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csapp之lab：shell lab

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Linux C++

实验目的

shell lab主要目的是为了熟悉进程控制和信号。具体来说需要比对16个test和rtest文件的输出，实现五个函数：

void eval(char *cmdline)：分析命令，并派生子进程执行 主要功能是解析cmdline并运行
int builtin_cmd(char **argv)：解析和执行bulidin命令，包括 quit, fg, bg, and jobs
void do_bgfg(char **argv) 执行bg和fg命令
void waitfg(pid_t pid)：实现阻塞等待前台程序运行结束
void sigchld_handler(int sig)：SIGCHID信号处理函数
void sigint_handler(int sig)：信号处理函数，响应 SIGINT (ctrl-c) 信号 
void sigtstp_handler(int sig)：信号处理函数，响应 SIGTSTP (ctrl-z) 信号

辅助函数

可用辅助函数：

int parseline(const char *cmdline,char **argv)：获取参数列表，返回是否为后台运行命令
void clearjob(struct job_t *job)：清除job结构体。
void initjobs(struct job_t *jobs)：初始化jobs链表。
void maxjid(struct job_t *jobs)：返回jobs链表中最大的jid号。
int addjob(struct job_t *jobs,pid_t pid,int state,char *cmdline)：在jobs链表中添加job
int deletejob(struct job_t *jobs,pid_t pid)：在jobs链表中删除pid的job。
pid_t fgpid(struct job_t *jobs)：返回当前前台运行job的pid号。
struct job_t *getjobpid(struct job_t *jobs,pid_t pid)：返回pid号的job。
struct job_t *getjobjid(struct job_t *jobs,int jid)：返回jid号的job。
int pid2jid(pid_t pid)：将pid号转化为jid。
void listjobs(struct job_t *jobs)：打印jobs。
void sigquit_handler(int sig)：处理SIGQUIT信号。

简介

shell是交互式的命令行解释器，打印提示符并在stdin上等待输入命令，并按照命令行的内容执行。命令行是ASCII单词组成的命令和参数序列。若首个单词是内置命令，shell会立即在当前进程中执行。否则是可执行文件路径，shell派生出子进程，然后在该子进程的上下文中加载和运行程序。解释单个命令行而创建的子进程叫作业，通常由Unix管道连接的多个子进程组成。**若命令行以&号“&”结束，则作业将在后台运行且不会等待作业结束。否则作业将在前台运行且等待作业终止。**故在任何时间点最多仅一个作业在前台运行。但可在后台运行任意数量的作业。

例如：tsh> /bin/ls -l -d

在前台运行程序，程序的入口是：int main(int argc,char *argv[])

则argc3，argv[0] == ‘‘/bin/ls’’，argv[1] ‘‘-l’’,argv[2]== ‘‘-d’’。若在命令行后加上&，则在后台运行ls程序。shell支持作业控制，允许用户在后台和前台移动作业，并更改作业中进程的状态（运行、停止或终止）。输入ctrl-c会向前台作业中的每个进程发送SIGINT信号，默认操作是终止进程。类似地，键入ctrl-z将向前台作业中的每个进程发送SIGTSTP信号，默认操作是将进程置于停止状态，直到收到SIGCONT信号将其唤醒。当然shell也提供内置命令支持作业控制：

jobs：列出运行和终止的后台作业
bg ：将终止的后台作业改为运行
fg ：将终止或运行的后台作业改为前台运行
kill ：发送特定信号给特定进程和进程组，默认动作是终止进程
quit：终止shell

有三点值得注意：

tsh不支持管道和I/O重定向
每个作业要么被process ID识别，要么被job ID识别，jid因该在命令行中用前缀“%”表示，“%5”表示jid 5，5表示PID 5
shell因该回收所有僵尸进程，若任何一个作业因为接收到它没有捕捉到的信号而终止，那么tsh应该识别该事件，并打印PID和错误描述消息

