CSAPP Lab5：Shell

一、实验简介

此实验目的是为了更加熟悉进程控制和信号。该实验内容是实现一个简单的Unix Shell程序，其支持job control。

准备步骤及基础要求：

• tar xvf shlab-handout.tar
• make，编译测试程序
• 每次修改，都需要make clean，再重新make
• 参考 tsh.c，掌握shell 程序的架构，实现其中的空函数
  • eval：解析命令行参数的主函数，大约70行
  • builtin_cmd：识别内置命令，包括quit、fg、bg、jobs，大约25行
  • do_bgfg：实现内置命令，fg和bg，大约50行
  • waitfg：等待前台任务完成的函数，大概20行
  • sigchld_handler：捕获 SIGCHILD 信号并处理，大约80行
  • sigint_handler：捕获 SIGINT(ctrl + c)信号并处理，大约15行
  • sigtstp_handler：捕获 SIGTSTP (ctrl + z)信号并处理，大约15行
• 每次更改过tsh.c之后都需要重新编译
• 命令行，第一个参数作为内置命令，如果不是，则作为要执行的job的路径，fork一个子进程去执行job。一个job可以由Uinx管道连接的多个子进程组成
• 如果命令行参数以 & 结尾，则表示该 job 后台执行，shell可以拥有一个前台 job 以及任意数量个后台 job 。
• shell 提供了一些内置命令：
  • jobs：列举所有的运行中的或者已经停止的 job
  • bg < job>：将一个前台 job 更改为 后台 job
  • fg < job>：将一个后台 job 更改为 前台 job
  • kill < job>：终止一个job
• 验证：make生成可执行程序之后，使用 make testx 得到自己程序的输出，make rtestx 得到正确答案，进行比对。

shell的具体要求：

• 提示符为 tsh>
• 暂不支持 管道 和 IO重定向
• 支持 ctrl +c、ctrl + z
• 支持 &
• 支持前面所述的内置命令
• 需要回收所有的僵尸子进程

二、实现

eval函数

首先解析命令行，调用builtin_cmd判断是否是内置命令
如果是内置命令，执行对应的内置命令
否则，fork一个子进程，execv 对应的代码，将其添加到 job 列表中
需要考虑信号的使用

void eval(char *cmdline) 
{
    char *argv[MAXARGS];//存储解析后的参数
    char buf[MAXLINE];
    int bg;//job 是否 后台运行
    pid_t pid;//创建的子进程的pid，用于构建job
    sigset_t mask_all, mask_chld, prev;

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);//解析命令行，返回值为该任务是否为一个前台任务
    if(argv[0] == NULL){
        return; //空命令行，不执行操作
    }
    //如果不是内置命令，则fork子进程
    if(!builtin_cmd(argv)){
        //注意，参考书籍P542页，因为我们设置了SIGCHLD信号的回调
        //为了防止，父进程fork子进程之后，还未执行到addjobs的逻辑，子进程就执行完毕，发送了SIGCHLD信号
        //导致父进程处理信号，先调用了deletejobs而导致错误，此时应该先屏蔽SIGCHLD信号
        sigemptyset(&mask_chld);
        sigaddset(&mask_chld, SIGCHLD);
        // Signal(SIGCHLD, sigchld_handler);//注册SIGCHLD信号的处理函数，用于父进程回收子进程的资源
        sigprocmask(SIG_BLOCK, &mask_chld, &prev);//父进程阻塞SIGCHLD信号
        //创建子进程,environ为父进程的环境变量
        if((pid == fork()) == 0){
            //由于子进程完全的复制了父进程，所以需要先解除子进程对SIGCHLD信号的阻塞
            sigprocmask(SIG_SETMASK, &prev, NULL);
            if(execve(argv[0], argv, environ)){
                printf("%s: Command not found\n", argv[0]);
                exit(0);
            }
        }

