问题描述
我正在检查'pthread_join'的行为,并具有以下代码:
I am checking the behavior of 'pthread_join' and have the following code:
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>
#include <errno.h>
#include <pthread.h>
void *thread(void *vargp)
{
pthread_detach(pthread_self());
pthread_exit((void *)42);
}
int main()
{
int i = 1;
pthread_t tid;
pthread_create(&tid, NULL, thread, NULL);
sleep(1);
pthread_join(tid, (void **)&i);
printf("%d\n", i);
printf("%d\n", errno);
}
我的平台上观察到的输出(Linux 3.2.0-32-通用#51-Ubuntu SMP x86_64 GNU/Linux):
Observed output on my platform (Linux 3.2.0-32-generic #51-Ubuntu SMP x86_64 GNU/Linux):
-
带有'sleep(1)'的注释:420
with the 'sleep(1)' commented out:420
带有sleep语句,它产生:1个0
with the sleep statement, it produces:10
根据pthread_join的手册页,当我们尝试连接不可连接的线程时,应该出现错误" EINVAL ",但是,以上两种情况都没有设置errno.而且在第一种情况下,似乎我们甚至可以得到分离线程的退出状态,我对结果感到困惑.有人可以解释吗?谢谢
according to the man page of pthread_join, we should get the error 'EINVAL' when we are trying to join a non-joinable thread, however, neither cases above got the errno set. And also in the first case, it seemed that we can even get the exit status of a detached thread, I am confused with the result. Could anyone explain this? Thanks
:我意识到第一个printf可能会重置'errno',但是即使我交换了两个'printf'语句的顺序,我仍然得到相同的结果.
: I realized that the first printf may reset 'errno', however, even after I swapped the order of the two 'printf' statements, I still got the same results.
推荐答案
您的期望是错误的.在分离的线程上调用pthread_join
会调用未定义的行为.不需要设置errno
,返回错误代码,甚至根本不需要返回.
Your expectation is wrong. Calling pthread_join
on a detached thread invokes undefined behavior. There is no requirement to set errno
, return the error code, or even return at all.
如果您需要引用,
来源: http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_join.html
此外,请注意,大多数pthread函数(包括pthread_join
)都不使用errno
.相反,他们返回错误代码作为其返回值.这样,即使您在调用pthread函数时未调用未定义的行为,也要在检查它后检查errno
是错误的.
Also, note that most pthread functions, including pthread_join
, do not use errno
. Instead they return the error code as their return value. As such, inspecting errno
after calling a pthread function is wrong even if you didn't invoke undefined behavior when you called it.
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