UNIX情形高级编程：system V信号量

信号量函数定义如下所示：

#include<sys/sem.h>

int semctl(int sem_id, int sem_num, int command, ...);//用来直接控制信号量信息

int semget(key_t key, int num_sems, int sem_flags);//创建一个新信号量或取得一个已有信号量的键

int semop(int sem_id, struct sembuf *sem_ops, size_t num_sem_ops);//用于改变信号量的值

/* 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 semaphoren");  
            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 - finishedn", 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 semaphoren");  
}  
      
/* 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 failedn");  
        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 failedn");  
        return(0);  
    }  
    return(1);  
}

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