        //此处父进程执行，子进程的逻辑已经被更改,会重新执行自己的main函数
        //父进程阻塞所有信号，防止被打断去执行别的处理
        sigfillset(&mask_all);
        sigprocmask(SIG_BLOCK, &mask_all, NULL);
        //添加任务，fg为1，bg为2
        addjob(jobs, pid, bg + 1, cmdline);
        //恢复原来的状态,不再阻塞SIGCHLD
        sigprocmask(SIG_SETMASK, &prev, NULL);
        if(!bg){
            //前台执行，父进程等待子进程执行完毕
            waitfg(pid);
        }else{
            printf("[%d] (%d) %s\n", pid2jid(pid), pid, cmdline);//返回main函数，等待下一个任务
        }
    }

    return;
}
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builtin_cmd

int builtin_cmd(char **argv) 
{
    if(!strcmp(argv[0], "quit")){
        exit(0); //quit 命令
    }else if(!strcmp(argv[0], "jobs")){
        listjobs(jobs);
    }else if(!(strcmp(argv[0], "fg")) || !(strcmp(argv[0], "bg"))){
        do_bgfg(argv);
    }

    return 0;     /* not a builtin command */
}
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waitfg

• 简单方式，循环判断整个 jobs 列表，判断有无前台作业
• 设置一个全局变量，wait_pid，当父进程等待一个子进程时，将其设置为等待的子进程pid，当父进程收到信号SIGCHLD时，在信号处理函数中将其置为-1，结束循环。
  此时需要注意，在调用该函数之前，父进程已经不阻塞SIGCHLD，因此可能在进入该函数之前，子进程就已经结束，wait_pid等于-1了，因此需要先判断wait_pid 是否等于 -1，如果是的话，重置为0，直接返回。（此时需要考虑，信号处理函数对于wait_pid置1的条件是什么呢？如何区分是前台任务还是后台任务发送的SIGCHLD信号呢？）

volatile sig_atomic_t wait_pid = 0;
void waitfg(pid_t pid)
{
    sigset_t mask, prev;
    sigemptyset(&mask);
    while(1){
        sigprocmask(SIG_SETMASK, &mask, &prev);
        if(!fgpid(jobs)){
            return;
        }
        sleep(1);
        sigprocmask(SIG_SETMASK, &prev, NULL);
    }
    
    // sigprocmask(SIG_SETMASK, &mask, &prev);
    // if(wait_pid == -1){
	// 	wait_pid = 0;
	// 	return;
	// }
    // wait_pid = pid;
    // while(wait_pid != -1){
    //     sleep(1);
    //     //sigsuspend(&prev);
    // }
    // sigprocmask(SIG_SETMASK, &prev, NULL);
    return;
}
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do_fgbg

取出第二个参数，得到要挪到前台/后台的 job 的 pid或者 jid，判断有无 %前缀，如果是%5，则表示 jid = 5，没有%则表示pid=5
向该pid进程发送SIGCONT信号，然后更新该任务的状态

    char* arg1 = argv[1];
    int pid;
    int jid;
    struct job_t *cur;//当前的任务
    if(arg1 == NULL){
        printf("%s command requires PID or %%jobid argument\n", argv[0]);
        return;
    }

    //参数为jid
    if(arg1[0] == '%'){
        jid = atoi(((char *)arg1 + 1));
        if(jid == 0 && strcmp(((char *)arg1 + 1),"0")){
            printf("%s argument must be a PID or %%jobid\n", argv[0]);
            return;
        }
        cur = getjobjid(jobs, jid);
        if(cur == NULL){
            printf("%s: No such job\n", arg1);
            return;
        }
        pid = cur->pid;
    }else{
        //参数为pid
        pid = atoi(arg1);
        if(pid == 0 && strcmp(arg1, "0")){
            printf("%s: argument must be a PID or %%jobid\n", argv[0]);
            return;
        }

        cur = getjobpid(jobs, pid);
        if(cur == NULL){
            printf("(%d): No such process\n", pid);
            return;
        }
    }

    //退出该进程，重新执行
    kill(-pid, SIGCONT);

    //匹配前台还是后台任务，strcmp匹配成功返回0
    if(strcmp(argv[0], "bg")){
        //前台任务
        cur->state = FG;
        waitfg(pid);
    }else{
        //后台任务
        cur->state = BG;
        printf("[%d] (%d) %s", pid2jid(pid), pid, cur->cmdline);
    }
    return;
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sigchld_handler

