模型

#include #include #include ftok() //獲取key semget() //創(chuàng)建/獲取信號(hào)量集 semctl() //初始化信號(hào)量集 semop() //操作信號(hào)量集 semctl() //刪除信號(hào)量集

ftok()

//獲取key值, key值是System V IPC的標(biāo)識(shí)符，成功返回key，失敗返回-1設(shè)errno//同pathname+同 proj_id==>同key_t;key_t ftok(const char *pathname, int proj_id);

pathname?:文件名
proj_id: 1~255的一個(gè)數(shù)，表示project_id

key_t key=ftok(".",100); //“.”就是一個(gè)存在且可訪問的路徑, 100是假設(shè)的proj_id if(-1==key) perror("ftok"),exit(-1);

semget():

//創(chuàng)建/獲取一個(gè)信號(hào)量集，成功返回semid，失敗返回-1int semget(key_t key, int nsems, int semflg);

nsems: 信號(hào)量集的大小/信號(hào)量的個(gè)數(shù)，0表示獲取已經(jīng)存在的信號(hào)量集
semflg

• IPC_CREAT?:若不存在則創(chuàng)建, 需要在msgflg中＂|權(quán)限信息＂; 若存在則打開
• IPC_EXCL?:若存在則創(chuàng)建失敗
• 0?:獲取已經(jīng)存在的信號(hào)量集

//create semsemid=semget(key,1,IPC_CREAT|IPC_EXCL|0664);if(-1==semid) perror("semid"),exit(-1);

semctl()

//主要用于對(duì)指定的信號(hào)量集/信號(hào)量執(zhí)行指定的操作,成功返回0,失敗返回-1設(shè)errnoint semctl(int semid, int semnum, int cmd, ...);

semid: 信號(hào)量集的編號(hào)(哪個(gè)信號(hào)量集)
semnum: 信號(hào)量集的下標(biāo)(這個(gè)信號(hào)量集里的哪個(gè)信號(hào)量)
cmd:具體的操作命令

• IPC_STAT?將內(nèi)核中與semid相關(guān)的信息拷貝到arg.buf指向的結(jié)構(gòu)體中
• IPC_SET將buf指向的semid_ds結(jié)構(gòu)體的部分內(nèi)容寫入到內(nèi)核中的相關(guān)數(shù)據(jù)結(jié)構(gòu)中，同時(shí)更新sem_ctime成員
• IPC_RMID?立即銷毀指定的信號(hào)量集，調(diào)用的進(jìn)程的的effective UID必須和信號(hào)量集的創(chuàng)建者或所有者相匹配，或者這個(gè)進(jìn)程有足夠的特權(quán)級(jí)別，此時(shí)第四個(gè)參數(shù)會(huì)被忽略
• IPC_INFO(Linux-specific)返回系統(tǒng)對(duì)信號(hào)量集的限制到__buf指向的結(jié)構(gòu)體seminfo中

//_GNU_SOURCEstruct seminfo { int semmap; /* Number of entries in semaphore map; unused within kernel */ int semmni; /* Maximum number of semaphore sets */ int semmns; /* Maximum number of semaphores in all semaphore sets */ int semmnu; /* System-wide maximum number of undo structures; unused within kernel */ int semmsl; /* Maximum number of semaphores in a set */ int semopm; /* Maximum number of operations for semop(2) */ int semume; /* Maximum number of undo entries per process; unused within kernel */ int semusz; /* Size of struct sem_undo */ int semvmx; /* Maximum semaphore value */ int semaem; /* Max. value that can be recorded for semaphore adjustment (SEM_UNDO) */};//semmsl, semmns, semopm, semmni可以通過/proc/sys/kernel/sem來設(shè)置