提示

仔细阅读CSAPP第八章的异常控制流和lab的writeup
make testn测试shell执行第n组测试数据的输出，make rtestn打印shell预期输出，tshref.out包含shell所有预期输出结果，先看文件输出，了解命令格式再编码，修改makefile文件中CFLAGS字段，加-g参数并去掉-O2参数
waitpid, kill, fork,execve, setpgid, sigprocmask 很常用，可通过命令手册查看使用细节，WUNTRACED和WNOHANG选项对waitpid也很有用
实现信息处理函数，确保发送SIGINT和SIGTSTP信号给整个前台进程组，用-pid代替pid作为kill参数
建议在waitfg的循环中用sleep函数，在sigchld_handler中对waitpid只调用一次
eval中进程在fork之前用sigprocmask阻塞SIGCHLD信号，之后在解除信号阻塞，之后在调用addjob添加孩子到作业列表用sigprocmask阻塞信号，因为子继承继承父进程的阻塞集合，所以子程序必须确保在执行新进程前解除阻塞SIGCHLD信号。父进程需以这种方式阻塞SIGCHLD信号，避免在父进程调用addjob之前，SIGCHLD处理器获取子进程(从而从任务列表中删除)的竞争状态。
不要直接调用常用命令，而应输入完整路径，如/bin/ls
当在标准Unix shell运行tsh时，tsh运行在前台进程组中。若tsh随后创建子进程，默认情况下，该子进程也是前台进程组的成员。因为按下ctrl-c会向前台组中的每个进程发送SIGINT信号，按下ctrl-c会向tsh及Unix shell创建的每个子进程，显然不正确。应该在fork后，但在execve前，子进程调用setpgid(0,0)，把子进程放到新进程组中，该进程组ID与子进程的PID相同。确保前台进程组中只有一个进程，即tsh进程。当按下ctrl-c时，tsh应捕获生成的SIGINT，然后将其转发给包含前台作业的进程组。

实验前环境配置

由于csapp都是运行在32位系统，即使安装32位系统所需的库，仍然无法运行tsh，在网上找到有人配置好的csapp的docker镜像，因此直接使用docker，环境配置如下：

安装docker，并配置加速
安装vscode和ssh插件
命令行中运行systemctl start docker启动docker和docker run --privileged -d -p 1221:22 --name shell yansongsongsong/csapp:shelllabshell lab的实验环境
通过ssh输入密码登录实验环境

实验

在vscode中打开shlab-handout文件夹，并打开tsh.c文件，可以看到在main函数中调用eval函数，而在书P525或20-ecf-sigs的P19可找到eval函数的整体代码框架：

void eval(char *cmdline) 
{
    char *argv[MAXARGS];/*Argument list execve() */
    char buf[MAXLINE];/*Holds modified command line */
    int bg;/*Should the job run in bg or fg? */
    pid_t pid;/*Process id */

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);
    if (argv[0] == NULL)
        return;/* Ignore empty lines */

    if (!builtin_cmd(argv)) {
        if ((pid = Fork()) == 0) {/* Child runs user job */
            Execve(argv[0], argv, environ);
        }/* Parent waits for foreground job to terminate */
        if (!bg) {
            int status;
            if (waitpid(pid, &status,0) < 0)
                unix_error("waitfg: waitpid error");
        }
        else
            printf("%d %s", pid, cmdline);
    }
    return;
}

尽管ppt上说有bug，暂时先不管，先搞好整体框架，完成简单的函数，到后面在考虑。另外值得一提的是这里将fork和execve都进行封装以处理错误情况。
运行make rtest01和make test01可以看到输出一样，已经达到要求。同样操作，可以看到test02未按照预期退出tsh，分析知需要实现builtin_cmd函数。同样在书上P525能找到基础代码，只需加上jobs、fg、bg3种情况即可。代码如下：

int builtin_cmd(char **argv) 
{
    if(!strcmp(argv[0],"quit")) /* quit command */
        exit(0);

    if (!strcmp(argv[0], "&")) /* Ignore singleton & */
        return 1; 
    
    if(!strcmp((argv[0]),"jobs"))/* jobs command */
    {
        listjobs(jobs);
        return 1;
    }

    if(!strcmp((argv[0]),"fg") || !strcmp((argv[0]),"bg"))/* bg/fg command */
    {
        do_bgfg(argv);
        return 1;
    }
    return 0;     /* not a builtin command */
}

这样就过了test02和test03，经过比较test04和rtest04的输出，确定只需修改输出格式即可：

printf("[%d] (%d) %s", pid2jid(pid),pid, cmdline);

接着发现test05是执行内部命令：jobs，打印job list，比对rtest05发现没有打印出job，参考上面的提示第6条，知应同步避免父子竞争，具体来说：父进程在fork前屏蔽信号，子进程在execve前还原信号，因为子进程回继承原来的屏蔽信号。同时前台job需要调用waitfg进行等待。如果不阻塞会出现子进程先结束从jobs中删除，然后再执行到主进程addjob的竞争问题。在书上P542和PPT P57页都有对应的参考代码：

int main(int argc, char **argv)
{
    int pid;
    sigset_t mask_all, mask_one, prev_one;
    int n = N; /* N = 5 */
    Sigfillset(&mask_all);
    Sigemptyset(&mask_one);
    Sigaddset(&mask_one, SIGCHLD);
    Signal(SIGCHLD, handler);
    initjobs(); /* Initialize the job list */
    
    while (n--) {
        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one); /* Block SIGCHLD */
        if ((pid = Fork()) == 0) { /* Child process */
            Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
            Execve("/bin/date", argv, NULL);
        }
        Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Parent process */
        addjob(pid); /* Add the child to the job list */
        Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
    }
    exit(0);
}

