3.编写C程序模拟实现单处理机系统中的进程调度算法，实现对多个进程的调度模拟，要求至少完成采用一种常见进程调度算法(如先来先服务、时间片轮转和优先级调度等算法)进行模拟调度。

在C程序中模拟单处理机系统的进程调度，可以使用结构体来表示进程，包含进程ID、状态（运行、就绪、等待）、优先级、时间片等相关信息。下面是一个基本的示例，我们将使用时间片轮转调度算法(Timeslice Scheduling)来模拟：

#include <stdio.h>
#include <stdlib.h>

// 定义进程结构体
typedef struct {
    int pid;       // 进程ID
    char state;    // 进程状态 (RUNNING, READY, WAITING)
    int priority;  // 进程优先级
    int time_slice; // 时间片大小
    int elapsed_time; // 已消耗的时间
} Process;

// 模拟函数
void simulate_timeslicing(Process* processes, int num_processes) {
    Process *current_process = &processes[0];
    while (true) {
        if (current_process->state == RUNNING) {
            // 运行时间片
            if (current_process->elapsed_time < current_process->time_slice) {
                current_process->elapsed_time++;
                printf("Process %d running for %d seconds.\n", current_process->pid, current_process->elapsed_time);
            } else {
                // 如果时间片结束，检查下一个进程
                switch_to_next_process(processes, num_processes);
            }
        }
        else if (current_process->state == READY) {
            // 当前进程变为就绪，有机会运行
            current_process->elapsed_time = 0;
            printf("Process %d becomes ready.\n", current_process->pid);
        }
    }
}

// 切换到下一个进程
void switch_to_next_process(Process* processes, int num_processes) {
    int next_pid = (current_process->pid + 1) % num_processes;
    Process *next = &processes[next_pid];
    
    if (next->state == WAITING) {
        printf("Switching to process %d from waiting.\n", next_pid);
    } else if (next->priority > current_process->priority) {
        printf("Switching to process %d with higher priority (%d vs %d).\n", next_pid, next->priority, current_process->priority);
    } else {
        printf("Switching to process %d in the order.\n", next_pid);
    }

    // 更新当前进程为下一个是就绪状态
    current_process = next;
    current_process->state = READY;
}

int main() {
    Process processes[] = {/* 初始化进程数组 */};
    int num_processes = sizeof(processes) / sizeof(processes[0]);

    simulate_timeslicing(processes, num_processes);

    return 0;
}

这个简单的例子展示了如何通过循环和条件判断实现一个基础的时间片轮转调度。你可以根据需要添加更多的功能，比如进程创建、进程阻塞/唤醒、进程结束等。