有人可以帮助我理解在子线程退出之后唤醒主线程的情况吗？ 我的理解是，join调用futex_wait，导致主线程等待，直到子线程完成。 大概有一个futex_wake调用，以允许主线程恢复，但在两个线程运行strace后，我不能看到系统调用唤醒父线程的位置。
#include <iostream> #include <thread> #include <string> void hello() { std::cout <<"Hello Concurrent World" << std::endl; } int main () { std::thread t(hello); t.join(); std::cout << "finished" << std::endl; } 从父线程strace
19:37:06.206727 brk(NULL) = 0x1380000 19:37:06.206804 brk(0x13b2000) = 0x13b2000 19:37:06.206968 futex(0x7fcf7953005c, FUTEX_WAKE_PRIVATE, 2147483647) = 0 19:37:06.207331 futex(0x7fcf79530068, FUTEX_WAKE_PRIVATE, 2147483647) = 0 19:37:06.207577 mmap(NULL, 8392704, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_STACK, -1, 0) = 0x7fcf780ba000 19:37:06.207726 mprotect(0x7fcf780ba000, 4096, PROT_NONE) = 0 19:37:06.207863 clone(child_stack=0x7fcf788b9fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7fcf788ba9d0, tls=0x7fcf788ba700, child_tidptr=0x7fcf788ba9d0) = 10204 19:37:06.208005 futex(0x7fcf788ba9d0, FUTEX_WAIT, 10204, NULL) = 0 19:37:06.209781 write(1, "finished\n", 9) = 9 19:37:06.210021 exit_group(0) = ? 19:37:06.210394 +++ exited with 0 +++
从子线程strace
19:07:09.837174 set_robust_list(0x7f123adba9e0, 24) = 0 19:07:09.837411 fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0 19:07:09.837508 mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0) = 0x7f12325ba000 19:07:09.837601 munmap(0x7f12325ba000, 27549696) = 0 19:07:09.837693 munmap(0x7f1238000000, 39559168) = 0 19:07:09.837818 mprotect(0x7f1234000000, 139264, PROT_READ|PROT_WRITE) = 0 19:07:09.837947 write(1, "Hello Concurrent World\n", 23) = 23 19:07:09.838082 madvise(0x7f123a5ba000, 8368128, MADV_DONTNEED) = 0 19:07:09.838164 exit(0) = ? 19:07:09.838285 +++ exited with 0 +++
目前使用Ubuntu 16.10编译，铿锵声版本为3.8.1-12，内核为4.8.0-34-通用
这个调用： set_robust_list(0x7f123adba9e0, 24) = 0设置一个健壮的futex，它会在线程退出时自动恢复。 所以没有系统调用来唤醒futex。 内核将唤醒线程退出时的futex（即使线程被终止）。
下面是Debian伸展中的一段描述，表明fuxes“自行”恢复：
[pid 21604] 20:38:49.614788 futex(0x7f0349dbc9d0, FUTEX_WAIT, 21605, NULL <unfinished ...> [pid 21605] 20:38:49.614851 set_robust_list(0x7f0349dbc9e0, 24) = 0 [pid 21605] 20:38:49.614948 nanosleep({tv_sec=0, tv_nsec=100000000}, NULL) = 0 [pid 21605] 20:38:49.715377 madvise(0x7f03495bc000, 8368128, MADV_DONTNEED) = 0 [pid 21605] 20:38:49.715554 exit(0) = ? [pid 21604] 20:38:49.715674 <... futex resumed> ) = 0 [pid 21605] 20:38:49.715701 +++ exited with 0 +++