加上图中对应代码，同时若子进程结束，需要delete job，在sigchld_handler中加上非阻塞循环等待子进程的代码：

void eval(char *cmdline) 
{
    char *argv[MAXARGS];/*Argument list execve() */
    char buf[MAXLINE];/*Holds modified command line */
    int bg;/*Should the job run in bg or fg? */
    pid_t pid;/*Process id */
    sigset_t mask_all,mask_one,prev_one;

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);
    if (argv[0] == NULL)
        return;/* Ignore empty lines */

    if (!builtin_cmd(argv)) {
        Sigfillset(&mask_all);/* add every signal number to set */
        Sigemptyset(&mask_one);/* create empty set */
        Sigaddset(&mask_one, SIGCHLD);/* add signal number to set */

        /*  block SIGINT and save previous blocked set */
        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one); /* Block SIGCHLD */
        if ((pid = Fork()) == 0) {/* Child runs user job */
            /* restore previous blocked set,unblocking SIGINT */
            Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
            //Setpgid(0,0);
            Execve(argv[0], argv, environ);
        }/* Parent waits for foreground job to terminate */

        Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Block SIGCHLD */
        int st = (bg==0) ? FG : BG;
        addjob(jobs,pid,st,cmdline);
        Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
        if (!bg) {
            //由于sigchld_handler上面被调用，而上面回调用waitpid，因此这里不用调用只需循环等待即可
            waitfg(pid);
        }
        else
            printf("[%d] (%d) %s", pid2jid(pid),pid, cmdline);
    }
    return;
}

void sigchld_handler(int sig) 
{
    int olderrno = errno;
    sigset_t mask_all,prev_all;
    pid_t pid;
    Sigfillset(&mask_all);
    /*改成非阻塞，否则test05中运行到此处，前端进程执行jobs会阻塞直到所有子进程都被回收，即两个后端进程都执行并delete才会离开，则jobs命令什么也没有打印*/
    while((pid = waitpid(-1,NULL,WNOHANG | WUNTRACED))>0){
        Sigprocmask(SIG_BLOCK,&mask_all,&prev_all);
        deletejob(jobs,pid);
        Sigprocmask(SIG_SETMASK,&prev_all,NULL);
    }
    errno = olderrno;
    return;
}

如果是前台命令，则调用waitfg循环等待，在注释中看到最好不要用waitpid(pid,NULL,0)，其次根据上面的提示，不要同时在sigchld_handler和waitfg函数中使用waitpid，因为在同一个程序的两个地方都回收僵死进程，虽然也行，但容易让人迷惑：

void waitfg(pid_t pid)
{
    while(fgpid(jobs))
        usleep(1000);//一秒
    return;
}

这样就完成test05，接下来test06和test07、test08就是实现SIGINT和SIGSTOP信号处理函数，注意前面提示的第4条用-pid作为kill的参数，同时最后一条在fork后execve前子进程应调用setpgid(0,0)，否则回报错No such process，注意sigint_handler和sigtstp_handler只需调用kill即可，将输出留到sigchld_handler中，这样就需修改前面的sigchld_handler以处理不同子进程退出状态：

void sigint_handler(int sig) 
{
    int olderrno = errno;
    pid_t fg = fgpid(jobs);
    if(fg){
        Kill(-fg,sig);
    }
    errno = olderrno;
    return;
}
void sigtstp_handler(int sig) 
{
    int olderrno = errno;
    pid_t fg = fgpid(jobs);
    if(fg){
        Kill(-fg,sig);
    }
    errno = olderrno;
    return;
}

