“在没有激活异常的情况下终止”在pthread_cancel之后

问题描述 投票:0回答:1

在探查this question的条件时,出现了问题,以下代码举例说明。

#include <iostream>
#include <thread>
#include <chrono>
#include <stdexcept>
#include <cxxabi.h>

using namespace std;

// mocking external library call stuck in an infinite loop
int buggy_function_simulation()
{
    // cout << "In buggy function" << endl; // (1)
    int counter = 0;
    while (true)
    {
        if ( ++counter == 1000000 ) { counter = 0; }
    }
    return 0;
}

int main(int argc, char **argv) {
    cout << "Hello, world!" << endl;

    auto lambda = []() {
        pthread_setcanceltype( PTHREAD_CANCEL_ASYNCHRONOUS, nullptr );
        // cout << "ID: "<<pthread_self() <<endl; // (2)
        try
        {
            cout << "ID: "<<pthread_self() <<endl; // (3)
            buggy_function_simulation();
        }
        catch ( abi::__forced_unwind& )
        {
            cout << "thread cancelled!" << endl; // (4)
            throw;
        }
    };

    std::thread th(lambda);

    pthread_t id = th.native_handle();
    cout << id << endl;

    this_thread::sleep_for(chrono::seconds(1));
    cout << "cancelling ID: "<< id << endl;

    pthread_cancel(id);
    th.join();

    cout << "cancelled: "<< id << endl;

    return 0;
}

编译并运行导致中止:

$ g++ -g -Og -std=c++11 -pthread -o test test.cpp -lpthread
$ ./test
Hello, world!
139841296869120
ID: 139841296869120
cancelling ID: 139841296869120
terminate called without an active exception
Aborted (core dumped)
$

请注意,不会显示诊断输出(4)。

如果我注释掉(3)并取消注释(2),结果是:

$ ./test
Hello, world!
139933357348608
ID: 139933357348608
cancelling ID: 139933357348608
cancelled: 139933357348608
$

同样,(4)的输出没有出现(为什么?),但已取消中止操作。

或者,如果我保留(3),将(2)保留为注释,然后取消注释(1),则结果最终符合预期:

$ ./test
Hello, world!
139998901511936
ID: 139998901511936
In buggy function
cancelling ID: 139998901511936
thread cancelled!
cancelled: 139998901511936
$

所以,问题是:

  • 在第一种情况下“终止而没有活动异常的终止”的原因是什么?
  • 为什么在第二种情况下未激活catch块?
  • 为什么在第三种情况下不加注释(1)会产生如此不同?

为了完整性,这是第一种情况下gdb的堆栈跟踪:

Program terminated with signal SIGABRT, Aborted.
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
51      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
[Current thread is 1 (Thread 0x7f5d9b49a700 (LWP 12130))]
(gdb) where
#0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1  0x00007f5d9b879801 in __GI_abort () at abort.c:79
#2  0x00007f5d9bece957 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#3  0x00007f5d9bed4ab6 in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#4  0x00007f5d9bed4af1 in std::terminate() () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#5  0x00007f5d9bed44ba in __gxx_personality_v0 () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#6  0x00007f5d9bc3a708 in ?? () from /lib/x86_64-linux-gnu/libgcc_s.so.1
#7  0x00007f5d9bc3acfc in _Unwind_ForcedUnwind () from /lib/x86_64-linux-gnu/libgcc_s.so.1
#8  0x00007f5d9c1dbf10 in __GI___pthread_unwind (buf=<optimized out>) at unwind.c:121
#9  0x00007f5d9c1d0d42 in __do_cancel () at ./pthreadP.h:297
#10 sigcancel_handler (sig=<optimized out>, si=0x7f5d9b499bb0, ctx=<optimized out>) at nptl-init.c:215
#11 <signal handler called>
#12 buggy_function_simulation () at test.cpp:15
#13 0x0000558865838227 in <lambda()>::operator() (__closure=<optimized out>) at test.cpp:29
#14 std::__invoke_impl<void, main(int, char**)::<lambda()> > (__f=...) at /usr/include/c++/7/bits/invoke.h:60
#15 std::__invoke<main(int, char**)::<lambda()> > (__fn=...) at /usr/include/c++/7/bits/invoke.h:95
#16 std::thread::_Invoker<std::tuple<main(int, char**)::<lambda()> > >::_M_invoke<0> (this=<optimized out>)
    at /usr/include/c++/7/thread:234
#17 std::thread::_Invoker<std::tuple<main(int, char**)::<lambda()> > >::operator() (this=<optimized out>)
    at /usr/include/c++/7/thread:243
#18 std::thread::_State_impl<std::thread::_Invoker<std::tuple<main(int, char**)::<lambda()> > > >::_M_run(void) (
    this=<optimized out>) at /usr/include/c++/7/thread:186
#19 0x00007f5d9beff66f in ?? () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#20 0x00007f5d9c1d26db in start_thread (arg=0x7f5d9b49a700) at pthread_create.c:463
#21 0x00007f5d9b95a88f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
c++ c++11 g++ pthreads stdthread
1个回答
0
投票

如果从标记为noexcept的函数内部抛出该消息,则可以触发该消息。所有析构函数都是隐式的noexcept,因此,如果在引发由pthread_cancel触发的异常时线程正在运行析构函数,则程序将终止并收到该消息。

operator<<std::cout是格式化输出操作,它构造一个sentry对象,该对象在退出时被破坏(请参阅https://en.cppreference.com/w/cpp/named_req/FormattedOutputFunction)。如果在处理sentry对象的析构函数的同时取消,则将终止您的应用程序。

请勿在C ++中使用PTHREAD_CANCEL_ASYNCHRONOUS。由于pthread_cancel子句会自动抛出,因此即使使用catch也可能会出现问题。

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