子进程终止时，会向父进程发送SIGCHLD信号，父进程收到信号后，使用waitpid去检查是否有结束的子进程
然后根据子进程的状态，判断子进程是怎么结束的，更新对应的job状态
为了保证原子性，父进程在deletejob时，屏蔽所有信号，删除完之后再打开信号

void sigchld_handler(int sig) 
{
    int olderrno = errno;
    pid_t pid;
    int status;//进程状态
    sigset_t mask_all, prev;
    sigfillset(&mask_all);//父进程进行处理时，屏蔽信息，保证原子性
    //设置WNOHANG选项，只等待所有的僵死进程
    while((pid = waitpid(-1, &status, WNOHANG | WUNTRACED)) > 0){
        sigprocmask(SIG_BLOCK, &mask_all, &prev);
        //如果子进程正常结束
        if(WIFEXITED(status)){
            deletejob(jobs, pid);
        }else if(WIFSTOPPED(status)){
            //如果进程状态为停止，更新job状态
            printf("Job [%d] (%d) stopped by signal 20 \n", pid2jid(pid), pid);
            (*getjobpid(jobs, pid)).state = ST;
        }else if(WIFSIGNALED(status)){
            //进程状态为终止状态
            printf("Job [%d] (%d) terminated by signal %d \n", pid2jid(pid), pid, WTERMSIG(status));
            deletejob(jobs, pid);
        }
        if(pid == wait_pid){
            wait_pid = -1;
        }
        sigprocmask(SIG_SETMASK, &prev, NULL);
    }

    errno = olderrno;
    return;
}
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sigint_handler

/* 
 * 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) 
{
    //终止前台子进程
    kill(-fgpid(jobs), sig);
    return;
}
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sigtstp_handler

/*
 * 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) 
{
    kill(-fgpid(jobs), sig);
    return;
}
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三、整体代码

/* 
 * tsh - A tiny shell program with job control
 * 
 * 
 */
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

/* 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 */


/* 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];//存储解析后的参数
    char buf[MAXLINE];
    int bg;//job 是否 后台运行
    pid_t pid;//创建的子进程的pid，用于构建job
    sigset_t mask_all, mask_chld, prev;

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);//解析命令行，返回值为该任务是否为一个前台任务
    if(argv[0] == NULL){
        return; //空命令行，不执行操作
    }
    //如果不是内置命令，则fork子进程
    if(!builtin_cmd(argv)){
        //注意，参考书籍P542页，因为我们设置了SIGCHLD信号的回调
        //为了防止，父进程fork子进程之后，还未执行到addjobs的逻辑，子进程就执行完毕，发送了SIGCHLD信号
        //导致父进程处理信号，先调用了deletejobs而导致错误，此时应该先屏蔽SIGCHLD信号
        sigemptyset(&mask_chld);
        sigaddset(&mask_chld, SIGCHLD);
        // Signal(SIGCHLD, sigchld_handler);//注册SIGCHLD信号的处理函数，用于父进程回收子进程的资源
        sigprocmask(SIG_BLOCK, &mask_chld, &prev);//父进程阻塞SIGCHLD信号
        //创建子进程,environ为父进程的环境变量
        if((pid = fork()) == 0){
            //由于子进程完全的复制了父进程，所以需要先解除子进程对SIGCHLD信号的阻塞
            sigprocmask(SIG_SETMASK, &prev, NULL);
            setpgid(0, 0);
            if(execve(argv[0], argv, environ) < 0){
                printf("%s: Command not found\n", argv[0]);
                exit(0);
            }
        }

        //此处父进程执行，子进程的逻辑已经被更改,会重新执行自己的main函数
        //父进程阻塞所有信号，防止被打断去执行别的处理
        sigfillset(&mask_all);
        sigprocmask(SIG_BLOCK, &mask_all, NULL);
        //添加任务，fg为1，bg为2
        addjob(jobs, pid, bg + 1, cmdline);
        //恢复原来的状态,不再阻塞SIGCHLD
        sigprocmask(SIG_SETMASK, &prev, NULL);
        if(!bg){
            //前台执行，父进程等待子进程执行完毕
            waitfg(pid);
        } else {
            printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);//返回main函数，等待下一个任务
        }
    }

