1 //! Support for capturing a stack backtrace of an OS thread
3 //! This module contains the support necessary to capture a stack backtrace of a
4 //! running OS thread from the OS thread itself. The `Backtrace` type supports
5 //! capturing a stack trace via the `Backtrace::capture` and
6 //! `Backtrace::force_capture` functions.
8 //! A backtrace is typically quite handy to attach to errors (e.g. types
9 //! implementing `std::error::Error`) to get a causal chain of where an error
12 //! > **Note**: this module is unstable and is designed in [RFC 2504], and you
13 //! > can learn more about its status in the [tracking issue].
15 //! [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
16 //! [tracking issue]: https://github.com/rust-lang/rust/issues/53487
20 //! Backtraces are attempted to be as accurate as possible, but no guarantees
21 //! are provided about the exact accuracy of a backtrace. Instruction pointers,
22 //! symbol names, filenames, line numbers, etc, may all be incorrect when
23 //! reported. Accuracy is attempted on a best-effort basis, however, and bugs
24 //! are always welcome to indicate areas of improvement!
26 //! For most platforms a backtrace with a filename/line number requires that
27 //! programs be compiled with debug information. Without debug information
28 //! filenames/line numbers will not be reported.
30 //! ## Platform support
32 //! Not all platforms that libstd compiles for support capturing backtraces.
33 //! Some platforms simply do nothing when capturing a backtrace. To check
34 //! whether the platform supports capturing backtraces you can consult the
35 //! `BacktraceStatus` enum as a result of `Backtrace::status`.
37 //! Like above with accuracy platform support is done on a best effort basis.
38 //! Sometimes libraries may not be available at runtime or something may go
39 //! wrong which would cause a backtrace to not be captured. Please feel free to
40 //! report issues with platforms where a backtrace cannot be captured though!
42 //! ## Environment Variables
44 //! The `Backtrace::capture` function may not actually capture a backtrace by
45 //! default. Its behavior is governed by two environment variables:
47 //! * `RUST_LIB_BACKTRACE` - if this is set to `0` then `Backtrace::capture`
48 //! will never capture a backtrace. Any other value this is set to will enable
49 //! `Backtrace::capture`.
51 //! * `RUST_BACKTRACE` - if `RUST_LIB_BACKTRACE` is not set, then this variable
52 //! is consulted with the same rules of `RUST_LIB_BACKTRACE`.
54 //! * If neither of the above env vars are set, then `Backtrace::capture` will
57 //! Capturing a backtrace can be a quite expensive runtime operation, so the
58 //! environment variables allow either forcibly disabling this runtime
59 //! performance hit or allow selectively enabling it in some programs.
61 //! Note that the `Backtrace::force_capture` function can be used to ignore
62 //! these environment variables. Also note that the state of environment
63 //! variables is cached once the first backtrace is created, so altering
64 //! `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` at runtime may not actually change
65 //! how backtraces are captured.
67 #![unstable(feature = "backtrace", issue = "53487")]
72 // NB: A note on resolution of a backtrace:
74 // Backtraces primarily happen in two steps, one is where we actually capture
75 // the stack backtrace, giving us a list of instruction pointers corresponding
76 // to stack frames. Next we take these instruction pointers and, one-by-one,
77 // turn them into a human readable name (like `main`).
79 // The first phase can be somewhat expensive (walking the stack), especially
80 // on MSVC where debug information is consulted to return inline frames each as
81 // their own frame. The second phase, however, is almost always extremely
82 // expensive (on the order of milliseconds sometimes) when it's consulting debug
85 // We attempt to amortize this cost as much as possible by delaying resolution
86 // of an address to a human readable name for as long as possible. When
87 // `Backtrace::create` is called to capture a backtrace it doesn't actually
88 // perform any symbol resolution, but rather we lazily resolve symbols only just
89 // before they're needed for printing. This way we can make capturing a
90 // backtrace and throwing it away much cheaper, but actually printing a
91 // backtrace is still basically the same cost.
93 // This strategy comes at the cost of some synchronization required inside of a
94 // `Backtrace`, but that's a relatively small price to pay relative to capturing
95 // a backtrace or actually symbolizing it.
97 use crate::backtrace_rs::{self, BytesOrWideString};
98 use crate::cell::UnsafeCell;
100 use crate::ffi::c_void;
102 use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
103 use crate::sync::Once;
104 use crate::sys_common::backtrace::{lock, output_filename};
107 /// A captured OS thread stack backtrace.
109 /// This type represents a stack backtrace for an OS thread captured at a
110 /// previous point in time. In some instances the `Backtrace` type may
111 /// internally be empty due to configuration. For more information see
112 /// `Backtrace::capture`.
113 pub struct Backtrace {
117 /// The current status of a backtrace, indicating whether it was captured or
118 /// whether it is empty for some other reason.
120 #[derive(Debug, PartialEq, Eq)]
121 pub enum BacktraceStatus {
122 /// Capturing a backtrace is not supported, likely because it's not
123 /// implemented for the current platform.
