UNIX环境高级编程——system V信号量

1. 信号量(semaphore)主要用于保护临界资源。
进程可以根据它判断是否能访问某些共享资源。
信号量除了用于访问控制外,还可用于进程同步,也就是进程间通信。

2. 信号量分类:
a. 二值信号量: 信号量的值只能取0或1,类似于互斥锁mutex,但两者又不同:

mutex 与 二值信号量的区别:
信号量强调共享资源,只要共享资源可用,其他进程同样可以修改信号量的值;
互斥锁更强调进程,占用资源的进程使用完资源后,必须由进程本身来接锁。

b. 计数信号量:信号量的值可以取任意非负值。

system V信号量通过定义如下概念给信号量增加了另外一级复杂度。

计数信号量集:一个或多个信号量(构成一个集合),其中每个都是计数信号量。每个集合的信号量数存在一个限制,一般在25个数量级。


3.semget函数(信号量的创建

    semget函数创建一个信号量集或访问一个已存在的信号量集。

#include 
int senget(key_t key,int nsems,int oflag);

nsems参数指定集合中的信号量数。如果我们不创建一个新的信号量集,而只是访问一个已存在的集合,那就可以把该参数指定为0。一旦创建完一个信号量集,我们就不能改变其中的信号量数。

    oflag值是SEM_R和SEM_A常值得组合。他们还可以与IPC _CREAT或IPC_CREAT | IPC_EXCL按位或。

    当实际操作为创建一个新的信号量集时,相应的semid_ds结构的以下成员将被初始化。

(1)sem_perm结构的uid和cuid成员被置为调用进程的有效用户ID,gid和cgid成员被置为调用进程的有效组ID。

(2)oflag参数中的读写权限位存入sem_perm.mode。

(3)sem_otime被置为0,sem_ctime则被置为当前时间。

(4)sem_nsems被置为nsems参数的值。

(5)与该集合中每个信号量关联的各个sem结构并不初始化。这些结构时在以SET_VAL或SETALL命令调用semctl时初始化的。


4.semop函数(操作信号量

     使用semget打开一个信号量集后,对其中一个或多个信号量的操作就使用semop函数来执行。

#include 
int semop(int semid,struct sembuf *opsptr,size_t nops);
  其中opsptr指向一个如下结构的数组:

 struct sembuf{
           unsigned short sem_num;  /* semaphore number */
           short          sem_op;   /* semaphore operation */
           short          sem_flg;  /* operation flags */
};


假定有一个信号量变量sv,

P(sv):用于等待,如果sv大于0,就给它减去1,如果它的值等于0,就挂起该进程的执行

V(sv):用于发送信号,如果有其他进程因等待sv而挂起,就让它恢复运行,如果没有进程因等待sv而被挂起,就给它加1


semaphore sv=1;

loop forever{

P(sv);

critical code section;

V(sv);

noncritical code section;

}


信号量函数定义如下所示:

#include

int semctl(int sem_id, int sem_num, int command, ...);//用来直接控制信号量信息

int semget(key_t key, int num_sems, int sem_flags);//创建一个新信号量或取得一个已有信号量的键

int semop(int sem_id, struct sembuf *sem_ops, size_t num_sem_ops);//用于改变信号量的值


/* After the #includes, the function prototypes and the global variable, we come to the
 main function. There the semaphore is created with a call to semget, which returns the
 semaphore ID. If the program is the first to be called (i.e. it's called with a parameter
 and argc > 1), a call is made to set_semvalue to initialize the semaphore and op_char is
 set to X. */

#include 
#include 
#include 

#include 

#include "semun.h"

static int set_semvalue(void);
static void del_semvalue(void);
static int semaphore_p(void);
static int semaphore_v(void);

static int sem_id;


int main(int argc, char *argv[])
{
    int i;
    int pause_time;
    char op_char = 'O';

    srand((unsigned int)getpid());

    sem_id = semget((key_t)1234, 1, 0666 | IPC_CREAT);

    if (argc > 1) {
        if (!set_semvalue()) {
            fprintf(stderr, "Failed to initialize semaphore\n");
            exit(EXIT_FAILURE);
        }
        op_char = 'X';
        sleep(2);
    }

/* Then we have a loop which enters and leaves the critical section ten times.
 There, we first make a call to semaphore_p which sets the semaphore to wait, as
 this program is about to enter the critical section. */

    for(i = 0; i < 10; i++) {

 if (!semaphore_p()) exit(EXIT_FAILURE);
        printf("%c", op_char);fflush(stdout);
        pause_time = rand() % 3;
        sleep(pause_time);
        printf("%c", op_char);fflush(stdout);

/* After the critical section, we call semaphore_v, setting the semaphore available,
 before going through the for loop again after a random wait. After the loop, the call
 to del_semvalue is made to clean up the code. */

        if (!semaphore_v()) exit(EXIT_FAILURE);

        pause_time = rand() % 2;
        sleep(pause_time);
    }

    printf("\n%d - finished\n", getpid());

    if (argc > 1) {
        sleep(10);
        del_semvalue();
    }

    exit(EXIT_SUCCESS);
}

/* The function set_semvalue initializes the semaphore using the SETVAL command in a
 semctl call. We need to do this before we can use the semaphore. */

static int set_semvalue(void)
{
    union semun sem_union;

    sem_union.val = 1;
    if (semctl(sem_id, 0, SETVAL, sem_union) == -1) return(0);
    return(1);
}

/* The del_semvalue function has almost the same form, except the call to semctl uses
 the command IPC_RMID to remove the semaphore's ID. */

static void del_semvalue(void)
{
    union semun sem_union;

if (semctl(sem_id, 0, IPC_RMID, sem_union) == -1)
        fprintf(stderr, "Failed to delete semaphore\n");
}

/* semaphore_p changes the semaphore by -1 (waiting). */

static int semaphore_p(void)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;
    sem_b.sem_op = -1; /* P() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_p failed\n");
        return(0);
    }
    return(1);
}

/* semaphore_v is similar except for setting the sem_op part of the sembuf structure to 1,
 so that the semaphore becomes available. */

static int semaphore_v(void)
{
    struct sembuf sem_b;

    sem_b.sem_num = 0;
    sem_b.sem_op = 1; /* V() */
    sem_b.sem_flg = SEM_UNDO;
    if (semop(sem_id, &sem_b, 1) == -1) {
        fprintf(stderr, "semaphore_v failed\n");
        return(0);
    }
    return(1);
}




转载于:https://www.cnblogs.com/hehehaha/p/6332667.html

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