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    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use io;
    use io::prelude::*;
    use libc;
    use mem;
    use sync::StaticMutex;

    use super::super::printing::print;
    use unwind as uw;

    #[inline(never)] // if we know this is a function call, we can skip it when
                     // tracing
    pub fn write(w: &mut Write) -> io::Result<()> {
        struct Context<'a> {
            idx: isize,
            writer: &'a mut (Write+'a),
            last_error: Option<io::Error>,
        }

        // When using libbacktrace, we use some necessary global state, so we
        // need to prevent more than one thread from entering this block. This
        // is semi-reasonable in terms of printing anyway, and we know that all
        // I/O done here is blocking I/O, not green I/O, so we don't have to
        // worry about this being a native vs green mutex.
        static LOCK: StaticMutex = StaticMutex::new();
        let _g = LOCK.lock();

        writeln!(w, "stack backtrace:")?;

        let mut cx = Context { writer: w, last_error: None, idx: 0 };
        return match unsafe {
            uw::_Unwind_Backtrace(trace_fn,
                                  &mut cx as *mut Context as *mut libc::c_void)
        } {
            uw::_URC_NO_REASON => {
                match cx.last_error {
                    Some(err) => Err(err),
                    None => Ok(())
                }
            }
            _ => Ok(()),
        };

        extern fn trace_fn(ctx: *mut uw::_Unwind_Context,
                           arg: *mut libc::c_void) -> uw::_Unwind_Reason_Code {
            let cx: &mut Context = unsafe { mem::transmute(arg) };
            let mut ip_before_insn = 0;
            let mut ip = unsafe {
                uw::_Unwind_GetIPInfo(ctx, &mut ip_before_insn) as *mut libc::c_void
            };
            if !ip.is_null() && ip_before_insn == 0 {
                // this is a non-signaling frame, so `ip` refers to the address
                // after the calling instruction. account for that.
                ip = (ip as usize - 1) as *mut _;
            }

            // dladdr() on osx gets whiny when we use FindEnclosingFunction, and
            // it appears to work fine without it, so we only use
            // FindEnclosingFunction on non-osx platforms. In doing so, we get a
            // slightly more accurate stack trace in the process.
            //
            // This is often because panic involves the last instruction of a
            // function being "call std::rt::begin_unwind", with no ret
            // instructions after it. This means that the return instruction
            // pointer points *outside* of the calling function, and by
            // unwinding it we go back to the original function.
            let symaddr = if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
                ip
            } else {
                unsafe { uw::_Unwind_FindEnclosingFunction(ip) }
            };

            // Don't print out the first few frames (they're not user frames)
            cx.idx += 1;
            if cx.idx <= 0 { return uw::_URC_NO_REASON }
            // Don't print ginormous backtraces
            if cx.idx > 100 {
                match write!(cx.writer, " ... <frames omitted>\n") {
                    Ok(()) => {}
                    Err(e) => { cx.last_error = Some(e); }
                }
                return uw::_URC_FAILURE
            }

            // Once we hit an error, stop trying to print more frames
            if cx.last_error.is_some() { return uw::_URC_FAILURE }

            match print(cx.writer, cx.idx, ip, symaddr) {
                Ok(()) => {}
                Err(e) => { cx.last_error = Some(e); }
            }

            // keep going
            uw::_URC_NO_REASON
        }
    }

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