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