鉴于这段代码:
#include
#include
void f(bool doThrow) {
if (doThrow) {
std::cout << "Throwing" << std::endl;
throw 42;
}
std::cout << "Not throwing" << std::endl;
}
std::once_flag flag;
void g(bool doThrow) {
try {
std::call_once(flag, f, doThrow);
std::cout << "Returning" << std::endl;
} catch (int i) {
std::cout << "Caught " << i << std::endl;
}
}
int main() {
std::once_flag flag;
g(true);
g(true);
g(false);
g(true);
g(false);
}
编译时g++ -std=c++11 -pthread -ggdb我得到输出:
Throwing
Caught 42
进程挂起之后:
#0 0x000003fff7277abf in futex_wait (private=0, expected=1, futex_word=0x2aaaacad144 ) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
#1 futex_wait_simple (private=0, expected=1, futex_word=0x2aaaacad144 ) at ../sysdeps/nptl/futex-internal.h:135
#2 __pthread_once_slow (once_control=0x2aaaacad144 , init_routine=0x3fff7a8d870 ) at pthread_once.c:105
#3 0x000002aaaaaab06f in __gthread_once (__once=0x2aaaacad144 , __func=0x3fff7a8d870 ) at /usr/lib/gcc/x86_64-pc-linux-gnu/5.4.0/include/g++-v5/x86_64-pc-linux-gnu/bits/gthr-default.h:699
#4 0x000002aaaaaab6c8 in std::call_once (__once=..., __f=@0x2aaaaaab08c: {void (bool)} 0x2aaaaaab08c ) at /usr/lib/gcc/x86_64-pc-linux-gnu/5.4.0/include/g++-v5/mutex:738
#5 0x000002aaaaaab192 in g (doThrow=true) at test.cpp:17
#6 0x000002aaaaaab287 in main () at test.cpp:27
但是当clang++ -std=c++11 -pthread -ggdb我编译时,我得到:
Throwing
Caught 42
Throwing
Caught 42
Not throwing
Returning
Returning
Returning
据我所知,这似乎是正确的行为.
这是一个GCC错误,只是我对语义感到困惑std::call_once,或者我的代码是不正确的？
这看起来是GNU C++库中的一个错误.
当然这是一个错误,因为如果你尝试使用提供的在线编译器(Coliru),即使是std::call_once cppreference的默认示例也会挂起:)
这个bug发生在使用pthreads的g ++ linux实现中.令我困惑的是,这个Wandbox示例运行得很好.我检查了以下版本libstdc++:
Wandbox: GLIBCXX: 20130411
Coliru: GLIBCXX: 20161221
因此,我认为这是一个libstdc++错误,也许这一次,或者更准确地说这一个.
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