src/libstd/sys/common/thread_local.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! OS-based thread local storage
  12 //!
  13 //! This module provides an implementation of OS-based thread local storage,
  14 //! using the native OS-provided facilities (think `TlsAlloc` or
  15 //! `pthread_setspecific`). The interface of this differs from the other types
  16 //! of thread-local-storage provided in this crate in that OS-based TLS can only
  17 //! get/set pointers,
  18 //!
  19 //! This module also provides two flavors of TLS. One is intended for static
  20 //! initialization, and does not contain a `Drop` implementation to deallocate
  21 //! the OS-TLS key. The other is a type which does implement `Drop` and hence
  22 //! has a safe interface.
  23 //!
  24 //! # Usage
  25 //!
  26 //! This module should likely not be used directly unless other primitives are
  27 //! being built on. types such as `thread_local::spawn::Key` are likely much
  28 //! more useful in practice than this OS-based version which likely requires
  29 //! unsafe code to interoperate with.
  30 //!
  31 //! # Examples
  32 //!
  33 //! Using a dynamically allocated TLS key. Note that this key can be shared
  34 //! among many threads via an `Arc`.
  35 //!
  36 //! ```rust,ignore
  37 //! let key = Key::new(None);
  38 //! assert!(key.get().is_null());
  39 //! key.set(1 as *mut u8);
  40 //! assert!(!key.get().is_null());
  41 //!
  42 //! drop(key); // deallocate this TLS slot.
  43 //! ```
  44 //!
  45 //! Sometimes a statically allocated key is either required or easier to work
  46 //! with, however.
  47 //!
  48 //! ```rust,ignore
  49 //! static KEY: StaticKey = INIT;
  50 //!
  51 //! unsafe {
  52 //!     assert!(KEY.get().is_null());
  53 //!     KEY.set(1 as *mut u8);
  54 //! }
  55 //! ```
  56
  57 #![allow(non_camel_case_types)]
  58 #![unstable(feature = "thread_local_internals")]
  59 #![allow(dead_code)] // sys isn't exported yet
  60
  61 use prelude::v1::*;
  62
  63 use sync::atomic::{self, AtomicUsize, Ordering};
  64
  65 use sys::thread_local as imp;
  66
  67 /// A type for TLS keys that are statically allocated.
  68 ///
  69 /// This type is entirely `unsafe` to use as it does not protect against
  70 /// use-after-deallocation or use-during-deallocation.
  71 ///
  72 /// The actual OS-TLS key is lazily allocated when this is used for the first
  73 /// time. The key is also deallocated when the Rust runtime exits or `destroy`
  74 /// is called, whichever comes first.
  75 ///
  76 /// # Examples
  77 ///
  78 /// ```ignore
  79 /// use tls::os::{StaticKey, INIT};
  80 ///
  81 /// static KEY: StaticKey = INIT;
  82 ///
  83 /// unsafe {
  84 ///     assert!(KEY.get().is_null());
  85 ///     KEY.set(1 as *mut u8);
  86 /// }
  87 /// ```
  88 pub struct StaticKey {
  89     /// Inner static TLS key (internals), created with by `INIT_INNER` in this
  90     /// module.
  91     pub inner: StaticKeyInner,
  92     /// Destructor for the TLS value.
  93     ///
  94     /// See `Key::new` for information about when the destructor runs and how
  95     /// it runs.
  96     pub dtor: Option<unsafe extern fn(*mut u8)>,
  97 }
  98
  99 /// Inner contents of `StaticKey`, created by the `INIT_INNER` constant.
 100 pub struct StaticKeyInner {
 101     key: AtomicUsize,
 102 }
 103
 104 /// A type for a safely managed OS-based TLS slot.
 105 ///
 106 /// This type allocates an OS TLS key when it is initialized and will deallocate
 107 /// the key when it falls out of scope. When compared with `StaticKey`, this
 108 /// type is entirely safe to use.
 109 ///
 110 /// Implementations will likely, however, contain unsafe code as this type only
 111 /// operates on `*mut u8`, an unsafe pointer.
 112 ///
 113 /// # Examples
 114 ///
 115 /// ```rust,ignore
 116 /// use tls::os::Key;
 117 ///
 118 /// let key = Key::new(None);
 119 /// assert!(key.get().is_null());
 120 /// key.set(1 as *mut u8);
 121 /// assert!(!key.get().is_null());
 122 ///
 123 /// drop(key); // deallocate this TLS slot.
 124 /// ```
 125 pub struct Key {
 126     key: imp::Key,
 127 }
 128
 129 /// Constant initialization value for static TLS keys.
 130 ///
 131 /// This value specifies no destructor by default.
 132 pub const INIT: StaticKey = StaticKey {
 133     inner: INIT_INNER,
 134     dtor: None,
 135 };
 136
 137 /// Constant initialization value for the inner part of static TLS keys.
 138 ///
 139 /// This value allows specific configuration of the destructor for a TLS key.
 140 pub const INIT_INNER: StaticKeyInner = StaticKeyInner {
 141     key: atomic::ATOMIC_USIZE_INIT,
 142 };
 143
 144 impl StaticKey {
 145     /// Gets the value associated with this TLS key
 146     ///
 147     /// This will lazily allocate a TLS key from the OS if one has not already
 148     /// been allocated.