125 /// Capturing a backtrace has been disabled through either the
126 /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
128 /// A backtrace has been captured and the `Backtrace` should print
129 /// reasonable information when rendered.
136 Captured(LazilyResolvedCapture),
142 frames: Vec<BacktraceFrame>,
145 fn _assert_send_sync() {
146 fn _assert<T: Send + Sync>() {}
147 _assert::<Backtrace>();
150 /// A single frame of a backtrace.
151 #[unstable(feature = "backtrace_frames", issue = "79676")]
152 pub struct BacktraceFrame {
154 symbols: Vec<BacktraceSymbol>,
159 Actual(backtrace_rs::Frame),
164 struct BacktraceSymbol {
165 name: Option<Vec<u8>>,
166 filename: Option<BytesOrWide>,
176 impl fmt::Debug for Backtrace {
177 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
178 let capture = match &self.inner {
179 Inner::Unsupported => return fmt.write_str("<unsupported>"),
180 Inner::Disabled => return fmt.write_str("<disabled>"),
181 Inner::Captured(c) => c.force(),
184 let frames = &capture.frames[capture.actual_start..];
186 write!(fmt, "Backtrace ")?;
188 let mut dbg = fmt.debug_list();
190 for frame in frames {
191 if frame.frame.ip().is_null() {
195 dbg.entries(&frame.symbols);
202 impl fmt::Debug for BacktraceFrame {
203 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
204 let mut dbg = fmt.debug_list();
205 dbg.entries(&self.symbols);
210 impl fmt::Debug for BacktraceSymbol {
211 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
212 // FIXME: improve formatting: https://github.com/rust-lang/rust/issues/65280
213 // FIXME: Also, include column numbers into the debug format as Display already has them.
214 // Until there are stable per-frame accessors, the format shouldn't be changed:
215 // https://github.com/rust-lang/rust/issues/65280#issuecomment-638966585
218 if let Some(fn_name) = self.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)) {
219 write!(fmt, "fn: \"{:#}\"", fn_name)?;
221 write!(fmt, "fn: <unknown>")?;
224 if let Some(fname) = self.filename.as_ref() {
225 write!(fmt, ", file: \"{:?}\"", fname)?;
228 if let Some(line) = self.lineno {
229 write!(fmt, ", line: {:?}", line)?;
236 impl fmt::Debug for BytesOrWide {
237 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
241 BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
242 BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
244 backtrace_rs::PrintFmt::Short,
245 crate::env::current_dir().as_ref().ok(),
251 /// Returns whether backtrace captures are enabled through environment
253 fn enabled() -> bool {
254 // Cache the result of reading the environment variables to make
255 // backtrace captures speedy, because otherwise reading environment
256 // variables every time can be somewhat slow.
257 static ENABLED: AtomicUsize = AtomicUsize::new(0);
258 match ENABLED.load(SeqCst) {
263 let enabled = match env::var("RUST_LIB_BACKTRACE") {
265 Err(_) => match env::var("RUST_BACKTRACE") {
270 ENABLED.store(enabled as usize + 1, SeqCst);
274 /// Capture a stack backtrace of the current thread.
276 /// This function will capture a stack backtrace of the current OS thread of
277 /// execution, returning a `Backtrace` type which can be later used to print
278 /// the entire stack trace or render it to a string.
280 /// This function will be a noop if the `RUST_BACKTRACE` or
281 /// `RUST_LIB_BACKTRACE` backtrace variables are both not set. If either
282 /// environment variable is set and enabled then this function will actually
283 /// capture a backtrace. Capturing a backtrace can be both memory intensive
284 /// and slow, so these environment variables allow liberally using
285 /// `Backtrace::capture` and only incurring a slowdown when the environment
286 /// variables are set.
288 /// To forcibly capture a backtrace regardless of environment variables, use
289 /// the `Backtrace::force_capture` function.
290 #[inline(never)] // want to make sure there's a frame here to remove
291 pub fn capture() -> Backtrace {
292 if !Backtrace::enabled() {
293 return Backtrace { inner: Inner::Disabled };
295 Backtrace::create(Backtrace::capture as usize)
298 /// Forcibly captures a full backtrace, regardless of environment variable
301 /// This function behaves the same as `capture` except that it ignores the
302 /// values of the `RUST_BACKTRACE` and `RUST_LIB_BACKTRACE` environment
303 /// variables, always capturing a backtrace.
305 /// Note that capturing a backtrace can be an expensive operation on some
306 /// platforms, so this should be used with caution in performance-sensitive
308 #[inline(never)] // want to make sure there's a frame here to remove
309 pub fn force_capture() -> Backtrace {
310 Backtrace::create(Backtrace::force_capture as usize)
313 /// Forcibly captures a disabled backtrace, regardless of environment
314 /// variable configuration.
315 pub const fn disabled() -> Backtrace {
316 Backtrace { inner: Inner::Disabled }
319 // Capture a backtrace which start just before the function addressed by
321 fn create(ip: usize) -> Backtrace {
322 // SAFETY: We don't attempt to lock this reentrantly.
