五 Shell Lab

一步一步教你写SHELL

这个LAB 是上完CMU CSAPP的14-15 LECTURE之后,就可以做了。
csapp 课程观看地址:https://search.bilibili.com/all?keyword=csapp&from_source=banner_search
lab 5 下载地址: http://csapp.cs.cmu.edu/3e/shlab-handout.tar
LAB 要求:http://csapp.cs.cmu.edu/3e/cachelab.pdf
课件PPT 在(需要抄代码)https://www.cs.cmu.edu/~213/lectures/15-ecf-signals.pdf

这个lab要求我们实现一个简易的unix shell程序,内容依托CSAPP第八章的异常控制流.
这个LAB 就按照TRACE 文件来一点点实现。 读WRITES UP很重要,理解课本里的每段代码也很重要是前置功课。

所给的文件中已经有了大致的框架,我们要做的就是完成一下几个函数:

函数原型 作用
void eval(char *cmdline) 分析命令,并派生子进程执行
int builtin_cmd(char **argv) 执行内建的命令
void do_bgfg(char **argv) 执行bg和fg命令
void waitfg(pid_t pid) 阻塞知道一个进程不在前台运行
void sigchld_handler(int sig) SIGCHID信号处理函数
void sigint_handler(int sig) SIGINT信号处理函数
void sigtstp_handler(int sig) SIGTSTP信号处理函数

TSH.C辅助函数

image.png

STEP 1

理解书本的SHELL基本代码,搭一个框架。因为PPT上有讲,我就不解释了,PPT逐步解释的很清楚了


image.png

因为这页PPT说了,这个代码有个问题,没有回收僵尸子进程。我们稍后来做。
有了这个框架,我们就可以填充我们的builtin_command, 书上也很友好的给了例子。


image.png

代码里用到了Fork,这是为了简洁代码,把出错的情况包装起来的,下面所有的Code,可能都会用到这种首字母大写的方法。根据PPT 找到CSAPP.C
http://csapp.cs.cmu.edu/3e/ics3/code/src/csapp.c
把需要的方法都抄出来。

image.png

这2个代码写好,TRACE2,3 应该可以过了。是测QUIT的。
TRACE4,需要修改下输出格式。也就过了。

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

TRACE5,因为辅助函数已经提供LISTJOBS所以很快可以实现。加在builtin_command

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

发现JOB没有打出来,看来是需要维护JOB LIST。根据PPT上的代码,有对应的,需要控制BLOCK。来保证父进程先ADD,子进程再DELETE。这里需要弄懂下面几个方法,可以MAN 方法名查看。


image.png

STEP 2

把下面的逻辑接进我们的EVAL,和CHILD HANDLER


image.png
image.png

大概代码如下图。


image.png

然后写CHD_HANLDER,如果子进程结束,需要去DELETE JOB


image.png

跑了之后发现都在打FRONTEND,这是为什么呢?


image.png

经过分析,是因为在EVAL,和SIGCHLD_HANDLER里有WAITPID的阻塞等待子进程退出的逻辑。
当时前端进程的时候,EVAL里的WAITPID 会抢到这个线程退出的状态,然后就没有DELETEJOBS。而后端进程,因为在EVAL中只是做了PRINT,所以会在CHLD_HANDLER里被DELETE。
这就解释了为什么只打印了前端进程。
解决方案是可以2块地方都加上DELETEJOB 的逻辑。

不过看write up里面有一句


image.png

实现代码

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

改了之后,运行MAKE TEST05,发现这次是啥也没打出来。


image.png

分析TRACE_05做的事。


image.png

代码是SIG_CHLD HANDLER里
会BLOCK WAITING,这个时候前端进程打印出那个TSH> JOBS,会BLOCK,一直等到所有子进程都被回收后,那么就等价于等到2个后端进程都做好,也DELETE,才会离开HANDLER。
那么最后JOBS就当然没有东西可以打印。

所以WAITPID要改成非阻塞的。
同时因为不阻塞了,所以ERRNO != ECHILD的判断可以删了。


image.png

SETP 3

从TRACE06 开始就是要实现SIGINT 和 SIGSTOP。
WRITEUP 里的一个HINT很重要。


image.png

image.png

实现如下代码

/* 
 * 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) 
{
    pid_t fg = fgpid(jobs);
    printf("Job [%d] (%d) terminated by signal %d \n",
                        pid2jid(fg), fg, sig);
    deletejob(jobs, fg); 
    Kill(-fg,sig);
}
image.png

image.png

TRACE06,07都可以过了
同样方式,实现SIGSTOP, pass trace08

/*
 * 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) 
{
    pid_t fg = fgpid(jobs);
    printf("Job [%d] (%d) stopped by signal %d \n",
                        pid2jid(fg), fg, sig);
    (*getjobpid(jobs, fg)).state = ST;
    Kill(-fg,sig);
}
image.png

