给定这段代码：
#include <mutex>
#include <iostream>
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);
}
当编译g -std = c 11 -pthread -ggdb时,我得到输出：
Throwing Caught 42
之后进程挂起：
#0 0x000003fff7277abf in futex_wait (private=0,expected=1,futex_word=0x2aaaacad144 <flag>) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
#1 futex_wait_simple (private=0,futex_word=0x2aaaacad144 <flag>) at ../sysdeps/nptl/futex-internal.h:135
#2 __pthread_once_slow (once_control=0x2aaaacad144 <flag>,init_routine=0x3fff7a8d870 <std::__once_proxy()>) at pthread_once.c:105
#3 0x000002aaaaaab06f in __gthread_once (__once=0x2aaaacad144 <flag>,__func=0x3fff7a8d870 <std::__once_proxy()>) 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<void (&)(bool),bool&> (__once=...,__f=@0x2aaaaaab08c: {void (bool)} 0x2aaaaaab08c <f(bool)>) 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
但是当编译时使用clang -std = c 11 -pthread -ggdb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.
解决方法
这看起来是GNU C库中的一个错误.
当然这是一个错误,因为即使默认std :: call_once example from cppreference将挂起,如果您尝试使用提供的在线编译器(Coliru):)
该错误发生在使用pthread的g linux实现中.令我困惑的是this Wandbox example跑得很好.我检查了libstdc的版本：
> WandBox：GLIBCXX：20130411
> Coliru：GLIBCXX：20161221