void sigchld_handler(int sig) 
{
    int olderrno = errno;
    sigset_t mask_all,prev;
    pid_t pid;
    int status;
    Sigfillset(&mask_all);
    /*改成非阻塞，否则test05中运行到此处，前端进程执行jobs会阻塞直到所有子进程都被回收，即两个后端进程都执行并delete才会离开，则jobs命令什么也没有打印*/
    while((pid = waitpid(-1,&status,WNOHANG | WUNTRACED))>0){
        // WNOHANG | WUNTRACED 是立即返回
        // 用WIFEXITED(status)，WIFSIGNALED(status)，WIFSTOPPED(status)等来补获终止或者
        // 被停止的子进程的退出状态。
    	if (WIFEXITED(status))  // 正常退出 delete
    	{
    		sigprocmask(SIG_BLOCK, &mask_all, &prev);
    		deletejob(jobs, pid);
    		sigprocmask(SIG_SETMASK, &prev, NULL);
    	}
    	else if (WIFSIGNALED(status))  // 信号退出 delete
    	{
    	    struct job_t* job = getjobpid(jobs, pid);
            sigprocmask(SIG_BLOCK, &mask_all, &prev);
            printf("Job [%d] (%d) terminated by signal %d\n", job->jid, job->pid, WTERMSIG(status));
            deletejob(jobs, pid);
            sigprocmask(SIG_SETMASK, &prev, NULL);
    	}
    	else  // 停止 只修改状态就行
    	{
            struct job_t* job = getjobpid(jobs, pid);
            sigprocmask(SIG_BLOCK, &mask_all, &prev);
            printf("Job [%d] (%d) stopped by signal %d\n", job->jid, job->pid, WSTOPSIG(status));
            job->state= ST;
            sigprocmask(SIG_SETMASK, &prev, NULL);
        }
    }
    errno = olderrno;  // 恢复
    return;
}

这样就完成test06和test07、test08。接下来test09和test10是测试fg和bg内置命令，先解析命令通过getjobjid或getjobpid获取job，再分情况对fg和bg命令做不同处理，输入%num 代表任务id，num代表进程id，分情况讨论即可，但要注意各种异常情况：

void do_bgfg(char **argv) 
{
    if(!argv[1]){
        printf("%s command requires PID or %%jobid argument\n", argv[0]);
        return;
    }

    if (!isdigit(argv[1][0]) && argv[1][0] != '%') {            // Checks if the second argument is valid
        printf("%s: argument must be a PID or %%jobid\n", argv[0]);
        return;
    }

    struct job_t* myjob;
    if(argv[1][0]=='%'){
        myjob = getjobjid(jobs,atoi(&argv[1][1]));
        if(!myjob){
            printf("%s: No such job\n", argv[1]);
            return;
        }
    }else{
        myjob = getjobpid(jobs,atoi(argv[1]));
        if (!myjob) {                                 // Checks if the given PID is there
            printf("(%d): No such process\n", atoi(argv[1]));
            return;
        }
    }

    Kill(-myjob->pid,SIGCONT);
    if(!strcmp(argv[0],"bg")){
        myjob->state = BG;
        printf("[%d] (%d) %s",myjob->jid,myjob->pid,myjob->cmdline);
    }else{
        myjob->state = FG;
        waitfg(myjob->pid);
    }

    return;
}

这样就过了test09和test10。接下来test11 和test12 、test13分别测试Forward SIGINT、Forward SIGTSTP、Restart stopped process都能正常通过，若每通过，因该是前面某些测试有问题，解决后即可。test14是测试JID或PID的错误输入的情况，较容易通过。test15将前面所有测试情况放一起，也顺利通过，而test16是测试tsh能否处理不是来自终端而是来自其他进程的SIGSTP和SIGINT信号，顺利通过。

总结

最终代码见下，该实验主要涉及加载、进程控制、信号等基础但很重要的知识，涉及到异常控制流、进程、系统调用、信号处理函数与非本地跳转等并发编程的知识。并发的同步问题是关键，利用信号屏蔽与还原就能解决。此外阅读 man 手册了解系统接口使用细节对完成实验很有帮助。

/* 
 * tsh - A tiny shell program with job control
 * 
 * <Put your name and login ID here>
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

/* Misc manifest constants */
#define MAXLINE    1024   /* max line size */
#define MAXARGS     128   /* max args on a command line */
#define MAXJOBS      16   /* max jobs at any point in time */
#define MAXJID    1<<16   /* max job ID */

/* Job states */
#define UNDEF 0 /* undefined */
#define FG 1    /* running in foreground */
#define BG 2    /* running in background */
#define ST 3    /* stopped */

/* 
 * Jobs states: FG (foreground), BG (background), ST (stopped)
 * Job state transitions and enabling actions:
 *     FG -> ST  : ctrl-z
 *     ST -> FG  : fg command
 *     ST -> BG  : bg command
 *     BG -> FG  : fg command
 * At most 1 job can be in the FG state.
 */

/* Global variables */
extern char **environ;      /* defined in libc */
char prompt[] = "tsh> ";    /* command line prompt (DO NOT CHANGE) */
int verbose = 0;            /* if true, print additional output */
int nextjid = 1;            /* next job ID to allocate */
char sbuf[MAXLINE];         /* for composing sprintf messages */

struct job_t {              /* The job struct */
    pid_t pid;              /* job PID */
    int jid;                /* job ID [1, 2, ...] */
    int state;              /* UNDEF, BG, FG, or ST */
    char cmdline[MAXLINE];  /* command line */
};
struct job_t jobs[MAXJOBS]; /* The job list */
/* End global variables */

