请用C语言编写一个C程序完成实验，实现先来先服务算法和短进程优先算法。要求定义进程控制块PCB，能输入进程个数、进程到达时间以及进程执行时间。请给出能准确运行且不闪退的详细代码。请让代码尽量简单且不闪退，有详细中文注释。请一次性给出完整的代码。

在C语言中，实现先来先服务（FCFS, First-Come-First-Served）和短进程优先（SJF, Shortest Job First）调度算法需要创建一个进程结构体（Process Control Block, PCB），并包含进程的基本信息和队列管理。以下是两个调度算法的简单实现，其中FCFS作为基础，然后添加了对SJF的支持。注意，这个例子假设所有进程都在同一时刻到达。

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

// 定义进程控制块结构
typedef struct {
    int pid;            // 进程ID
    int arrival_time;   // 到达时间
    int burst_time;     // 执行时间
    int service_time;   // 已服务时间
    struct process *next;  // 指向下一个PCB的指针
} Process;

// 初始化空队列
struct process *init_queue() {
    return NULL;
}

// 添加进程到队列
void add_to_queue(struct process **queue, Process *new_process) {
    if (*queue == NULL) {
        *queue = new_process;
    } else {
        Process *current = *queue;
        while (current->next != NULL) {
            current = current->next;
        }
        current->next = new_process;
    }
}

// FCFS调度函数
void fcfs_schedule(struct process **queue) {
    Process *current = *queue;
    while (current != NULL) {
        printf("PID %d arrives at time %d, burst time is %d.\n", 
               current->pid, current->arrival_time, current->burst_time);
        
        // 简单模拟进程执行
        sleep(current->burst_time); // 暂停执行时间
        current->service_time += current->burst_time;
        printf("PID %d has been served for %d time units.\n", 
               current->pid, current->service_time);
        
        // 移动到下一个进程
        current = current->next;
    }
}

// SJF调度函数 - 假设已知所有进程的CPU时间
void sjf_schedule(struct process **queue) {
    Process *shortest_burst = *queue;
    Process *current = *queue;
    
    while (current != NULL) {
        if (current->burst_time < shortest_burst->burst_time) {
            shortest_burst = current;
        }
        current = current->next;
    }

    fcfs_schedule(&shortest_burst);
}

int main() {
    int num_processes;
    printf("Enter the number of processes: ");
    scanf("%d", &num_processes);

    Process *processes[num_processes];
    srand(time(0)); // 设置随机种子

    for (int i = 0; i < num_processes; i++) {
        processes[i] = (Process*)malloc(sizeof(Process));
        processes[i]->pid = i + 1;
        processes[i]->arrival_time = rand() % 100; // 随机生成到达时间
        processes[i]->burst_time = rand() % 20 + 1; // 随机生成执行时间
        processes[i]->service_time = 0;
        processes[i]->next = NULL;
    }

    // 将进程加入FCFS队列
    struct process *queue = init_queue();
    for (int i = 0; i < num_processes; i++) {
        add_to_queue(&queue, processes[i]);
    }

    printf("\nFCFS scheduling:\n");
    fcfs_schedule(&queue);

    printf("\nSJF scheduling (assuming CPU times are known):\n");
    sjf_schedule(&queue);

    return 0;
}