操作系统：高优先级调度算法C语言实现

该算法用于作业调度时，系统从后备作业队列中选择若干个优先级最高的，且系统能满足资源要求的作业装入内存运行。当该算法用于进程调度时，将把处理机分配给就绪进程队列中优先级最高的进程。高优先级调度算法分为抢占的高优先级调度算法和非抢占的高优先级调度算法。非抢占式优先级算法，这种调度方式下，系统一旦把处理机分配给就绪队列中优先级最高的进程后，该进程就能一直执行下去，直至完成；或因等待某事件的发生使该进程不得不放弃处理机时，系统才能将处理机分配给另一个优先级高的就绪进程。抢占式优先级调度算法，在这种调度方式下，进程调度程序把处理机分配给当时优先级最高的就绪进程，使之执行。一旦出现了另一个优先级更高的就绪进程时，进程调度程序就停止正在执行的进程，将处理机分配给新出现的优先级最高的就绪进程。
  设置进程控制块结构体，以链表的形式构成进程组，包括进程名、需求时间、优先级、状态等信息。在运行时，选取优先级最高的进程执行，之后将此进程优先级降低，然后对所有进程排序，再重复此过程，直到所有进程执行完毕。
  流程图如下;

#include 
#include 

typedef struct pcb
{
    char name[5];
    struct pcb *next;
    int needtime;
    int priority;
    char state[5];
}NODE;

NODE *create_process(int n)
{
    NODE * head, * s, * t;
    int time,i=0,j;
    char pname[5];
    head=(NODE *)malloc(sizeof(NODE));
    printf("please input process name:");
    scanf("%s",pname);
    strcpy(head->name,pname);
    printf("please input need time:");
    scanf("%d",&time);
    head->needtime=time;
    printf("please input priority:");
    scanf("%d",&j);
    head->priority=j;
    strcpy(head->state,"ready");
    head->next=NULL;
    t=head;

    for(i=1;iname,pname);
        printf("please input need time :");
        scanf("%d",&time);
        s->needtime=time;
        printf("please input priority:");
        scanf("%d",&j);
        s->priority=j;
        strcpy(s->state,"ready");
        s->next=NULL;
        t->next=s;
        t=s;
    }
    return head;
}

void pri_process (NODE * p)
{
    int i;
    NODE * q;
    q=p-> next;
    printf ("nnamet needtimet priorityt staten");
    while (q!=NULL)
    {
        printf("%4st %2d tt %2d tt %5s n", q->name,q->needtime,q->priority,q->state);
        q=q-> next;
    }
}

NODE *order (NODE *head_sort)
{
    NODE * p, * s, * q, * head, * r, * t;
    int m, pr;
    char name[5];
    head=head_sort;
    p=head->next;
    r=p;
    t=p;
    q=p->next;
    while(r!=NULL)
    {
        while(q!=NULL)
        {
            if(p->prioritypriority)
            {
                m=p->priority;
                p->priority=q->priority;
                q->priority=m;
                strcpy(name,p->name);
                strcpy(p->name,q->name);
                strcpy(q->name,name);
                pr=p->needtime;
                p->needtime=q->needtime;
                q->needtime=pr;
            }
            p=q;
            q=q->next;
        }
        r=r->next;
        p=t;
        q=p->next;
    }
    return(head_sort);
}

void main ()
{
    NODE * p=NULL, * head=NULL, * m=NULL, * z=NULL, * n=NULL;
    int j, time, x=0;
    char c, pname [5];
    printf (" please input process number)!");
    scanf("%d",&x);
    head = (NODE *)malloc(sizeof(NODE)); // need to alloc area!
    p=create_process(x);
    head->next=p;
    pri_process(head);

    getchar();
    while(x>0)
    {
        order(head);
        m=head->next;
        strcpy(m->state,"run");
        if(m->priority>=2)
            m->priority--;
        m->needtime--;
        if(head->next!=NULL)
            pri_process(head);
        if(m->needtime==0)
        {
            head->next=m->next;
            printf("%s has finishedn",m->name);
            free(m);
            x--;
        }
        getchar();
    }
    printf("over!");
    getchar();
}