 149     #[inline]
 150     pub unsafe fn get(&self) -> *mut u8 { imp::get(self.key()) }
 151
 152     /// Sets this TLS key to a new value.
 153     ///
 154     /// This will lazily allocate a TLS key from the OS if one has not already
 155     /// been allocated.
 156     #[inline]
 157     pub unsafe fn set(&self, val: *mut u8) { imp::set(self.key(), val) }
 158
 159     /// Deallocates this OS TLS key.
 160     ///
 161     /// This function is unsafe as there is no guarantee that the key is not
 162     /// currently in use by other threads or will not ever be used again.
 163     ///
 164     /// Note that this does *not* run the user-provided destructor if one was
 165     /// specified at definition time. Doing so must be done manually.
 166     pub unsafe fn destroy(&self) {
 167         match self.inner.key.swap(0, Ordering::SeqCst) {
 168             0 => {}
 169             n => { imp::destroy(n as imp::Key) }
 170         }
 171     }
 172
 173     #[inline]
 174     unsafe fn key(&self) -> imp::Key {
 175         match self.inner.key.load(Ordering::Relaxed) {
 176             0 => self.lazy_init() as imp::Key,
 177             n => n as imp::Key
 178         }
 179     }
 180
 181     unsafe fn lazy_init(&self) -> usize {
 182         // POSIX allows the key created here to be 0, but the compare_and_swap
 183         // below relies on using 0 as a sentinel value to check who won the
 184         // race to set the shared TLS key. As far as I know, there is no
 185         // guaranteed value that cannot be returned as a posix_key_create key,
 186         // so there is no value we can initialize the inner key with to
 187         // prove that it has not yet been set. As such, we'll continue using a
 188         // value of 0, but with some gyrations to make sure we have a non-0
 189         // value returned from the creation routine.
 190         // FIXME: this is clearly a hack, and should be cleaned up.
 191         let key1 = imp::create(self.dtor);
 192         let key = if key1 != 0 {
 193             key1
 194         } else {
 195             let key2 = imp::create(self.dtor);
 196             imp::destroy(key1);
 197             key2
 198         };
 199         assert!(key != 0);
 200         match self.inner.key.compare_and_swap(0, key as usize, Ordering::SeqCst) {
 201             // The CAS succeeded, so we've created the actual key
 202             0 => key as usize,
 203             // If someone beat us to the punch, use their key instead
 204             n => { imp::destroy(key); n }
 205         }
 206     }
 207 }
 208
 209 impl Key {
 210     /// Creates a new managed OS TLS key.
 211     ///
 212     /// This key will be deallocated when the key falls out of scope.
 213     ///
 214     /// The argument provided is an optionally-specified destructor for the
 215     /// value of this TLS key. When a thread exits and the value for this key
 216     /// is non-null the destructor will be invoked. The TLS value will be reset
 217     /// to null before the destructor is invoked.
 218     ///
 219     /// Note that the destructor will not be run when the `Key` goes out of
 220     /// scope.
 221     #[inline]
 222     pub fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
 223         Key { key: unsafe { imp::create(dtor) } }
 224     }
 225
 226     /// See StaticKey::get
 227     #[inline]
 228     pub fn get(&self) -> *mut u8 {
 229         unsafe { imp::get(self.key) }
 230     }
 231
 232     /// See StaticKey::set
 233     #[inline]
 234     pub fn set(&self, val: *mut u8) {
 235         unsafe { imp::set(self.key, val) }
 236     }
 237 }
 238
 239 impl Drop for Key {
 240     fn drop(&mut self) {
 241         unsafe { imp::destroy(self.key) }
 242     }
 243 }
 244
 245 #[cfg(test)]
 246 mod tests {
 247     use prelude::v1::*;
 248     use super::{Key, StaticKey, INIT_INNER};
 249
 250     fn assert_sync<T: Sync>() {}
 251     fn assert_send<T: Send>() {}
 252
 253     #[test]
 254     fn smoke() {
 255         assert_sync::<Key>();
 256         assert_send::<Key>();
 257
 258         let k1 = Key::new(None);
 259         let k2 = Key::new(None);
 260         assert!(k1.get().is_null());
 261         assert!(k2.get().is_null());
 262         k1.set(1 as *mut _);
 263         k2.set(2 as *mut _);
 264         assert_eq!(k1.get() as usize, 1);
 265         assert_eq!(k2.get() as usize, 2);
 266     }
 267
 268     #[test]
 269     fn statik() {
 270         static K1: StaticKey = StaticKey { inner: INIT_INNER, dtor: None };
 271         static K2: StaticKey = StaticKey { inner: INIT_INNER, dtor: None };
 272
 273         unsafe {
 274             assert!(K1.get().is_null());
 275             assert!(K2.get().is_null());
 276             K1.set(1 as *mut _);
 277             K2.set(2 as *mut _);
 278             assert_eq!(K1.get() as usize, 1);
 279             assert_eq!(K2.get() as usize, 2);
 280         }
 281     }
 282 }