CSAPP-lab：shell lab

overview

此次实验的目的是做一个tiny shell，实现的tsh有以下要求：

• 提示符应该是字符串“ tsh >”
• 用户键入的命令行应该由一个名称和零个或多个参数组成，所有这些参数都由一个或多个空格隔开。如果 name 是一个内置命令，那么 tsh 应该立即处理它并等待下一个命令行。否则，tsh 应该假定 name 是一个可执行文件的路径，它在初始子进程的上下文中加载和运行该文件(在此上下文中，术语 job 指的是这个初始子进程)
• tsh 不需要支持管道(|)或 I/O 重定向(< 和 >)
• 键入 ctrl-c (ctrl-z)会导致 SIGINT (SIGTSTP)信号被发送到当前的前台作业，以及该作业的任何子进程(例如，它fork的任何子进程)。如果没有前台作业，那么信号应该没有效果
• 如果命令行以 & 结尾，那么 tsh 应该在后台运行作业。否则，它应该在前台运行作业
• 每个作业可以通过进程 ID (PID)或作业 ID (JID)来标识，JID 是由 tsh 分配的正整数。JID 应该在命令行中用前缀“%”表示。例如，“% 5”表示 JID5，而“5”表示 PID 5
• tsh 应该支持以下内置命令
  • quit 终止shell
  • jobs 列出所有后台任务
  • bg < job > 命令重启 < job > ，发送一个 SIGCONT 信号，然后在后台运行它。< job > 参数可以是 PID 或 JID
  • fg < job > 命令重启 < job > ，发送一个 SIGCONT 信号 ，然后在前台运行它。< job > 参数可以是 PID 或 JID
• tsh 应该reap它所有的僵尸子进程。如果任何作业因为接收到未捕获的信号而终止，那么 tsh 应该识别此事件，并使用作业的 PID 和违规信号的描述打印消息

每次修改 tsh.c 文件时，键入 make 以重新编译

要运行 shell，需要在命令行中键入 ./tsh

unix> ./tsh

tsh> [type commands to your shell here]

关于shell：

• 命令行是由空格分隔的 ASCII 文本单词序列。命令行中的第一个单词是内置命令的名称或可执行文件的路径名。其余的单词是命令行参数。如果第一个单词是内置命令，则 shell 将立即在当前进程中执行该命令。否则，该单词被假定为可执行程序的路径名。在这种情况下，shell 分支一个子进程，然后在子进程的上下文中加载并运行程序。由于解释单个命令行而创建的子进程统称为作业。通常，一个作业可以由多个子进程组成，这些子进程由 Unix 管道连接。

• 如果命令行以“ &”结尾，那么作业将在后台运行，这意味着 shell 在打印提示符并等待下一个命令行之前不会等待作业终止。否则，作业将在前台运行，这意味着 shell 在等待下一个命令行之前等待作业终止。因此，在任何时候，最多只能有一个作业在前台运行。但是，任意数量的作业都可以在后台运行。例如，键入命令行

• Unix shell 支持作业控制的概念，允许用户在后台和前台之间来回移动作业，并更改作业中进程的进程状态(运行、停止或终止)。键入 ctrl-c 会将 SIGINT 信号传递给前台作业中的每个进程。SIGINT 的默认操作是终止进程。类似地，键入 ctrl-z 将导致 SIGTSTP 信号传递到前台作业中的每个进程。SIGTSTP 的默认操作是将进程置于已停止状态，直到收到 SIGCONT 信号将其唤醒为止。UnixShell 还提供各种支持作业控制的内置命令。例如:

• jobs: List the running and stopped background jobs.
• bg <job>: Change a stopped background job to a running background job.
• fg <job>: Change a stopped or running background job to a running in the foreground.
• kill <job>: Terminate a job.

我们需要完成tsh.c文件下面的函数以实现shell的功能

• eval: Main routine that parses and interprets the command line. [70 lines]
• builtin cmd: Recognizes and interprets the built-in commands: quit, fg, bg, and jobs. [25 lines]
• do bgfg: Implements the bg and fg built-in commands. [50 lines]
• waitfg: Waits for a foreground job to complete. [20 lines]
• sigchld handler: Catches SIGCHILD signals. 80 lines]
• sigint handler: Catches SIGINT (ctrl-c) signals. [15 lines]
• sigtstp handler: Catches SIGTSTP (ctrl-z) signals. [15 lines]

builtin_cmd需要处理以下内置命令：

• quit
• jobs
• bg
• fg

程序执行流

main

调用了signal函数进行了信号的自定义安装

进入while无限循环

• 输出命令提示符
• 然后进行了命令行字符串的读取，如果不成功或者eof也进行了处理
• 调用eval函数对输入的命令进行处理

eval

eval的实现可以参考PPT和csapp P543的内容

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

• 调用parseline对输入的命令cmdline进行处理，将命令行解析后的结果放到argv中，parseline最终返回值为是否在后台运行
• 调用builtin_cmd函数，builtin_cmd内部需要处理以下内置命令：quit，jobs，bg，fg，使用strcmp函数进行字符串的比较，并分别进行处理，如果不是内置命令说明要fork出子进程进行处理
• 由于父子进程的执行顺序是不固定的，有可能子进程的执行先于父进程，那么会造成竞争条件的问题，使程序出现bug。所以这里需要信号屏蔽的一些操作
  • 父进程先阻塞SIGCHLD信号，然后执行fork函数。如果返回值为0，说明是子进程，子进程需要使用Sigprocmask(SIG_SETMASK, &prev_one, NULL); 解除SIGCHLD的阻塞，之后使用Setpgid(0,0);设置pgid为子进程的pid，并用execve执行程序
  • 如果返回值不是0，说明是父进程，父进程在addjob的时候由于要防止竞争条件问题，因此在addjob函数前后需要信号的阻塞与还原
  • 父进程判断是前台任务还使后台任务，如果是前台任务那么需要等待前台任务执行完成才可以操作，这里相当于被阻塞了，因此如果判断是前台任务那么需要调用waitfg函数，参考lab指导书的hint可以使用while循环一直获取前台id的值，如果是0说明没有前台任务，如果不是0的话说明有前台任务，使用线程安全的usleep函数等待，相当于实现了wait

do_bgfg

要求：

• bg < job > 命令重启 < job > ，发送一个 SIGCONT 信号，然后在后台运行它。< job > 参数可以是 PID 或 JID
• fg < job > 命令重启 < job > ，发送一个 SIGCONT 信号 ，然后在前台运行它。< job > 参数可以是 PID 或 JID
• 报错处理：比如格式不正确或者虽然格式正确但是没有此进程 / job

对于重启的实现，可以使用kill函数的sigcont参数，hint提示在使用-pid而非pid

改变job的状态，直接对job->state赋值即可

如果是前台运行，那么需要调用waitfg函数进行阻塞

sigchld_handler

SIGCHLD：当父进程的某一子进程终止（或者因为调用了 exit()，或者因为被信号杀死）时，（内核）将向父进程发送该信号

为了防止竞争条件和信号默认不排队，在sigchld_handler函数中delete前后需要对信号掩码进行操作

sigchld_handler需要包含对进程的回收，这里使用while循环，防止有信号没有处理的情况

sigint_handler

sigint指的是使用ctrl+c对进程发出中断信号，这时候需要使用Kill(-fg,sig);对前台进程发出中断信号

sigtstp_handler

使用Kill(-fg,sig);对前台进程进行挂起

代码

/* 
 * 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;
}