IPC之信号量

无名信号量

POSIX标准提出了有名信号量和无名信号量来同步进程和线程，而linux（2.6以前）只实现了无名信号量。

sem_overview中有详细介绍：man 7 sem_overview.

System V semaphores(semget, segop, etc.)是旧的信号量API，但应用广泛。 posix信号量简单易用。

命令行ipcs可提供ipc相关信息，如显示调用进程的信号灯，ipcs -s。

• Posix Sem

#include <semaphore.h>

int sem_init(sem_t *sem, int pshared, unsigned int value);

int sem_destroy(sem_t *sem);

int sem_wait(sem_t *sem);

int sem_trywait(sem_t *sem);

int sem_post(sem_t *sem);

int sem_getvalue(sem_t *sem);

pshared决定了信号量是否在几个进程间共享，0信号量在线程间共享；非0表信号量在进程间共享.

编译上面几个函数程序要加上-lrt选项，以连接the real-time library, librt.so库。

加锁步骤：值大于0，值减1，锁成功；值小于等于0，则锁不成功阻塞。

解锁步骤：值加1，唤醒所有阻塞线程。

• System sem

#include <sys/types.h>

#include <sys/ipc.h>

#include <sys/sem.h>

key_t ftok(char *pathname, int proj);

int semget(key_t key, int nsems, int semflg);

int semctl(int semid, int semnum, int cmd, union semun arg);  // semnum指出感兴趣的特定信号灯。

int semop(int semid, struct sembuf *spos, int nspos);  //nspos为spos指向的数组的元素个数

struct sembuf{

    short sem_num; /*使用哪一个信号*/  

    short sem_op; /*进行什么操作*/

    short sem_flg; /*操作的标志*/    

};

sembuf结构中，sem_num是信号灯的编号，其值从0到nsems-1，sem_op是执行的操作，而sem_flg调整semop的行为。sem_op能够为正值、负值和0。
>>如果sem_op为正，信号灯控制的资源被释放，而且信号灯的值增加。
>>如果sem_op为负，调用进程表示它将等待直到受控资源被释放，此时信号灯的值减小而资源被调用进程加锁。
>>如果sem_op为0，调用进程阻塞直到信号灯变为0；如果信号灯已经是0，调用立即返回。
sem_flg可以为IPC_NOWAIT，不等待直接返回，或者SEM_UNDO，这意味着当调用semop的进程退出后执行的操作将被撤销。

cmd可以取值：GETVAL，SETVAL，GETPID，GETNCNT（在信号灯上等待的进程数），GETZCNT（等待信号灯的值为0的进程数），GETALL，SETALL，
             IPC_RMID（删除带有semid的信号灯），IPC_SET（在信号灯上设置模式--权限位）， IPC_STAT（每个信号灯都有一个数据结构semid_ds，
             这个数据结构完整地描述它的配置和行为。IPC_STAT把这些配置信息复制到semun结构的成员arg.buf中）

创建信号量集
The semget() system call returns the semaphore set identifier associated with the argument key.  A new set of nsems semaphores is created 
if key has the  value  IPC_PRIVATE  or  if  no existing semaphore set is associated with key and IPC_CREAT is specified in semflg.

If  semflg  specifies both IPC_CREAT and IPC_EXCL and a semaphore set already exists for key, then semget() fails with errno set to EEXIST.  
(This is analogous to the effect of the combi‐nation O_CREAT | O_EXCL for open(2).)
假如指定参数key为IPC_PRIVATE，或与key相关的信号集不存在且semflg为IPC_CREAT，则信号集被创建。
If successful, the return value will be the semaphore set identifier (a nonnegative integer), otherwise -1 is returned, with errno indicating the error.

创建key
ftok - convert a pathname and a project identifier to a System V IPC key
The ftok() function uses the identity of the file named by the given pathname (which must refer to an existing, accessible file) 
and the least significant 8 bits of proj_id (which must be nonzero) to generate a key_t type System V IPC key, suitable for use with msgget(2), semget(2), or shmget(2). 
The resulting value is the same for all pathnames that name the same file, when the same value of proj_id is used.  
The value returned should be different when the (simultaneously existing)files or the project IDs differ.
On success, the generated key_t value is returned. On failure -1 is returned, with errno indicatine the error as for the stat(2) system call.

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>

int main(void)
{
    int semid;
    int nsems = 1;
    int flags = 0666;
    struct sembuf buf;

    semid = semget(IPC_PRIVATE, nsems, flags);
    if(semid < 0)
    {   
        perror("semget");
        exit(EXIT_FAILURE);
    }   
    printf("semaphore created: %d
", semid);

    buf.sem_num = 0;
    buf.sem_op = 1;
    buf.sem_flg = IPC_NOWAIT;

    if((semop(semid, &buf, nsems)) < 0)
    {
        perror("semop");
        exit(EXIT_FAILURE);
    }

    system("ipcs -s");
    exit(EXIT_SUCCESS);
}

多次运行结果如下：

~$./a.out 
semaphore created: 131076

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x00000000 0          yu       666        1         
0x00000000 32769      yu       666        1         
0x00000000 65538      yu       666        1         
0x00000000 98307      yu       666        1         
0x00000000 131076     yu       666        1 

include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int semid;

    if(argc != 2)
    {   
        puts("USAGE: sctl <semaphore id>");
        exit(EXIT_FAILURE);
    }   
    semid = atoi(argv[1]);

    if((semctl(semid, 0, IPC_RMID)) < 0)
    {   
        perror("semctl IPC_RMID");
        exit(EXIT_FAILURE);
    } else {
        puts("semaphore removed");
        system("ipcs -s");
    }

    exit(EXIT_SUCCESS);
}

~$./a.out 
USAGE: sctl <semaphore id>
~$./a.out 0
semaphore removed

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x00000000 32769      yu       666        1         
0x00000000 65538      yu       666        1         
0x00000000 98307      yu       666        1         
0x00000000 131076     yu       666        1  

主要应用于保护临界资源（同一时刻只有一个进程可以访问资源）。