/*error handling function */
pid_t Fork(void);
void Execve(const char *filename, char *const argv[], char *const environ[]);
void Kill(pid_t pid, int signum);
void Sigemptyset(sigset_t *set);
void Sigaddset(sigset_t *set, int signum);
void Sigfillset(sigset_t *set);
void Setpgid(pid_t pid, pid_t pgid);
void Sigprocmask(int how, sigset_t *set, sigset_t *oldset);
/* Function prototypes */

/* Here are the functions that you will implement */
void eval(char *cmdline);
int builtin_cmd(char **argv);
void do_bgfg(char **argv);
void waitfg(pid_t pid);

void sigchld_handler(int sig);
void sigtstp_handler(int sig);
void sigint_handler(int sig);

/* Here are helper routines that we've provided for you */
int parseline(const char *cmdline, char **argv); 
void sigquit_handler(int sig);

void clearjob(struct job_t *job);
void initjobs(struct job_t *jobs);
int maxjid(struct job_t *jobs); 
int addjob(struct job_t *jobs, pid_t pid, int state, char *cmdline);
int deletejob(struct job_t *jobs, pid_t pid); 
pid_t fgpid(struct job_t *jobs);
struct job_t *getjobpid(struct job_t *jobs, pid_t pid);
struct job_t *getjobjid(struct job_t *jobs, int jid); 
int pid2jid(pid_t pid); 
void listjobs(struct job_t *jobs);

void usage(void);
void unix_error(char *msg);
void app_error(char *msg);
typedef void handler_t(int);
handler_t *Signal(int signum, handler_t *handler);

/*
 * main - The shell's main routine 
 */
int main(int argc, char **argv) 
{
    char c;
    char cmdline[MAXLINE];
    int emit_prompt = 1; /* emit prompt (default) */

    /* Redirect stderr to stdout (so that driver will get all output
     * on the pipe connected to stdout) */
    dup2(1, 2);

    /* Parse the command line */
    while ((c = getopt(argc, argv, "hvp")) != EOF) {
        switch (c) {
        case 'h':             /* print help message */
            usage();
	    break;
        case 'v':             /* emit additional diagnostic info */
            verbose = 1;
	    break;
        case 'p':             /* don't print a prompt */
            emit_prompt = 0;  /* handy for automatic testing */
	    break;
	default:
            usage();
	}
    }

    /* Install the signal handlers */

    /* These are the ones you will need to implement */
    Signal(SIGINT,  sigint_handler);   /* ctrl-c */
    Signal(SIGTSTP, sigtstp_handler);  /* ctrl-z */
    Signal(SIGCHLD, sigchld_handler);  /* Terminated or stopped child */

    /* This one provides a clean way to kill the shell */
    Signal(SIGQUIT, sigquit_handler); 

    /* Initialize the job list */
    initjobs(jobs);

    /* Execute the shell's read/eval loop */
    while (1) {

	/* Read command line */
	if (emit_prompt) {
	    printf("%s", prompt);
	    fflush(stdout);
	}
	if ((fgets(cmdline, MAXLINE, stdin) == NULL) && ferror(stdin))
	    app_error("fgets error");
	if (feof(stdin)) { /* End of file (ctrl-d) */
	    fflush(stdout);
	    exit(0);
	}

	/* Evaluate the command line */
	eval(cmdline);
	fflush(stdout);
	fflush(stdout);
    } 

    exit(0); /* control never reaches here */
}
  
/* 
 * eval - Evaluate the command line that the user has just typed in
 * 
 * If the user has requested a built-in command (quit, jobs, bg or fg)
 * then execute it immediately. Otherwise, fork a child process and
 * run the job in the context of the child. If the job is running in
 * the foreground, wait for it to terminate and then return.  Note:
 * each child process must have a unique process group ID so that our
 * background children don't receive SIGINT (SIGTSTP) from the kernel
 * when we type ctrl-c (ctrl-z) at the keyboard.  
*/
void eval(char *cmdline) 
{
    char *argv[MAXARGS];/*Argument list execve() */
    char buf[MAXLINE];/*Holds modified command line */
    int bg;/*Should the job run in bg or fg? */
    pid_t pid;/*Process id */
    sigset_t mask_all,mask_one,prev_one;

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);
    if (argv[0] == NULL)
        return;/* Ignore empty lines */

    if (!builtin_cmd(argv)) {
        //blocking SIGCHLD in if status,otherewise it maybe has bugs
        Sigfillset(&mask_all);/* add every signal number to set */
        Sigemptyset(&mask_one);/* create empty set */
        Sigaddset(&mask_one, SIGCHLD);/* add signal number to set */

