Question

在Beej Guide上，有如下声明

“当你第一次创建一些信号量时，它们都是未初始化的;需要另一次调用才能将它们标记为空闲。”

最初我用信号量计数创建为5。

#define SEM_COUNT 5
semget(sem_key,SEM_COUNT,IPC_CREAT|IPC_EXCL|0600);

现在根据beej指南的上述声明，我必须通过调用semop将5个信号量标记为空闲。

将信号量指示为自由，将sem_op传递为1

sem_op[0].sem_op=1;

查询1. sem_op值是否可以多于一个？例如，如果它传递为3，那么这意味着什么？

我对查询1的理解：一次可以访问关键部分或共享资源的资源数量（线程或进程）。

查询2.如果我对查询1的理解是正确的，那么我为semget传递的信号量计数是可以用于sevaral关键部分的单个信号量。如果我创建信号量计数为5.那么这意味着我可以访问5个关键部分。在semop调用期间，可以说并行地访问这5个关键部分的可靠资源。我的理解是否正确？

注意：我正在使用系统V（sys / sem.h）

的实现

这个查询背后的原因：我把自己混淆了，就像sem_op值是否为信号量（或）单个信号量中的资源数量是免费的。

示例代码

#include <sys/sem.h>
#include <cstdio>
#include <unistd.h>

#define SEMAPHORE_COUNT 1

void semaphore_wait(int sem_id);
void semaphore_signal(int sem_id);


int main()
{
   /* 1.Getting the semaphore Key */

   key_t sem_key=ftok("key.txt",'S');

   if(0 >= sem_key)
   {
      perror("Error in getting key: ");
   }
   else
   {
      printf("\nSucessfully generated semaphore key. Value %d ",sem_key);
   }

   /* 2.Attaching the key to semaphore */

   int sem_id = semget(sem_key,SEMAPHORE_COUNT,0);

   struct sembuf sem_inp;
   sem_inp.sem_op=1; /*Semaphore  operation: 1:Increment -1:Decrement: Query1: Can this value can be more than 1? what does that means if greater than 1? */
   sem_inp.sem_num=0;
   sem_inp.sem_flg=SEM_UNDO;

   semop(sem_id,&sem_inp,1);
   /* 3.Utilizing the semaphore on critical sections */

   printf("\n Waiting for semaphore...");
   semaphore_wait(sem_id);
   printf("\n Acquired semaphore...");
   sleep(4);
   semaphore_signal(sem_id);
   printf("\n Released semaphore...");


   return 0;
}


void semaphore_wait(int sem_id)
{
   struct sembuf sem_inp;
   sem_inp.sem_op=-1; /*Semaphore  operation: 1:Increment -1:Decrement */
   sem_inp.sem_num=0;
   sem_inp.sem_flg=SEM_UNDO;

   semop(sem_id,&sem_inp,1);

}

void semaphore_signal(int sem_id)
{
   struct sembuf sem_inp;
   sem_inp.sem_op=1; /*Semaphore  operation: 1:Increment -1:Decrement */
   sem_inp.sem_num=0;
   sem_inp.sem_flg=SEM_UNDO;

   semop(sem_id,&sem_inp,1);
}

Answer 1

查询1：

sem_op可以不止一个。在这种情况下，该数字被添加到信号量具有的许可证中

查询2：

您对查询1的假设是错误的。这不是许可证的绝对数量，而是增加许可证的数量。信号量允许的含义＆＃39;是多少实体可以同时持有该信号量。这意味着，例如，如果你有5个线程在一个受信号量保护的关键部分上竞争3个许可证，那么只有3个能够同时进入。

从手册：

The variable sem_op specifies one of three semaphore operations:

    1. If sem_op is a negative integer and the calling process has alter permission, one of the following shall occur:

        *  If  semval(see <sys/sem.h>) is greater than or equal to the absolute value of sem_op, the absolute value of sem_op is subtracted from sem‐
           val.  Also, if (sem_flg &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for
           the specified semaphore.

        *  If semval is less than the absolute value of sem_op and (sem_flg &IPC_NOWAIT) is non-zero, semop() shall return immediately.

        *  If  semval  is  less than the absolute value of sem_op and (sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semncnt associated with
           the specified semaphore and suspend execution of the calling thread until one of the following conditions occurs:

           --  The value of semval becomes greater than or equal to the absolute value of sem_op.  When this occurs, the value of semncnt  associated
               with  the  specified  semaphore  shall  be  decremented, the absolute value of sem_op shall be subtracted from semval and, if (sem_flg
               &SEM_UNDO) is non-zero, the absolute value of sem_op shall be added to the semadj value of the calling process for the specified sema‐
               phore.

           --  The  semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set to [EIDRM]
               and −1 shall be returned.

           --  The calling thread receives a signal that is to be caught. When this occurs, the value of semncnt associated with the specified  sema‐
               phore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction().

    2. If  sem_op  is  a positive integer and the calling process has alter permission, the value of sem_op shall be added to semval and, if (sem_flg
       &SEM_UNDO) is non-zero, the value of sem_op shall be subtracted from the semadj value of the calling process for the specified semaphore.

    3. If sem_op is 0 and the calling process has read permission, one of the following shall occur:

        *  If semval is 0, semop() shall return immediately.

        *  If semval is non-zero and (sem_flg &IPC_NOWAIT) is non-zero, semop() shall return immediately.

        *  If semval is non-zero and (sem_flg &IPC_NOWAIT) is 0, semop() shall increment the semzcnt associated with the specified semaphore and sus‐
           pend execution of the calling thread until one of the following occurs:

           --  The value of semval becomes 0, at which time the value of semzcnt associated with the specified semaphore shall be decremented.

           --  The  semid for which the calling thread is awaiting action is removed from the system. When this occurs, errno shall be set to [EIDRM]
               and −1 shall be returned.

           --  The calling thread receives a signal that is to be caught. When this occurs, the value of semzcnt associated with the specified  sema‐
               phore shall be decremented, and the calling thread shall resume execution in the manner prescribed in sigaction().

   Upon successful completion, the value of sempid for each semaphore specified in the array pointed to by sops shall be set to the process ID of the
   calling process. Also, the sem_otime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description.

在semop调用期间传递的信号量计数和操作值之间的差异

1 个答案: