semop()失败了errno 4.剂量semop()支持线程在进程内的竞争？

Question

我在一个过程中有两个线程.这两个线程竞争共享内存,尝试通过信号量进行同步.但是当一个线程紧挨着另一个线程调用semop函数时,我随机地使用errno 4失败了.我做了一点挖掘,发现似乎呼叫被系统调用打断了.

EINTR在此系统调用中被阻止时,该过程发出信号; 见信号(7).errno 4是这个吗？

请注意第583和601行.

哪个系统调用打断了它？函数semop()本身？有什么方法可以忽略这个系统调用中断或恢复/重启这个功能？

semop可以在多线程环境中使用吗？

[Switching to Thread -1208269120 (LWP 4501)]
GetMyQue2Wait (MyModule=RM, wait_shm_ptr=0xbf8a5cf4) at tdm_ipc.c:247
247                                             TDM_SEM_P( MyModule );
(gdb) s
tdm_sem_p (mid=RM) at tdm_ipc.c:579
579             sem_b.sem_num = 0;
(gdb) s
580             sem_b.sem_op = -1;
(gdb) s
581             sem_b.sem_flg = SEM_UNDO;
(gdb) s
583             if (semop(TDM_M[mid].semid, &sem_b, 1) == -1)
(gdb) s
[Switching to Thread -1208480880 (LWP 4506)]

GetMyQue2Send (MyModule=RM, send_shm_ptr=0xb7f7ff54) at tdm_ipc.c:180
180             DMINT           TryTimes = SEND_TIMES;
(gdb) s
353             TDM_SEM_V( DstModule );
(gdb) s
tdm_sem_v (mid=RM) at tdm_ipc.c:597
597             sem_b.sem_num = 0;
(gdb) s
598             sem_b.sem_op = 1;
(gdb) s
599             sem_b.sem_flg = SEM_UNDO;
(gdb) s
601             if (semop(TDM_M[mid].semid, &sem_b, 1) == -1)
(gdb) s
606             return SUCC;
(gdb) s
607     }


(gdb) s
RM:4501: V operation on Semaphore .
SEND_MSG (SrcModule=51, DstModule=RM, msg_ptr=0xb7f7ff94, MsgLength=28) at tdm_ipc.c:368
368             printf("%s:%d: SEND_MSG: succeeded.\n",
(gdb) s
RM:4501: SEND_MSG: succeeded.
[Switching to Thread -1208269120 (LWP 4501)]
tdm_sem_p (mid=RM) at tdm_ipc.c:585
585                     printf("thread %u: errno = %d\n", (unsigned int)pthread_self(),errno);
(gdb) s
thread 3086698176: errno = 4
[Switching to Thread -1208480880 (LWP 4506)]



 main thread:

        ...
         while(1)
           {
                 if ((RetVal = WAIT_MSG( p1, &Msg )) !=SUCC)
                 {
                     switch ( RetVal )
                     {
                     ...
                      }
                  }
            }
     ------------------------------------  
        thread1:
        ...
        send(src, dst, &msg, lenght);

        /* both SEND_MSG() and WAIT_MSG() have an operation P and V on semid by calling the following */

        DMINT tdm_sem_p( key_t semid )
        {
           struct sembuf sem_b;

           sem_b.sem_num = 0;
           sem_b.sem_op = -1;
           sem_b.sem_flg = SEM_UNDO;

           if (semop(semid, &sem_b, 1) == -1)
           {
              printf("thread %u: errno = %d\n", (unsigned int)pthread_self(),errno);

              return S_PFAIL;
           }

           return SUCC;
        }

        DMINT tdm_sem_v( key_t semid )
        {
           struct sembuf sem_b;

           sem_b.sem_num = 0;
           sem_b.sem_op = 1;
           sem_b.sem_flg = SEM_UNDO;

           if (semop(semid, &sem_b, 1) == -1)
           {
              return S_VFAIL;
           }

           return SUCC;
        }

        /* semid is init by the following */
        DMINT tdm_set_sem(key_t semid)
        {
           union semun sem_union;
           sem_union.val = 1;

           if (semctl(semid, 0, SETVAL, sem_union) == -1)
           {
              return FAILURE;
           }
           return SUCC;
        }

这个问题有另一个链接可能有一个不好的问题描述. P信号量失败了

谢谢.

Answer 1

Errno 4确实如此EINTR.当您收到该错误时,表示您正在运行的系统调用(semop在这种情况下)被信号中断.

在这种情况下,您负责重新启动系统调用.只有一组有限的系统调用会自动重启,然后才会使用该SA_RESTART标志设置信号处理程序.有关signal(7)详细信息,请参阅"信号处理程序中断系统调用和库函数"部分.无论信号处理程序的处置方式如何,您都会注意到semop系统调用列表中永远不会重新启动.

你如何重新开始通话取决于你.其中一种方法是做一些事情:

int rc;

while ((rc = semop(...)) == -1) {
  if (errno != EINTR) {
    break;
  } else {
    // decide whether to restart the call after interruption
    // or not
  }
}
// here, if rc == 0, semop worked, otherwise an error different from
// EINTR happened (or you decided not to restart)

除非您有该信号的处理程序,否则您不知道哪个信号中断了给定的系统调用.gdb确实有信号处理选项,所以你可以尝试找出它.尝试handle all print从也许开始.