在主线程上调用pthread_join是否有效？

Question

这段代码的行为是否定义明确？

#include <stdio.h>
#include <pthread.h>

pthread_t mt;

void *start(void *x)
{
    void *y;
    pthread_join(mt, &y);
    printf("joined main thread\n");
    return 0;
}

int main()
{
    pthread_t t;
    mt = pthread_self();
    pthread_create(&t, 0, start, 0);
    pthread_exit(0);
}

Answer 1

是的,这是可能的.实际上,这种可能性是pthread_detach()存在的主要原因之一.从POSIX文档中可以pthread_detach()看到(参见参考资料man pthread_detach):

   It  has  been suggested that a "detach" function is not necessary; the
   detachstate thread creation attribute is sufficient,  since  a  thread
   need  never  be dynamically detached. However, need arises in at least
   two cases:

    1. In a cancellation handler for a pthread_join() it is nearly essen-
       tial  to  have  a pthread_detach() function in order to detach the
       thread on which pthread_join() was waiting. Without it,  it  would
       be  necessary  to  have  the  handler do another pthread_join() to
       attempt to detach the thread, which would both delay the cancella-
       tion  processing  for an unbounded period and introduce a new call
       to pthread_join(), which might itself need a cancellation handler.
       A dynamic detach is nearly essential in this case.


    2. In  order  to  detach the "initial thread" (as may be desirable in
       processes that set up server threads).

所以根据标准,你提出的建议是好的.

编辑:只是为了进一步确认,状态的POSIX描述exec():

新进程映像中的初始线程应该是可连接的,就像使用detachstate属性设置为PTHREAD_CREATE_JOINABLE一样创建的.