        /*  block SIGINT and save previous blocked set */
        /* avoid parent process run to addjob exited,before fork child process block sigchild signal,after call addjob unblock  */
        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one); /* Block SIGCHLD */
        if ((pid = Fork()) == 0) {/* Child runs user job */
            /* restore previous blocked set,unblocking SIGINT */
            /* child process inherit parent process' blocking sets,avoid it can't receive itself child process signal,so we must unblock */
            Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
            Setpgid(0,0);// set child's group to a new process group (this is identical to the child's PID)
            Execve(argv[0], argv, environ);//this function not return ,so must call exit,otherewise it will run forever
        }/* Parent waits for foreground job to terminate */

        Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Block SIGCHLD */
        int st = (bg==0) ? FG : BG;
        addjob(jobs,pid,st,cmdline);
        Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
        if (!bg) {
            //because sigchld_handler was called above，it call waitpid，so don't call and circular wait wait
            waitfg(pid);
        }
        else
            printf("[%d] (%d) %s", pid2jid(pid),pid, cmdline);
    }
    return;
}

/* 
 * parseline - Parse the command line and build the argv array.
 * 
 * Characters enclosed in single quotes are treated as a single
 * argument.  Return true if the user has requested a BG job, false if
 * the user has requested a FG job.  
 */
int parseline(const char *cmdline, char **argv) 
{
    static char array[MAXLINE]; /* holds local copy of command line */
    char *buf = array;          /* ptr that traverses command line */
    char *delim;                /* points to first space delimiter */
    int argc;                   /* number of args */
    int bg;                     /* background job? */

    strcpy(buf, cmdline);
    buf[strlen(buf)-1] = ' ';  /* replace trailing '\n' with space */
    while (*buf && (*buf == ' ')) /* ignore leading spaces */
	buf++;

    /* Build the argv list */
    argc = 0;
    if (*buf == '\'') {
	buf++;
	delim = strchr(buf, '\'');
    }
    else {
	delim = strchr(buf, ' ');
    }

    while (delim) {
	argv[argc++] = buf;
	*delim = '\0';
	buf = delim + 1;
	while (*buf && (*buf == ' ')) /* ignore spaces */
	       buf++;

	if (*buf == '\'') {
	    buf++;
	    delim = strchr(buf, '\'');
	}
	else {
	    delim = strchr(buf, ' ');
	}
    }
    argv[argc] = NULL;
    
    if (argc == 0)  /* ignore blank line */
	return 1;

    /* should the job run in the background? */
    if ((bg = (*argv[argc-1] == '&')) != 0) {
	argv[--argc] = NULL;
    }
    return bg;
}

/* 
 * builtin_cmd - If the user has typed a built-in command then execute
 *    it immediately.  
 */
int builtin_cmd(char **argv) 
{
    if(!strcmp(argv[0],"quit")) /* quit command */
        exit(0);

    if (!strcmp(argv[0], "&")) /* Ignore singleton & */
        return 1; 
    
    if(!strcmp((argv[0]),"jobs"))/* jobs command */
    {
        listjobs(jobs);
        return 1;
    }

    if(!strcmp((argv[0]),"fg") || !strcmp((argv[0]),"bg"))/* bg/fg command */
    {
        do_bgfg(argv);
        return 1;
    }
    return 0;     /* not a builtin command */
}

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    if(!argv[1]){
        printf("%s command requires PID or %%jobid argument\n", argv[0]);
        return;
    }

    if (!isdigit(argv[1][0]) && argv[1][0] != '%') {            // Checks if the second argument is valid
        printf("%s: argument must be a PID or %%jobid\n", argv[0]);
        return;
    }

    struct job_t* myjob;
    if(argv[1][0]=='%'){//jid
        myjob = getjobjid(jobs,atoi(&argv[1][1]));
        if(!myjob){
            printf("%s: No such job\n", argv[1]);
            return;
        }
    }else{//pid
        myjob = getjobpid(jobs,atoi(argv[1]));
        if (!myjob) {                                 // Checks if the given PID is there
            printf("(%d): No such process\n", atoi(argv[1]));
            return;
        }
    }

    Kill(-myjob->pid,SIGCONT);//send continue signal 
    if(!strcmp(argv[0],"bg")){
        myjob->state = BG;
        printf("[%d] (%d) %s",myjob->jid,myjob->pid,myjob->cmdline);
    }else{
        myjob->state = FG;
        waitfg(myjob->pid);
    }

    return;
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid)
{
    while(fgpid(jobs))
        usleep(1000);//sleep one second
    return;
}

