src/libstd/io/stdio.rs

   1 // Copyright 2015 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 use prelude::v1::*;
  12 use io::prelude::*;
  13
  14 use cell::{RefCell, BorrowState};
  15 use cmp;
  16 use fmt;
  17 use io::lazy::Lazy;
  18 use io::{self, BufReader, LineWriter};
  19 use sync::{Arc, Mutex, MutexGuard};
  20 use sys::stdio;
  21 use sys_common::io::{read_to_end_uninitialized};
  22 use sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
  23 use thread::LocalKeyState;
  24
  25 /// Stdout used by print! and println! macros
  26 thread_local! {
  27     static LOCAL_STDOUT: RefCell<Option<Box<Write + Send>>> = {
  28         RefCell::new(None)
  29     }
  30 }
  31
  32 /// A handle to a raw instance of the standard input stream of this process.
  33 ///
  34 /// This handle is not synchronized or buffered in any fashion. Constructed via
  35 /// the `std::io::stdio::stdin_raw` function.
  36 struct StdinRaw(stdio::Stdin);
  37
  38 /// A handle to a raw instance of the standard output stream of this process.
  39 ///
  40 /// This handle is not synchronized or buffered in any fashion. Constructed via
  41 /// the `std::io::stdio::stdout_raw` function.
  42 struct StdoutRaw(stdio::Stdout);
  43
  44 /// A handle to a raw instance of the standard output stream of this process.
  45 ///
  46 /// This handle is not synchronized or buffered in any fashion. Constructed via
  47 /// the `std::io::stdio::stderr_raw` function.
  48 struct StderrRaw(stdio::Stderr);
  49
  50 /// Constructs a new raw handle to the standard input of this process.
  51 ///
  52 /// The returned handle does not interact with any other handles created nor
  53 /// handles returned by `std::io::stdin`. Data buffered by the `std::io::stdin`
  54 /// handles is **not** available to raw handles returned from this function.
  55 ///
  56 /// The returned handle has no external synchronization or buffering.
  57 fn stdin_raw() -> io::Result<StdinRaw> { stdio::Stdin::new().map(StdinRaw) }
  58
  59 /// Constructs a new raw handle to the standard output stream of this process.
  60 ///
  61 /// The returned handle does not interact with any other handles created nor
  62 /// handles returned by `std::io::stdout`. Note that data is buffered by the
  63 /// `std::io::stdout` handles so writes which happen via this raw handle may
  64 /// appear before previous writes.
  65 ///
  66 /// The returned handle has no external synchronization or buffering layered on
  67 /// top.
  68 fn stdout_raw() -> io::Result<StdoutRaw> { stdio::Stdout::new().map(StdoutRaw) }
  69
  70 /// Constructs a new raw handle to the standard error stream of this process.
  71 ///
  72 /// The returned handle does not interact with any other handles created nor
  73 /// handles returned by `std::io::stderr`.
  74 ///
  75 /// The returned handle has no external synchronization or buffering layered on
  76 /// top.
  77 fn stderr_raw() -> io::Result<StderrRaw> { stdio::Stderr::new().map(StderrRaw) }
  78
  79 impl Read for StdinRaw {
  80     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> { self.0.read(buf) }
  81 }
  82 impl Write for StdoutRaw {
  83     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.0.write(buf) }
  84     fn flush(&mut self) -> io::Result<()> { Ok(()) }
  85 }
  86 impl Write for StderrRaw {
  87     fn write(&mut self, buf: &[u8]) -> io::Result<usize> { self.0.write(buf) }
  88     fn flush(&mut self) -> io::Result<()> { Ok(()) }
  89 }
  90
  91 enum Maybe<T> {
  92     Real(T),
  93     Fake,
  94 }
  95
  96 impl<W: io::Write> io::Write for Maybe<W> {
  97     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
  98         match *self {
  99             Maybe::Real(ref mut w) => handle_ebadf(w.write(buf), buf.len()),
 100             Maybe::Fake => Ok(buf.len())
 101         }
 102     }
 103
 104     fn flush(&mut self) -> io::Result<()> {
 105         match *self {
 106             Maybe::Real(ref mut w) => handle_ebadf(w.flush(), ()),
 107             Maybe::Fake => Ok(())
 108         }
 109     }
 110 }
 111
 112 impl<R: io::Read> io::Read for Maybe<R> {
 113     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
 114         match *self {
 115             Maybe::Real(ref mut r) => handle_ebadf(r.read(buf), 0),
 116             Maybe::Fake => Ok(0)
 117         }
 118     }
 119 }
 120
 121 fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
 122     #[cfg(windows)]
 123     const ERR: i32 = ::sys::c::ERROR_INVALID_HANDLE as i32;
 124     #[cfg(not(windows))]
 125     const ERR: i32 = ::libc::EBADF as i32;
 126
 127     match r {
 128         Err(ref e) if e.raw_os_error() == Some(ERR) => Ok(default),
 129         r => r
 130     }
 131 }
 132
 133 /// A handle to the standard input stream of a process.