• SEM_INFO?(Linux-specific)返回和IPC_INFO一樣的信息，除了以下方面：semusz成員返回當(dāng)前系統(tǒng)中存在的信號(hào)量集的數(shù)目，semaem返回系統(tǒng)中所有信號(hào)量集中的信號(hào)量總數(shù)
• SEM_STAT(Linux-specific)返回semid_ds結(jié)構(gòu)，類似與IPC_STAT
• GETALL?返回所有信號(hào)量的semval到arg.array中，忽略semnum
• GETNCNT?返回信號(hào)量集第semnum個(gè)信號(hào)量的semcnt值
• GETPID?返回信號(hào)量集第semnum個(gè)信號(hào)量的sempid值
• GETVAL?返回信號(hào)量集第semnum個(gè)信號(hào)量的semval值
• GETZCNT?返回信號(hào)量集第semnum個(gè)信號(hào)量的semzcnt值
• SETALL?使用arg.array設(shè)置信號(hào)量集里的所有的信號(hào)量的semval值，同時(shí)更新信號(hào)量集的semid_ds結(jié)構(gòu)體的sem_ctime成員的值
• SETVAL返回信號(hào)量集第semnum個(gè)信號(hào)量的semval的值到arg.val中，同時(shí)更新信號(hào)量集的semid_ds結(jié)構(gòu)體的sem_ctime成員的值

the fourth argument:

union semun { int val; /* Value for SETVAL */ struct semid_ds* buf; /* Buffer for IPC_STAT, IPC_SET */ unsigned short* array; /* Array for GETALL, SETALL */ struct seminfo* __buf; /* Buffer for IPC_INFO(Linux-specific) */};//struct semid_ds { struct ipc_perm sem_perm; /* Ownership and permissions */ time_t sem_otime; /* Last semop time */ time_t sem_ctime; /* Last change time */ unsigned short sem_nsems; /* No. of semaphores in set */};//struct ipc_perm { key_t __key; /* Key supplied to semget(2) */ uid_t uid; /* Effective UID of owner */ gid_t gid; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short mode; /* Permissions */ unsigned short __seq; /* Sequence number */};

int res=semctl(semid,0,SETVAL,5);if(-1==res) perror("semctl"),exit(-1);int res=semctl(semid,0,IPC_RMID);if(-1==res) perror("semctl"),exit(-1);

semop():

//操作指定的信號(hào)量集，成功返回0,失敗返回-1設(shè)errnoint semop(int semid, struct sembuf *sops, unsigned nsops);

semid:信號(hào)集的ID(returned by semget())
sops:結(jié)構(gòu)體指針, 既可以指向結(jié)構(gòu)體變量, 也可以指向結(jié)構(gòu)體數(shù)組?信號(hào)量集本質(zhì)上是若干個(gè)信號(hào)量的集合, 可以實(shí)現(xiàn)對(duì)信號(hào)量的批處理

struct sembuf{ unsigned short sem_num; //信號(hào)量集的下標(biāo) short sem_op; //正數(shù)表示增加, 0表示不變, 負(fù)數(shù)表示減小 short sem_flg; //操作標(biāo)志,默認(rèn)給0}

nsops:結(jié)構(gòu)體指針指向的元素個(gè)數(shù), 也就是數(shù)組的大小

例子

Sys V IPC sem#include#include#include#include#include#include#includeint main(){ //get key key_t key=ftok(".",200); if(-1==key) perror("ftok"),exit(-1); printf("key=%d\n",key); //create sem int semid=semget(key,0,0); if(-1==semid) perror("semget"),exit(-1); printf("semid=%d\n",semid); //creat 10 children to take the shared resource int i=0; for(i=0;i<10;i++){ //創(chuàng)建10個(gè)進(jìn)程, 當(dāng)然,需要只給parent或child單獨(dú)fork(), 否則就是2^10個(gè)進(jìn)程 pid_t pid=fork(); if(-1==pid) perror("fork"),exit(-1); if(0==pid){ struct sembuf buf; //準(zhǔn)備占用資源, sem_op-1 buf.sem_num=0; //信號(hào)量集下標(biāo) buf.sem_op=-1; //信號(hào)量-1 buf.sem_flg=0; //操作標(biāo)志 int res=semop(semid,&buf,1/*結(jié)構(gòu)體變量的個(gè)數(shù)*/); if(-1==res) perror("semop"),exit(-1); sleep(20); //模擬正在占用共享資源 buf.sem_op=1; //占用完了, sem_op+1 res=semop(semid,&buf,1); if(-1==res) perror("semop"),exit(-1); exit(0); //終止子進(jìn)程, 自然也就跳出了循環(huán),防止再fork() // break; } } return 0;}//出現(xiàn)搶占的效果, 還沒有全部釋放完畢的時(shí)候就有進(jìn)程搶到了已經(jīng)釋放的進(jìn)程