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P操作 负责把当前进程由执行状态转换为堵塞状态,直到另外一个进程唤醒它。
操作为:申请一个空暇资源(把信号量减1),若成功,则退出;若失败,则该进程被堵塞;
V操作 负责把一个被堵塞的进程唤醒,它有一个參数表,存放着等待被唤醒的进程信息。
操作为:释放一个被占用的资源(把信号量加1),假设发现有被堵塞的进程,则选择一个唤醒之。
补充:查看共享信息的内存的命令是ipcs [-m|-s|-q] (所有的话是ipcs -a) ;查看共享信息的内存的命令是ipcs [-m|-s|-q]。
int semget(key_t key, int num_sems, int sem_flags);
int semop(int sem_id, struct sembuf *sem_opa, size_t num_sem_ops);
struct sembuf{ short sem_num;//除非使用一组信号量,否则它为0 short sem_op;//信号量在一次操作中须要改变的数据,一般是两个数,一个是-1,即P(等待)操作, //一个是+1,即V(发送信号)操作。 short sem_flg;//通常为SEM_UNDO,使操作系统跟踪信号, //并在进程没有释放该信号量而终止时,操作系统释放信号量 };
int semctl(int sem_id, int sem_num, int command, ...);
union semun{ int val; struct semid_ds *buf; unsigned short *arry; };
#if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED) /* union semun is defined by including <sys/sem.h> */ #else /* according to X/OPEN we have to define it ourselves */ union semun { int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_STAT, IPC_SET */ unsigned short int *array; /* array for GETALL, SETALL */ struct seminfo *__buf; /* buffer for IPC_INFO */ }; #endif
/* 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 <unistd.h> #include <stdlib.h> #include <stdio.h> #include <sys/sem.h> #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); }
linux进程间通讯-System V IPC 信号量,布布扣,bubuko.com
标签:des style blog http color 使用
原文地址:http://www.cnblogs.com/hrhguanli/p/3817222.html