/*****************
 * Signal handlers
 *****************/

/* 
 * sigchld_handler - The kernel sends a SIGCHLD to the shell whenever
 *     a child job terminates (becomes a zombie), or stops because it
 *     received a SIGSTOP or SIGTSTP signal. The handler reaps all
 *     available zombie children, but doesn't wait for any other
 *     currently running children to terminate.  
 */
void sigchld_handler(int sig) 
{
    int olderrno = errno;
    sigset_t mask_all,prev;
    pid_t pid;
    int status;
    Sigfillset(&mask_all);
    while((pid = waitpid(-1,&status,WNOHANG | WUNTRACED))>0){
        // WNOHANG | WUNTRACED return immediately
    	if (WIFEXITED(status))  // normally exited,delete job
    	{
    		sigprocmask(SIG_BLOCK, &mask_all, &prev);
    		deletejob(jobs, pid);
    		sigprocmask(SIG_SETMASK, &prev, NULL);
    	}
    	else if (WIFSIGNALED(status))  //terminated by signal, delete job and print message
    	{
    	    struct job_t* job = getjobpid(jobs, pid);
            sigprocmask(SIG_BLOCK, &mask_all, &prev);
            printf("Job [%d] (%d) terminated by signal %d\n", job->jid, job->pid, WTERMSIG(status));
            deletejob(jobs, pid);
            sigprocmask(SIG_SETMASK, &prev, NULL);
    	}
    	else  //stopped,change the status
    	{
            struct job_t* job = getjobpid(jobs, pid);
            sigprocmask(SIG_BLOCK, &mask_all, &prev);
            printf("Job [%d] (%d) stopped by signal %d\n", job->jid, job->pid, WSTOPSIG(status));
            job->state= ST;
            sigprocmask(SIG_SETMASK, &prev, NULL);
        }
        //actually there is WIFCONTINUED,but we don't care about
    }
    errno = olderrno;  
    return;
}

/* 
 * sigint_handler - The kernel sends a SIGINT to the shell whenver the
 *    user types ctrl-c at the keyboard.  Catch it and send it along
 *    to the foreground job.  
 */
void sigint_handler(int sig) 
{
    int olderrno = errno;
    pid_t fg = fgpid(jobs);
    if(fg){
        Kill(-fg,sig);
    }
    errno = olderrno;
    return;
}

/*
 * sigtstp_handler - The kernel sends a SIGTSTP to the shell whenever
 *     the user types ctrl-z at the keyboard. Catch it and suspend the
 *     foreground job by sending it a SIGTSTP.  
 */
void sigtstp_handler(int sig) 
{
    int olderrno = errno;
    pid_t fg = fgpid(jobs);
    if(fg){
        Kill(-fg,sig);
    }
    errno = olderrno;
    return;
}

/*********************
 * End signal handlers
 *********************/

/***********************************************
 * Helper routines that manipulate the job list
 **********************************************/

/* clearjob - Clear the entries in a job struct */
void clearjob(struct job_t *job) {
    job->pid = 0;
    job->jid = 0;
    job->state = UNDEF;
    job->cmdline[0] = '\0';
}

/* initjobs - Initialize the job list */
void initjobs(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++)
	clearjob(&jobs[i]);
}

/* maxjid - Returns largest allocated job ID */
int maxjid(struct job_t *jobs) 
{
    int i, max=0;

    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].jid > max)
	    max = jobs[i].jid;
    return max;
}

/* addjob - Add a job to the job list */
int addjob(struct job_t *jobs, pid_t pid, int state, char *cmdline) 
{
    int i;
    
    if (pid < 1)
	return 0;

    for (i = 0; i < MAXJOBS; i++) {
	if (jobs[i].pid == 0) {
	    jobs[i].pid = pid;
	    jobs[i].state = state;
	    jobs[i].jid = nextjid++;
	    if (nextjid > MAXJOBS)
		nextjid = 1;
	    strcpy(jobs[i].cmdline, cmdline);
  	    if(verbose){
	        printf("Added job [%d] %d %s\n", jobs[i].jid, jobs[i].pid, jobs[i].cmdline);
            }
            return 1;
	}
    }
    printf("Tried to create too many jobs\n");
    return 0;
}

/* deletejob - Delete a job whose PID=pid from the job list */
int deletejob(struct job_t *jobs, pid_t pid) 
{
    int i;

    if (pid < 1)
	    return 0;

    for (i = 0; i < MAXJOBS; i++) {
        if (jobs[i].pid == pid) {
            clearjob(&jobs[i]);
            nextjid = maxjid(jobs)+1;
            return 1;
        }
    }
    return 0;
}

/* fgpid - Return PID of current foreground job, 0 if no such job */
pid_t fgpid(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].state == FG)
	    return jobs[i].pid;
    return 0;
}