 134 ///
 135 /// Each handle is a shared reference to a global buffer of input data to this
 136 /// process. A handle can be `lock`'d to gain full access to [`BufRead`] methods
 137 /// (e.g. `.lines()`). Writes to this handle are otherwise locked with respect
 138 /// to other writes.
 139 ///
 140 /// This handle implements the `Read` trait, but beware that concurrent reads
 141 /// of `Stdin` must be executed with care.
 142 ///
 143 /// Created by the [`io::stdin`] method.
 144 ///
 145 /// [`io::stdin`]: fn.stdin.html
 146 /// [`BufRead`]: trait.BufRead.html
 147 #[stable(feature = "rust1", since = "1.0.0")]
 148 pub struct Stdin {
 149     inner: Arc<Mutex<BufReader<Maybe<StdinRaw>>>>,
 150 }
 151
 152 /// A locked reference to the `Stdin` handle.
 153 ///
 154 /// This handle implements both the [`Read`] and [`BufRead`] traits, and
 155 /// is constructed via the [`Stdin::lock`] method.
 156 ///
 157 /// [`Read`]: trait.Read.html
 158 /// [`BufRead`]: trait.BufRead.html
 159 /// [`Stdin::lock`]: struct.Stdin.html#method.lock
 160 #[stable(feature = "rust1", since = "1.0.0")]
 161 pub struct StdinLock<'a> {
 162     inner: MutexGuard<'a, BufReader<Maybe<StdinRaw>>>,
 163 }
 164
 165 /// Constructs a new handle to the standard input of the current process.
 166 ///
 167 /// Each handle returned is a reference to a shared global buffer whose access
 168 /// is synchronized via a mutex. If you need more explicit control over
 169 /// locking, see the [`lock() method`][lock].
 170 ///
 171 /// [lock]: struct.Stdin.html#method.lock
 172 ///
 173 /// # Examples
 174 ///
 175 /// Using implicit synchronization:
 176 ///
 177 /// ```
 178 /// use std::io::{self, Read};
 179 ///
 180 /// # fn foo() -> io::Result<String> {
 181 /// let mut buffer = String::new();
 182 /// try!(io::stdin().read_to_string(&mut buffer));
 183 /// # Ok(buffer)
 184 /// # }
 185 /// ```
 186 ///
 187 /// Using explicit synchronization:
 188 ///
 189 /// ```
 190 /// use std::io::{self, Read};
 191 ///
 192 /// # fn foo() -> io::Result<String> {
 193 /// let mut buffer = String::new();
 194 /// let stdin = io::stdin();
 195 /// let mut handle = stdin.lock();
 196 ///
 197 /// try!(handle.read_to_string(&mut buffer));
 198 /// # Ok(buffer)
 199 /// # }
 200 /// ```
 201 #[stable(feature = "rust1", since = "1.0.0")]
 202 pub fn stdin() -> Stdin {
 203     static INSTANCE: Lazy<Mutex<BufReader<Maybe<StdinRaw>>>> = Lazy::new(stdin_init);
 204     return Stdin {
 205         inner: INSTANCE.get().expect("cannot access stdin during shutdown"),
 206     };
 207
 208     fn stdin_init() -> Arc<Mutex<BufReader<Maybe<StdinRaw>>>> {
 209         let stdin = match stdin_raw() {
 210             Ok(stdin) => Maybe::Real(stdin),
 211             _ => Maybe::Fake
 212         };
 213
 214         // The default buffer capacity is 64k, but apparently windows
 215         // doesn't like 64k reads on stdin. See #13304 for details, but the
 216         // idea is that on windows we use a slightly smaller buffer that's
 217         // been seen to be acceptable.