STEP 4

之后的TRACE,发现要求要实现FG,BG了
那么思路是首先要在BUILTIN_CMD 里把这个命令加上,随后实现DO_FGBG


image.png

在实现DO_FGBG,就是按照这个命令的功能,首先解析参数,拿到第二个看有没有%,如果有,从JID拿到PID。然后就是发送SIG_CONG的命令,让程序做起来(如果STOP的话,如果没STOP这个命令会被忽略掉(幂等))
最后别忘记要模仿EVAL的对前后端进程的处理,前端要调用WAITFG进行阻塞,后端要打印。
大概代码

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    char* next_arg = argv[1];
    int pid;
    struct job_t *cur;
    if (next_arg[0] == '%') {
        int jid = atoi(((char *)next_arg) + 1);
        cur = getjobjid(jobs,jid);
        pid = cur->pid;
    }else {
        pid = atoi(next_arg);
        cur = getjobpid(jobs,pid);
    }
    
    Kill(-pid,SIGCONT);
    if (!strcmp(argv[0], "bg")) {
        cur->state = BG;
        printf("[%d] (%d) %s",pid2jid(pid), pid, cur->cmdline);
    } else {
        cur->state = FG;
        waitfg(pid);
    }
    return;
}

到这里TRACE9和10就过了。TRACE 11,12,13是另外一个SIGINT和SIGSTOP的CASE,我们之前的代码似乎也能过,不需要作啥修改。

STEP 5

根据TRACE 14,需要对FG,BG 做一些ERROR HANDLING,就是各种JID,PID不合法的CORNER CASE 做输出处理。

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    char* next_arg = argv[1];
    if (next_arg == NULL) {
        printf("%s command requires PID or %%jobid argument\n",argv[0]);
        return;
    }
    int pid;
    struct job_t *cur;
    if (next_arg[0] == '%') {
        int jid = atoi(((char *)next_arg) + 1);
        if (jid == 0 && strcmp(((char *)next_arg) + 1, "0")) {
            printf("%s: argument must be a PID or %%jobid\n",argv[0]);
            return;
        }
        cur = getjobjid(jobs,jid);
        pid = cur->pid;
        if (cur == NULL) {
            printf("%s: No such job\n",next_arg);
            return;
        }
    }else {
        pid = atoi(next_arg);
        if (pid == 0 && strcmp(next_arg, "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;
        }
    }
    
    Kill(-pid,SIGCONT);
    if (!strcmp(argv[0], "bg")) {
        cur->state = BG;
        printf("[%d] (%d) %s",pid2jid(pid), pid, cur->cmdline);
    } else {
        cur->state = FG;
        waitfg(pid);
    }
    return;
}
image.png

TRACE 15 也赠送过了。之前代码写的可以哈


image.png

TRACE 16 失败了。发现我们的SHELL 没法停止。


image.png

看一下TRACE 16在干嘛

STEP 6

image.png

原来是程序里面在即调了SIG_INT 和 SIG_STOP。因为这个程序用代码发的,我们无法用HANDLER机制去捕捉到。并且因为是STOP状态,所以我们的SIGCHLD HANDLER里面是会忽略的。那么就没有HANDLER去DELETE JOB 或者去修改进程的状态从FG变为ST,那么WAIT FG就会无限等待。

思考了一下,这下WUNTRACED需要登场了。这个可以监听到STOP的子进程的情况,这下问题就变成,我们必须用不同分支去隔离开STOP还是TERMINATE的。

image.png

这个时候发现SIG INT 会堵住。


image.png

看来还需要加个判断,因为我们的代码只判断了正常退出,没有判断SIG_INT的退出。


image.png

最后发现有2句话没打印,就补一下,


image.png

但是再做REGRESSION TEST的时候,发现,这样的话SIG_INT(SIG_STOP)的提示语言 会被打印2次,一次是SIG_INT(SIG_STOP) HANDLER里,一次在SIG_CHLD HANDLER里。


image.png

但是我们可以看到JID,一次是2,一次是0。我们可以用JID是否为0来过滤。当然我们也可以去掉SIG_INT HANDLER里的那次打印。同时要把DELETE也移动过去,因为我们要确保,先打印后DELETE JOB,不然JID会变成0.