323 let _lock = unsafe { lock() };
324 let mut frames = Vec::new();
325 let mut actual_start = None;
327 backtrace_rs::trace_unsynchronized(|frame| {
328 frames.push(BacktraceFrame {
329 frame: RawFrame::Actual(frame.clone()),
332 if frame.symbol_address() as usize == ip && actual_start.is_none() {
333 actual_start = Some(frames.len());
339 // If no frames came out assume that this is an unsupported platform
340 // since `backtrace` doesn't provide a way of learning this right now,
341 // and this should be a good enough approximation.
342 let inner = if frames.is_empty() {
345 Inner::Captured(LazilyResolvedCapture::new(Capture {
346 actual_start: actual_start.unwrap_or(0),
355 /// Returns the status of this backtrace, indicating whether this backtrace
356 /// request was unsupported, disabled, or a stack trace was actually
358 pub fn status(&self) -> BacktraceStatus {
360 Inner::Unsupported => BacktraceStatus::Unsupported,
361 Inner::Disabled => BacktraceStatus::Disabled,
362 Inner::Captured(_) => BacktraceStatus::Captured,
368 /// Returns an iterator over the backtrace frames.
369 #[unstable(feature = "backtrace_frames", issue = "79676")]
370 pub fn frames(&'a self) -> &'a [BacktraceFrame] {
371 if let Inner::Captured(c) = &self.inner { &c.force().frames } else { &[] }
375 impl fmt::Display for Backtrace {
376 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
377 let capture = match &self.inner {
378 Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
379 Inner::Disabled => return fmt.write_str("disabled backtrace"),
380 Inner::Captured(c) => c.force(),
383 let full = fmt.alternate();
384 let (frames, style) = if full {
385 (&capture.frames[..], backtrace_rs::PrintFmt::Full)
387 (&capture.frames[capture.actual_start..], backtrace_rs::PrintFmt::Short)
390 // When printing paths we try to strip the cwd if it exists, otherwise
391 // we just print the path as-is. Note that we also only do this for the
392 // short format, because if it's full we presumably want to print
394 let cwd = crate::env::current_dir();
395 let mut print_path = move |fmt: &mut fmt::Formatter<'_>, path: BytesOrWideString<'_>| {
396 output_filename(fmt, path, style, cwd.as_ref().ok())
399 let mut f = backtrace_rs::BacktraceFmt::new(fmt, style, &mut print_path);
401 for frame in frames {
402 let mut f = f.frame();
403 if frame.symbols.is_empty() {
404 f.print_raw(frame.frame.ip(), None, None, None)?;
406 for symbol in frame.symbols.iter() {
407 f.print_raw_with_column(
409 symbol.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)),
410 symbol.filename.as_ref().map(|b| match b {
411 BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
412 BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
425 struct LazilyResolvedCapture {
427 capture: UnsafeCell<Capture>,
430 impl LazilyResolvedCapture {
431 fn new(capture: Capture) -> Self {
432 LazilyResolvedCapture { sync: Once::new(), capture: UnsafeCell::new(capture) }
435 fn force(&self) -> &Capture {
436 self.sync.call_once(|| {
437 // SAFETY: This exclusive reference can't overlap with any others
438 // `Once` guarantees callers will block until this closure returns
439 // `Once` also guarantees only a single caller will enter this closure
440 unsafe { &mut *self.capture.get() }.resolve();
443 // SAFETY: This shared reference can't overlap with the exclusive reference above
444 unsafe { &*self.capture.get() }
448 // SAFETY: Access to the inner value is synchronized using a thread-safe `Once`
449 // So long as `Capture` is `Sync`, `LazilyResolvedCapture` is too
450 unsafe impl Sync for LazilyResolvedCapture where Capture: Sync {}
453 fn resolve(&mut self) {
454 // If we're already resolved, nothing to do!
458 self.resolved = true;
460 // Use the global backtrace lock to synchronize this as it's a
461 // requirement of the `backtrace` crate, and then actually resolve
463 // SAFETY: We don't attempt to lock this reentrantly.
464 let _lock = unsafe { lock() };
465 for frame in self.frames.iter_mut() {
466 let symbols = &mut frame.symbols;
467 let frame = match &frame.frame {
468 RawFrame::Actual(frame) => frame,
470 RawFrame::Fake => unimplemented!(),
473 backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
474 symbols.push(BacktraceSymbol {
475 name: symbol.name().map(|m| m.as_bytes().to_vec()),
476 filename: symbol.filename_raw().map(|b| match b {
477 BytesOrWideString::Bytes(b) => BytesOrWide::Bytes(b.to_owned()),
478 BytesOrWideString::Wide(b) => BytesOrWide::Wide(b.to_owned()),
480 lineno: symbol.lineno(),
481 colno: symbol.colno(),
490 fn ip(&self) -> *mut c_void {
492 RawFrame::Actual(frame) => frame.ip(),
494 RawFrame::Fake => 1 as *mut c_void,