 218         Arc::new(Mutex::new(if cfg!(windows) {
 219             BufReader::with_capacity(8 * 1024, stdin)
 220         } else {
 221             BufReader::new(stdin)
 222         }))
 223     }
 224 }
 225
 226 impl Stdin {
 227     /// Locks this handle to the standard input stream, returning a readable
 228     /// guard.
 229     ///
 230     /// The lock is released when the returned lock goes out of scope. The
 231     /// returned guard also implements the [`Read`] and [`BufRead`] traits for
 232     /// accessing the underlying data.
 233     ///
 234     /// [`Read`]: trait.Read.html
 235     /// [`BufRead`]: trait.BufRead.html
 236     #[stable(feature = "rust1", since = "1.0.0")]
 237     pub fn lock(&self) -> StdinLock {
 238         StdinLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
 239     }
 240
 241     /// Locks this handle and reads a line of input into the specified buffer.
 242     ///
 243     /// For detailed semantics of this method, see the documentation on
 244     /// [`BufRead::read_line`].
 245     ///
 246     /// [`BufRead::read_line`]: trait.BufRead.html#method.read_line
 247     ///
 248     /// # Examples
 249     ///
 250     /// ```no_run
 251     /// use std::io;
 252     ///
 253     /// let mut input = String::new();
 254     /// match io::stdin().read_line(&mut input) {
 255     ///     Ok(n) => {
 256     ///         println!("{} bytes read", n);
 257     ///         println!("{}", input);
 258     ///     }
 259     ///     Err(error) => println!("error: {}", error),
 260     /// }
 261     /// ```
 262     ///
 263     /// You can run the example one of two ways:
 264     ///
 265     /// - Pipe some text to it, e.g. `printf foo | path/to/executable`
 266     /// - Give it text interactively by running the executable directly,
 267     ///   in which case it will wait for the Enter key to be pressed before
 268     ///   continuing
 269     #[stable(feature = "rust1", since = "1.0.0")]
 270     pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
 271         self.lock().read_line(buf)
 272     }
 273 }
 274
 275 #[stable(feature = "rust1", since = "1.0.0")]
 276 impl Read for Stdin {
 277     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
 278         self.lock().read(buf)
 279     }
 280     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
 281         self.lock().read_to_end(buf)
 282     }
 283     fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
 284         self.lock().read_to_string(buf)
 285     }
 286     fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
 287         self.lock().read_exact(buf)
 288     }
 289 }
 290
 291 #[stable(feature = "rust1", since = "1.0.0")]
 292 impl<'a> Read for StdinLock<'a> {
 293     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
 294         self.inner.read(buf)
 295     }
 296     fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
 297         unsafe { read_to_end_uninitialized(self, buf) }
 298     }
 299 }
 300
 301 #[stable(feature = "rust1", since = "1.0.0")]
 302 impl<'a> BufRead for StdinLock<'a> {
 303     fn fill_buf(&mut self) -> io::Result<&[u8]> { self.inner.fill_buf() }
 304     fn consume(&mut self, n: usize) { self.inner.consume(n) }
 305 }
 306
 307 // As with stdin on windows, stdout often can't handle writes of large
 308 // sizes. For an example, see #14940. For this reason, don't try to
 309 // write the entire output buffer on windows. On unix we can just
 310 // write the whole buffer all at once.
 311 //
 312 // For some other references, it appears that this problem has been
 313 // encountered by others [1] [2]. We choose the number 8KB just because
 314 // libuv does the same.
 315 //
 316 // [1]: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/1232
 317 // [2]: http://www.mail-archive.com/log4net-dev@logging.apache.org/msg00661.html
 318 #[cfg(windows)]
 319 const OUT_MAX: usize = 8192;
 320 #[cfg(unix)]
 321 const OUT_MAX: usize = ::usize::MAX;
 322
 323 /// A handle to the global standard output stream of the current process.
