c - 信号量计数与 semop 调用期间传递的操作值之间的差异

Question

关于Beej Guide ，有一个说法如下

“当您第一次创建一些信号量时，它们都未初始化；需要再次调用才能将它们标记为空闲。”

最初我创建的信号量计数为 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 初始化 5 个信号量以将它们标记为空闲。

为了将信号量指示为空闲，将 sem_op 传递为 1

sem_op[0].sem_op=1;

查询1. sem_op 值可以大于1吗？例如，如果它通过了 3，那么这意味着什么？

我对查询1的理解:它是一次可以访问临界区或共享资源的资源(线程或进程)的数量。

查询 2。如果我对查询 1 的理解是正确的，那么我传递给 semget 的信号量计数就是可用于多个关键部分的单个信号量。如果我创建的信号量计数为 5。那么这意味着我可以访问 5 个关键部分。可以并行访问这 5 个临界区的实时资源将在 semop 调用期间说明。我的理解正确吗？

注意:我正在使用系统 V(sys/sem.h) 的实现

此查询背后的原因:我自己很困惑，比如 sem_op 值是否作为免费信号量的数量(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 可以多个。在这种情况下，该数字将添加到信号量拥有的许可中

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，而是许可证数量的总和。 “信号量许可”的含义是有多少实体可以同时持有该信号量。这意味着，例如，如果您有 5 个线程在受具有 3 个许可的信号量保护的关键部分上竞争，则只有 3 个线程能够同时进入。

来自手册:

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.