在Beej Guide上,有以下陈述
“当您第一次创建某些信号时,它们都尚未初始化;需要再次调用才能将它们标记为空闲。”
我最初创建的信号量为5。
#define SEM_COUNT 5
semget(sem_key,SEM_COUNT,IPC_CREAT|IPC_EXCL|0600);
现在,根据上述《 beej指南》的声明,我必须初始化5个信号量,以通过调用semop将其标记为免费。
为了将信号量表示为免费,请将sem_op传递为1
sem_op[0].sem_op=1;
查询1. sem_op值可以大于一吗?例如,如果它以3传递,那意味着什么?
我对查询1的理解:是一次可以访问关键部分或共享资源的资源(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的信号量就是可以用于关键部分的单个信号量。如果我创建的信号量为5,则意味着我可以访问5个关键部分。可以在semop调用中说到可以并行访问这5个关键部分的同步资源。我的理解正确吗?
注意:我正在使用系统V(sys / sem.h)的实现
该查询背后的原因:我感到困惑,就像sem_op值是否等于免费的信号灯数量(或单个信号灯中的资源数量)一样。
样本代码
#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);
}
最佳答案
查询1:
sem_op可以是多个。在这种情况下,该数字将添加到信号灯具有的许可中
查询2:
您对查询1的假设是错误的。它不是许可的绝对数量,而是许可数量的增加。 “信号灯许可”的含义是有多少个实体可以同时持有该信号灯。例如,这意味着如果您有5个线程在具有3个许可的信号量保护的关键部分上竞争,那么只有3个线程可以同时进入。
从手册中:
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.
关于c - 在信号量调用期间传递的信号量计数与操作值之间的差异,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/31299962/