 324 ///
 325 /// Each handle shares a global buffer of data to be written to the standard
 326 /// output stream. Access is also synchronized via a lock and explicit control
 327 /// over locking is available via the `lock` method.
 328 ///
 329 /// Created by the [`io::stdout`] method.
 330 ///
 331 /// [`io::stdout`]: fn.stdout.html
 332 #[stable(feature = "rust1", since = "1.0.0")]
 333 pub struct Stdout {
 334     // FIXME: this should be LineWriter or BufWriter depending on the state of
 335     //        stdout (tty or not). Note that if this is not line buffered it
 336     //        should also flush-on-panic or some form of flush-on-abort.
 337     inner: Arc<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>>,
 338 }
 339
 340 /// A locked reference to the `Stdout` handle.
 341 ///
 342 /// This handle implements the [`Write`] trait, and is constructed via
 343 /// the [`Stdout::lock`] method.
 344 ///
 345 /// [`Write`]: trait.Write.html
 346 /// [`Stdout::lock`]: struct.Stdout.html#method.lock
 347 #[stable(feature = "rust1", since = "1.0.0")]
 348 pub struct StdoutLock<'a> {
 349     inner: ReentrantMutexGuard<'a, RefCell<LineWriter<Maybe<StdoutRaw>>>>,
 350 }
 351
 352 /// Constructs a new handle to the standard output of the current process.
 353 ///
 354 /// Each handle returned is a reference to a shared global buffer whose access
 355 /// is synchronized via a mutex. If you need more explicit control over
 356 /// locking, see the [Stdout::lock] method.
 357 ///
 358 /// [Stdout::lock]: struct.Stdout.html#method.lock
 359 ///
 360 /// # Examples
 361 ///
 362 /// Using implicit synchronization:
 363 ///
 364 /// ```
 365 /// use std::io::{self, Write};
 366 ///
 367 /// # fn foo() -> io::Result<()> {
 368 /// try!(io::stdout().write(b"hello world"));
 369 ///
 370 /// # Ok(())
 371 /// # }
 372 /// ```
 373 ///
 374 /// Using explicit synchronization:
 375 ///
 376 /// ```
 377 /// use std::io::{self, Write};
 378 ///
 379 /// # fn foo() -> io::Result<()> {
 380 /// let stdout = io::stdout();
 381 /// let mut handle = stdout.lock();
 382 ///
 383 /// try!(handle.write(b"hello world"));
 384 ///
 385 /// # Ok(())
 386 /// # }
 387 /// ```
 388 #[stable(feature = "rust1", since = "1.0.0")]
 389 pub fn stdout() -> Stdout {
 390     static INSTANCE: Lazy<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>>
 391         = Lazy::new(stdout_init);
 392     return Stdout {
 393         inner: INSTANCE.get().expect("cannot access stdout during shutdown"),
 394     };
 395
 396     fn stdout_init() -> Arc<ReentrantMutex<RefCell<LineWriter<Maybe<StdoutRaw>>>>> {
 397         let stdout = match stdout_raw() {
 398             Ok(stdout) => Maybe::Real(stdout),
 399             _ => Maybe::Fake,
 400         };
 401         Arc::new(ReentrantMutex::new(RefCell::new(LineWriter::new(stdout))))
 402     }
 403 }
 404
 405 impl Stdout {
 406     /// Locks this handle to the standard output stream, returning a writable
 407     /// guard.
 408     ///
 409     /// The lock is released when the returned lock goes out of scope. The
 410     /// returned guard also implements the `Write` trait for writing data.
 411     #[stable(feature = "rust1", since = "1.0.0")]
 412     pub fn lock(&self) -> StdoutLock {
 413         StdoutLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
 414     }
 415 }
 416
 417 #[stable(feature = "rust1", since = "1.0.0")]
 418 impl Write for Stdout {
 419     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
 420         self.lock().write(buf)
 421     }
 422     fn flush(&mut self) -> io::Result<()> {
 423         self.lock().flush()
 424     }
 425     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
 426         self.lock().write_all(buf)
 427     }
 428     fn write_fmt(&mut self, args: fmt::Arguments) -> io::Result<()> {
 429         self.lock().write_fmt(args)
 430     }
 431 }
 432 #[stable(feature = "rust1", since = "1.0.0")]
 433 impl<'a> Write for StdoutLock<'a> {
 434     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
 435         self.inner.borrow_mut().write(&buf[..cmp::min(buf.len(), OUT_MAX)])
 436     }
 437     fn flush(&mut self) -> io::Result<()> {
 438         self.inner.borrow_mut().flush()
 439     }
 440 }
 441
 442 /// A handle to the standard error stream of a process.