    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")){
        exit(0); //quit 命令
    }else if(!strcmp(argv[0], "jobs")){
        listjobs(jobs);
        return 1;
    }else if(!(strcmp(argv[0], "fg")) || !(strcmp(argv[0], "bg"))){
        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) 
{
    char* arg1 = argv[1];
    int pid;
    int jid;
    struct job_t *cur;//当前的任务
    if(arg1 == NULL){
        printf("%s command requires PID or %%jobid argument\n", argv[0]);
        return;
    }

    //参数为jid
    if(arg1[0] == '%'){
        jid = atoi(((char *)arg1 + 1));
        if(jid == 0 && strcmp(((char *)arg1 + 1),"0")){
            printf("%s argument must be a PID or %%jobid\n", argv[0]);
            return;
        }
        cur = getjobjid(jobs, jid);
        if(cur == NULL){
            printf("%s: No such job\n", arg1);
            return;
        }
        pid = cur->pid;
    }else{
        //参数为pid
        pid = atoi(arg1);
        if(pid == 0 && strcmp(arg1, "0")){
            printf("%s: argument must be a PID or %%jobid\n", argv[0]);
            return;
        }

        cur = getjobpid(jobs, pid);
        if(cur == NULL){
            printf("(%d): No such process\n", pid);
            return;
        }
    }

    //退出该进程，重新执行
    kill(-pid, SIGCONT);

    //匹配前台还是后台任务，strcmp匹配成功返回0
    if(strcmp(argv[0], "bg")){
        //前台任务
        cur->state = FG;
        waitfg(pid);
    }else{
        //后台任务
        cur->state = BG;
        printf("[%d] (%d) %s", pid2jid(pid), pid, cur->cmdline);
    }
    return;
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
volatile sig_atomic_t wait_pid;
void waitfg(pid_t pid)
{
    sigset_t mask, prev;
    sigemptyset(&mask);
    while(1){
        sigprocmask(SIG_SETMASK, &mask, &prev);
        if(!fgpid(jobs)){
            return;
        }
        sleep(1);
        sigprocmask(SIG_SETMASK, &prev, NULL);
    }
    
    // sigprocmask(SIG_SETMASK, &mask, &prev);
    // if(wait_pid == -1){
	// 	wait_pid = 0;
	// 	return;
	// }
    // wait_pid = pid;
    // while(wait_pid != -1){
    //     sleep(1);
    //     //sigsuspend(&prev);
    // }
    // sigprocmask(SIG_SETMASK, &prev, NULL);
    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;
    pid_t pid;
    int status;//进程状态
    sigset_t mask_all, prev;
    sigfillset(&mask_all);//父进程进行处理时，屏蔽信息，保证原子性
    //设置WNOHANG选项，只等待所有的僵死进程
    while((pid = waitpid(-1, &status, WNOHANG | WUNTRACED)) > 0){
        sigprocmask(SIG_BLOCK, &mask_all, &prev);
        //如果子进程正常结束
        if(WIFEXITED(status)){
            deletejob(jobs, pid);
        }else if(WIFSTOPPED(status)){
            //如果进程状态为停止，更新job状态
            printf("Job [%d] (%d) stopped by signal 20 \n", pid2jid(pid), pid);
            (*getjobpid(jobs, pid)).state = ST;
        }else if(WIFSIGNALED(status)){
            //进程状态为终止状态
            printf("Job [%d] (%d) terminated by signal %d \n", pid2jid(pid), pid, WTERMSIG(status));
            deletejob(jobs, pid);
        }
        if(pid == wait_pid){
            wait_pid = -1;
        }
        sigprocmask(SIG_SETMASK, &prev, NULL);
    }

    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) 
{
    //终止前台子进程
    kill(-fgpid(jobs), sig);
    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) 
{
    kill(-fgpid(jobs), sig);
    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);
}




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