IPC之信号量
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);
有名信号量:open() close() unlink()
无名信号量:init() destroy()
sem_init():shared为0时,信号量是在线程间共享的。shared非零时,是在进程间共享的,该信号量必须存放在某种类型的共享内存内。
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\n", 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)。
参考:
1. POSIX信号量