 443 ///
 444 /// For more information, see the [`io::stderr`] method.
 445 ///
 446 /// [`io::stderr`]: fn.stderr.html
 447 #[stable(feature = "rust1", since = "1.0.0")]
 448 pub struct Stderr {
 449     inner: Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>>,
 450 }
 451
 452 /// A locked reference to the `Stderr` handle.
 453 ///
 454 /// This handle implements the `Write` trait and is constructed via
 455 /// the [`Stderr::lock`] method.
 456 ///
 457 /// [`Stderr::lock`]: struct.Stderr.html#method.lock
 458 #[stable(feature = "rust1", since = "1.0.0")]
 459 pub struct StderrLock<'a> {
 460     inner: ReentrantMutexGuard<'a, RefCell<Maybe<StderrRaw>>>,
 461 }
 462
 463 /// Constructs a new handle to the standard error of the current process.
 464 ///
 465 /// This handle is not buffered.
 466 ///
 467 /// # Examples
 468 ///
 469 /// Using implicit synchronization:
 470 ///
 471 /// ```
 472 /// use std::io::{self, Write};
 473 ///
 474 /// # fn foo() -> io::Result<()> {
 475 /// try!(io::stderr().write(b"hello world"));
 476 ///
 477 /// # Ok(())
 478 /// # }
 479 /// ```
 480 ///
 481 /// Using explicit synchronization:
 482 ///
 483 /// ```
 484 /// use std::io::{self, Write};
 485 ///
 486 /// # fn foo() -> io::Result<()> {
 487 /// let stderr = io::stderr();
 488 /// let mut handle = stderr.lock();
 489 ///
 490 /// try!(handle.write(b"hello world"));
 491 ///
 492 /// # Ok(())
 493 /// # }
 494 /// ```
 495 #[stable(feature = "rust1", since = "1.0.0")]
 496 pub fn stderr() -> Stderr {
 497     static INSTANCE: Lazy<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> = Lazy::new(stderr_init);
 498     return Stderr {
 499         inner: INSTANCE.get().expect("cannot access stderr during shutdown"),
 500     };
 501
 502     fn stderr_init() -> Arc<ReentrantMutex<RefCell<Maybe<StderrRaw>>>> {
 503         let stderr = match stderr_raw() {
 504             Ok(stderr) => Maybe::Real(stderr),
 505             _ => Maybe::Fake,
 506         };
 507         Arc::new(ReentrantMutex::new(RefCell::new(stderr)))
 508     }
 509 }
 510
 511 impl Stderr {
 512     /// Locks this handle to the standard error stream, returning a writable
 513     /// guard.
 514     ///
 515     /// The lock is released when the returned lock goes out of scope. The
 516     /// returned guard also implements the `Write` trait for writing data.
 517     #[stable(feature = "rust1", since = "1.0.0")]
 518     pub fn lock(&self) -> StderrLock {
 519         StderrLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
 520     }
 521 }
 522
 523 #[stable(feature = "rust1", since = "1.0.0")]
 524 impl Write for Stderr {
 525     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
 526         self.lock().write(buf)
 527     }
 528     fn flush(&mut self) -> io::Result<()> {
 529         self.lock().flush()
 530     }
 531     fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
 532         self.lock().write_all(buf)
 533     }
 534     fn write_fmt(&mut self, args: fmt::Arguments) -> io::Result<()> {
 535         self.lock().write_fmt(args)
 536     }
 537 }
 538 #[stable(feature = "rust1", since = "1.0.0")]
 539 impl<'a> Write for StderrLock<'a> {
 540     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
 541         self.inner.borrow_mut().write(&buf[..cmp::min(buf.len(), OUT_MAX)])
 542     }
 543     fn flush(&mut self) -> io::Result<()> {
 544         self.inner.borrow_mut().flush()
 545     }
 546 }
 547
 548 /// Resets the thread-local stderr handle to the specified writer
 549 ///
 550 /// This will replace the current thread's stderr handle, returning the old
 551 /// handle. All future calls to `panic!` and friends will emit their output to
 552 /// this specified handle.
