信号量计数与 semop 调用期间传递的操作值之间的差异
Difference between semaphore count and operation value which is passed during semop call
在Beej Guide上，有如下说法
"When you first create some semaphores, they're all uninitialized; it takes another call to mark them as free."
最初我创建时信号量计数为 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。
要将信号量指示为空闲，我将 sem_op 作为 1
传递
sem_op[0].sem_op=1;
疑问 1.sem_op值是否可以大于1？例如，如果它传递为 3，那是什么意思？
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，那么我为 semget 传递的信号量计数是可用于多个临界区的单个信号量。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 个临界区的同时资源将在 semop 调用期间说到。我的理解正确吗？
注意：我正在使用为系统 V(sys/sem.h)
完成的实现
此查询背后的原因：我很困惑自己是否 sem_op 值作为空闲信号量的数量（或）单个信号量中的资源数量是否空闲。
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);
}
查询 1：
sem_op可以不止一个。在这种情况下，该数字将添加到信号量具有的许可中
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，而是许可证数量的增加。 'semaphore permit' 的意思是有多少个实体可以同时持有那个信号量。这意味着，例如，如果您有 5 个线程在一个由具有 3 个许可的信号量保护的临界区上竞争，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.
在Beej Guide上，有如下说法
"When you first create some semaphores, they're all uninitialized; it takes another call to mark them as free."
最初我创建时信号量计数为 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。
要将信号量指示为空闲，我将 sem_op 作为 1
传递sem_op[0].sem_op=1;
疑问 1.sem_op值是否可以大于1？例如，如果它传递为 3，那是什么意思？
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，那么我为 semget 传递的信号量计数是可用于多个临界区的单个信号量。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 个临界区的同时资源将在 semop 调用期间说到。我的理解正确吗？
注意：我正在使用为系统 V(sys/sem.h)
完成的实现此查询背后的原因：我很困惑自己是否 sem_op 值作为空闲信号量的数量（或）单个信号量中的资源数量是否空闲。
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);
}
查询 1：
sem_op可以不止一个。在这种情况下，该数字将添加到信号量具有的许可中
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，而是许可证数量的增加。 'semaphore permit' 的意思是有多少个实体可以同时持有那个信号量。这意味着，例如，如果您有 5 个线程在一个由具有 3 个许可的信号量保护的临界区上竞争，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.