Linux的訊號量機制
#include<sys/sem.h>
int semctl(int sem_id , int sem_num , int command , ...);
int semget(key_t key , int num_set , int sem_flags);
int semop(int sem_id struct sembuf *sem_ops , size_t num_sem_ops);
標頭檔sys/sem.h通常依賴於另外兩個有檔案sys/types.h和sys/ipc.h。一般情況下,他們會被sys/sem.h自動包含,因此不需要為它們明確添加相應的#include語句。
參數key的作用很像一個檔案名稱,它代表程式可能要使用的某個資源,如果多個程式使用相同的key值,它將負責協調工作。
1.semget函數
semget函數的作用是建立一個新訊號量或取得一個已有訊號量的鍵:
int semget(key_t key , int num_set , int sem_flags);
第一個參數是整數值,不相關的進程可以通過它訪問同一個訊號量。只有semget函數才直接使用訊號量鍵,所有其他的訊號量函數都是使用有semget函數返回的訊號量標識符。
num_sems指定需要的訊號量的數目,它幾乎總是取值為1。
sem_flags參數是一組標誌,它與open函數的標誌非常相識。
2.semop函數
semop函數用於改變訊號量的值,它的定義如下所示:
int semop(int sem_id , struct sembuf *sem_ops , size_t num_sem_ops);
第一個參數sem_id是由semget返回的訊號量標識符。第二個參數sem_ops是指向一個結構數組的指標,每個數組元素至少包含一下幾個成員:
struct sembuf
{
short sem_num;
short sem_op;
short sem_flg;
}
第一個成員sem_num是訊號量編寫,除非你需要使用一組訊號量,否則它的取值一般是0。
sem_op成員的值是訊號量在一次操作中需要改變的數值。通常只會用到兩個值,一個是-1,也就是P操作,它等待訊號量變為可用。另外一個是V操作,它是+1,它發送訊號表示訊號量現在可用。
最後一個成員sem_flg通常被設定為SEM_UNDO。它將是的作業系統跟蹤當前進程對這個訊號量的修改情況。如果這個進程在沒有釋放該訊號量的情況下終止,作業系統將自動釋放該進程持有的訊號量。
3.semctl函數
int semctl(int sem_id , int sem_num , int command , ...);
第一個參數sem_id是由semget返回的訊號量標識符。sem_num參數是訊號量編寫,當需要用到成組的訊號量時,就要用到這個參數,它一般取值為0,表示這是第一個唯一的一個訊號量。command參數是將要採取的動作。如果還有第四個參數,它將會是一個union semun結構,它至少包含一下幾個成員:
union semun
{
int val;
struct semid_ds *buf;
unsigned short *array;
}
semctl函數中的command參數可以設定許多不同的值,但只有下面介紹的兩個值最常用。
SETVAL:用來把訊號量初始化為一個已知的值。這個用過union semun中的val成員設定。
IPC_RMID:用於刪除一個已經無需繼續使用的訊號量標識符。
/* 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);}