 553 ///
 554 /// Note that this does not need to be called for all new threads; the default
 555 /// output handle is to the process's stderr stream.
 556 #[unstable(feature = "set_stdio",
 557            reason = "this function may disappear completely or be replaced \
 558                      with a more general mechanism",
 559            issue = "0")]
 560 #[doc(hidden)]
 561 pub fn set_panic(sink: Box<Write + Send>) -> Option<Box<Write + Send>> {
 562     use panicking::LOCAL_STDERR;
 563     use mem;
 564     LOCAL_STDERR.with(move |slot| {
 565         mem::replace(&mut *slot.borrow_mut(), Some(sink))
 566     }).and_then(|mut s| {
 567         let _ = s.flush();
 568         Some(s)
 569     })
 570 }
 571
 572 /// Resets the thread-local stdout handle to the specified writer
 573 ///
 574 /// This will replace the current thread's stdout handle, returning the old
 575 /// handle. All future calls to `print!` and friends will emit their output to
 576 /// this specified handle.
 577 ///
 578 /// Note that this does not need to be called for all new threads; the default
 579 /// output handle is to the process's stdout stream.
 580 #[unstable(feature = "set_stdio",
 581            reason = "this function may disappear completely or be replaced \
 582                      with a more general mechanism",
 583            issue = "0")]
 584 #[doc(hidden)]
 585 pub fn set_print(sink: Box<Write + Send>) -> Option<Box<Write + Send>> {
 586     use mem;
 587     LOCAL_STDOUT.with(move |slot| {
 588         mem::replace(&mut *slot.borrow_mut(), Some(sink))
 589     }).and_then(|mut s| {
 590         let _ = s.flush();
 591         Some(s)
 592     })
 593 }
 594
 595 #[unstable(feature = "print",
 596            reason = "implementation detail which may disappear or be replaced at any time",
 597            issue = "0")]
 598 #[doc(hidden)]
 599 pub fn _print(args: fmt::Arguments) {
 600     // As an implementation of the `println!` macro, we want to try our best to
 601     // not panic wherever possible and get the output somewhere. There are
 602     // currently two possible vectors for panics we take care of here:
 603     //
 604     // 1. If the TLS key for the local stdout has been destroyed, accessing it
 605     //    would cause a panic. Note that we just lump in the uninitialized case
 606     //    here for convenience, we're not trying to avoid a panic.
 607     // 2. If the local stdout is currently in use (e.g. we're in the middle of
 608     //    already printing) then accessing again would cause a panic.
 609     //
 610     // If, however, the actual I/O causes an error, we do indeed panic.
 611     let result = match LOCAL_STDOUT.state() {
 612         LocalKeyState::Uninitialized |
 613         LocalKeyState::Destroyed => stdout().write_fmt(args),
 614         LocalKeyState::Valid => {
 615             LOCAL_STDOUT.with(|s| {
 616                 if s.borrow_state() == BorrowState::Unused {
 617                     if let Some(w) = s.borrow_mut().as_mut() {
 618                         return w.write_fmt(args);
 619                     }
 620                 }
 621                 stdout().write_fmt(args)
 622             })
 623         }
 624     };
 625     if let Err(e) = result {
 626         panic!("failed printing to stdout: {}", e);
 627     }
 628 }
 629
 630 #[cfg(test)]
 631 mod tests {
 632     use thread;
 633     use super::*;
 634
 635     #[test]
 636     fn panic_doesnt_poison() {
 637         thread::spawn(|| {
 638             let _a = stdin();
 639             let _a = _a.lock();
 640             let _a = stdout();
 641             let _a = _a.lock();
 642             let _a = stderr();
 643             let _a = _a.lock();
 644             panic!();
 645         }).join().unwrap_err();
 646
 647         let _a = stdin();
 648         let _a = _a.lock();
 649         let _a = stdout();
 650         let _a = _a.lock();
 651         let _a = stderr();
 652         let _a = _a.lock();
 653     }
 654 }