffer.as_mut_ptr() as *mut c_char;
-    let len = buffer.len() as size_t;
+    let len = buffer.capacity() as size_t;
 
     let res = unsafe { libc::gethostname(ptr, len) };
     Errno::result(res).map(|_| {
-        buffer[len - 1] = 0; // ensure always null-terminated
-        unsafe { CStr::from_ptr(buffer.as_ptr() as *const c_char) }
+        unsafe {
+            buffer.as_mut_ptr().wrapping_add(len - 1).write(0); // ensure always null-terminated
+            let len = CStr::from_ptr(buffer.as_ptr() as *const c_char).len();
+            buffer.set_len(len);
+        }
+        OsString::from_vec(buffer)
     })
 }
 }