/* getjobpid  - Find a job (by PID) on the job list */
struct job_t *getjobpid(struct job_t *jobs, pid_t pid) {
    int i;

    if (pid < 1)
	return NULL;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].pid == pid)
	    return &jobs[i];
    return NULL;
}

/* getjobjid  - Find a job (by JID) on the job list */
struct job_t *getjobjid(struct job_t *jobs, int jid) 
{
    int i;

    if (jid < 1)
	return NULL;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].jid == jid)
	    return &jobs[i];
    return NULL;
}

/* pid2jid - Map process ID to job ID */
int pid2jid(pid_t pid) 
{
    int i;

    if (pid < 1)
	return 0;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].pid == pid) {
            return jobs[i].jid;
        }
    return 0;
}

/* listjobs - Print the job list */
void listjobs(struct job_t *jobs) 
{
    int i;
    
    for (i = 0; i < MAXJOBS; i++) {
	if (jobs[i].pid != 0) {
	    printf("[%d] (%d) ", jobs[i].jid, jobs[i].pid);
	    switch (jobs[i].state) {
		case BG: 
		    printf("Running ");
		    break;
		case FG: 
		    printf("Foreground ");
		    break;
		case ST: 
		    printf("Stopped ");
		    break;
	    default:
		    printf("listjobs: Internal error: job[%d].state=%d ", 
			   i, jobs[i].state);
	    }
	    printf("%s", jobs[i].cmdline);
	}
    }
}
/******************************
 * end job list helper routines
 ******************************/


/***********************
 * Other helper routines
 ***********************/

/*
 * usage - print a help message
 */
void usage(void) 
{
    printf("Usage: shell [-hvp]\n");
    printf("   -h   print this message\n");
    printf("   -v   print additional diagnostic information\n");
    printf("   -p   do not emit a command prompt\n");
    exit(1);
}

/*
 * unix_error - unix-style error routine
 */
void unix_error(char *msg)
{
    fprintf(stdout, "%s: %s\n", msg, strerror(errno));
    exit(1);
}

/*
 * app_error - application-style error routine
 */
void app_error(char *msg)
{
    fprintf(stdout, "%s\n", msg);
    exit(1);
}

/*
 * Signal - wrapper for the sigaction function
 */
handler_t *Signal(int signum, handler_t *handler) 
{
    struct sigaction action, old_action;

    action.sa_handler = handler;  
    sigemptyset(&action.sa_mask); /* block sigs of type being handled */
    action.sa_flags = SA_RESTART; /* restart syscalls if possible */

    if (sigaction(signum, &action, &old_action) < 0)
	unix_error("Signal error");
    return (old_action.sa_handler);
}

/*
 * sigquit_handler - The driver program can gracefully terminate the
 *    child shell by sending it a SIGQUIT signal.
 */
void sigquit_handler(int sig) 
{
    printf("Terminating after receipt of SIGQUIT signal\n");
    exit(1);
}

/******************************
 * my functions with error handling
 ******************************/

/*
 * fork error handling
 */
pid_t Fork(void)
{
    pid_t pid;

    if ((pid = fork()) < 0)
        unix_error("Fork error");
    return pid;
}

/*
 * execve error handling
 */
void Execve(const char *filename, char *const argv[], char *const environ[])
{
    if (execve(filename, argv, environ) < 0) {
        printf("%s: Command not found.\n", argv[0]);
        exit(0);
    }
}

/*
 * kill error handling
 */
void Kill(pid_t pid, int signum) 
{
    int kr;

    if ((kr = kill(pid, signum)) < 0)
        unix_error("Kill error");
    return;
}

/*
 * sigemptyset error handling
 */
void Sigemptyset(sigset_t *set)
{
    if(sigemptyset(set)<0)
        unix_error("Sigemptyset error");
    return;
}
/*
 * sigaddset error handling
 */
void Sigaddset(sigset_t *set,int sign)
{
    if(sigaddset(set,sign)<0)
        unix_error("Sigaddset error");
    return;
}

/*
 * sigprocmask error handling
 */ 
void Sigprocmask(int how, sigset_t *set, sigset_t *oldset)
{
    if(sigprocmask(how,set,oldset)<0)
        unix_error("Sigprocmask error");
    return;
}

/*
 * sigfillset error handling
 */
void Sigfillset(sigset_t *set)
{
    if(sigfillset(set)<0)
        unix_error("Sigfillset error");
    return;
}

/*
 * setpgid error handling
 */
void Setpgid(pid_t pid, pid_t pgid) {
    int rc;

    if ((rc = setpgid(pid, pgid)) < 0)
        unix_error("Setpgid error");
    return;
}