最终代码

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

    Sigfillset(&mask_all);
    Sigemptyset(&mask_one);
    Sigaddset(&mask_one, SIGCHLD);
    if (!builtin_cmd(argv)) {
        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one); /* Block SIGCHLD */
        if ((pid = Fork()) == 0) { /* Child runs user job */
            Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
            Setpgid(0,0);
            Execve(argv[0],argv,environ);
        }
        Sigprocmask(SIG_BLOCK, &mask_all, NULL); /* Parent process */
        addjob(jobs, pid, bg + 1, cmdline); /* Add the child to the job list FG 1 BG 2*/
        Sigprocmask(SIG_SETMASK, &prev_one, NULL); /* Unblock SIGCHLD */
        /* Parent waits for foreground job to terminate */
        if (!bg) {
            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], "jobs")){/* jobs command */
        listjobs(jobs);
        return 1;
    }
    if (!strcmp(argv[0], "bg") || !strcmp(argv[0], "fg")){/* bg/fg command */
        do_bgfg(argv);
        return 1;
    }
    if (!strcmp(argv[0], "quit")) /* quit command */
        exit(0);
    if (!strcmp(argv[0], "&")) /* Ignore singleton & */
        return 1;
    return 0;     /* not a builtin command */
}

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    char* next_arg = argv[1];
    if (next_arg == NULL) {
        printf("%s command requires PID or %%jobid argument\n",argv[0]);
        return;
    }
    int pid;
    struct job_t *cur;
    if (next_arg[0] == '%') {
        int jid = atoi(((char *)next_arg) + 1);
        if (jid == 0 && strcmp(((char *)next_arg) + 1, "0")) {
            printf("%s: argument must be a PID or %%jobid\n",argv[0]);
            return;
        }
        cur = getjobjid(jobs,jid);
        pid = cur->pid;
        if (cur == NULL) {
            printf("%s: No such job\n",next_arg);
            return;
        }
    }else {
        pid = atoi(next_arg);
        if (pid == 0 && strcmp(next_arg, "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;
        }
    }
    
    Kill(-pid,SIGCONT);
    if (!strcmp(argv[0], "bg")) {
        cur->state = BG;
        printf("[%d] (%d) %s",pid2jid(pid), pid, cur->cmdline);
    } else {
        cur->state = FG;
        waitfg(pid);
    }
    return;
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid)
{
    while (fgpid(jobs)) {
        usleep(1000);
    }
    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_all;
    pid_t pid;
    Sigfillset(&mask_all);
    int status;
    while ((pid = waitpid(-1, &status, WNOHANG|WUNTRACED)) > 0) { /* Reap a zombie child */
        Sigprocmask(SIG_BLOCK, &mask_all, &prev_all);
        if (WIFEXITED(status)) { 
            deletejob(jobs, pid);
        } else if (WIFSTOPPED(status)) {
            printf("Job [%d] (%d) stopped by signal %d \n",
                        pid2jid(fg), fg, sig);
            (*getjobpid(jobs, fg)).state = ST;
        } else if (WIFSIGNALED(status)) {
            printf("Job [%d] (%d) terminated by signal %d \n",
                        pid2jid(pid), pid, WTERMSIG(status));
            deletejob(jobs, pid); 
        } 

        Sigprocmask(SIG_SETMASK, &prev_all, NULL);
    }
    errno = olderrno;
}

/* 
 * 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) 
{
    pid_t fg = fgpid(jobs);
    
    Kill(-fg,sig);
}

/*
 * 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) 
{
    pid_t fg = fgpid(jobs);
    
    Kill(-fg,sig);
}

/*********************
 * 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);
}

/* error handling function defined by yixuaz */
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);
    }
}
pid_t Fork(void)
{
    pid_t pid;

    if ((pid = fork()) < 0)
        unix_error("Fork error");
    return pid;
}
void Sigemptyset(sigset_t *set)
{
    if(sigemptyset(set)<0)
        unix_error("Sigemptyset error");
    return;
}
void Sigaddset(sigset_t *set,int sign)
{
    if(sigaddset(set,sign)<0)
        unix_error("Sigaddset error");
    return;
}
void Sigprocmask(int how, sigset_t *set, sigset_t *oldset)
{
    if(sigprocmask(how,set,oldset)<0)
        unix_error("Sigprocmask error");
    return;
}
void Sigfillset(sigset_t *set)
{
    if(sigfillset(set)<0)
        unix_error("Sigfillset error");
    return;
}
void Kill(pid_t pid, int signum) 
{
    int rc;

    if ((rc = kill(pid, signum)) < 0)
    unix_error("Kill error");
}
void Setpgid(pid_t pid, pid_t pgid) {
    int rc;

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



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