一步一步教你寫SHELL

這個(gè)LAB 是上完CMU CSAPP的14-15 LECTURE之后闷沥，就可以做了。
csapp 課程觀看地址：https://search.bilibili.com/all?keyword=csapp&from_source=banner_search
lab 5 下載地址: http://csapp.cs.cmu.edu/3e/shlab-handout.tar
LAB 要求：http://csapp.cs.cmu.edu/3e/cachelab.pdf
課件PPT 在（需要抄代碼）https://www.cs.cmu.edu/~213/lectures/15-ecf-signals.pdf

這個(gè)lab要求我們實(shí)現(xiàn)一個(gè)簡(jiǎn)易的unix shell程序咐容，內(nèi)容依托CSAPP第八章的異秤咛樱控制流.
這個(gè)LAB 就按照TRACE 文件來一點(diǎn)點(diǎn)實(shí)現(xiàn)。 讀WRITES UP很重要戳粒，理解課本里的每段代碼也很重要是前置功課路狮。

所給的文件中已經(jīng)有了大致的框架，我們要做的就是完成一下幾個(gè)函數(shù)：

函數(shù)原型 作用
void eval(char *cmdline) 分析命令享郊，并派生子進(jìn)程執(zhí)行
int builtin_cmd(char **argv) 執(zhí)行內(nèi)建的命令
void do_bgfg(char **argv) 執(zhí)行bg和fg命令
void waitfg(pid_t pid) 阻塞知道一個(gè)進(jìn)程不在前臺(tái)運(yùn)行
void sigchld_handler(int sig) SIGCHID信號(hào)處理函數(shù)
void sigint_handler(int sig) SIGINT信號(hào)處理函數(shù)
void sigtstp_handler(int sig) SIGTSTP信號(hào)處理函數(shù)

TSH.C輔助函數(shù)

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STEP 1

理解書本的SHELL基本代碼，搭一個(gè)框架孝鹊。因?yàn)镻PT上有講炊琉，我就不解釋了，PPT逐步解釋的很清楚了

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因?yàn)檫@頁P(yáng)PT說了又活，這個(gè)代碼有個(gè)問題苔咪，沒有回收僵尸子進(jìn)程。我們稍后來做柳骄。
有了這個(gè)框架团赏，我們就可以填充我們的builtin_command， 書上也很友好的給了例子耐薯。

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代碼里用到了Fork,這是為了簡(jiǎn)潔代碼舔清，把出錯(cuò)的情況包裝起來的，下面所有的Code,可能都會(huì)用到這種首字母大寫的方法曲初。根據(jù)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來体谒。

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這2個(gè)代碼寫好，TRACE2,3 應(yīng)該可以過了臼婆。是測(cè)QUIT的抒痒。
TRACE4,需要修改下輸出格式。也就過了颁褂。

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

TRACE5故响，因?yàn)檩o助函數(shù)已經(jīng)提供LISTJOBS所以很快可以實(shí)現(xiàn)傀广。加在builtin_command

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

發(fā)現(xiàn)JOB沒有打出來，看來是需要維護(hù)JOB LIST彩届。根據(jù)PPT上的代碼伪冰，有對(duì)應(yīng)的，需要控制BLOCK惨缆。來保證父進(jìn)程先ADD糜值，子進(jìn)程再DELETE。這里需要弄懂下面幾個(gè)方法坯墨，可以MAN 方法名查看寂汇。

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STEP 2

把下面的邏輯接進(jìn)我們的EVAL，和CHILD HANDLER

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大概代碼如下圖捣染。

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然后寫CHD_HANLDER骄瓣，如果子進(jìn)程結(jié)束，需要去DELETE JOB

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跑了之后發(fā)現(xiàn)都在打FRONTEND耍攘，這是為什么呢榕栏？

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經(jīng)過分析，是因?yàn)樵贓VAL蕾各，和SIGCHLD_HANDLER里有WAITPID的阻塞等待子進(jìn)程退出的邏輯扒磁。
當(dāng)時(shí)前端進(jìn)程的時(shí)候，EVAL里的WAITPID 會(huì)搶到這個(gè)線程退出的狀態(tài)式曲，然后就沒有DELETEJOBS妨托。而后端進(jìn)程，因?yàn)樵贓VAL中只是做了PRINT吝羞，所以會(huì)在CHLD_HANDLER里被DELETE兰伤。
這就解釋了為什么只打印了前端進(jìn)程。
解決方案是可以2塊地方都加上DELETEJOB 的邏輯钧排。

不過看write up里面有一句

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實(shí)現(xiàn)代碼

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

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改了之后敦腔，運(yùn)行MAKE TEST05，發(fā)現(xiàn)這次是啥也沒打出來恨溜。

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分析TRACE_05做的事符衔。

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代碼是SIG_CHLD HANDLER里
會(huì)BLOCK WAITING，這個(gè)時(shí)候前端進(jìn)程打印出那個(gè)TSH> JOBS糟袁，會(huì)BLOCK柏腻，一直等到所有子進(jìn)程都被回收后，那么就等價(jià)于等到2個(gè)后端進(jìn)程都做好系吭，也DELETE五嫂，才會(huì)離開HANDLER。
那么最后JOBS就當(dāng)然沒有東西可以打印。

所以WAITPID要改成非阻塞的沃缘。
同時(shí)因?yàn)椴蛔枞饲啵訣RRNO ！= ECHILD的判斷可以刪了槐臀。

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SETP 3

從TRACE06 開始就是要實(shí)現(xiàn)SIGINT 和 SIGSTOP锄蹂。
WRITEUP 里的一個(gè)HINT很重要。

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實(shí)現(xiàn)如下代碼

/* 
 * 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);
}

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TRACE06,07都可以過了
同樣方式水慨，實(shí)現(xiàn)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);
}

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STEP 4

之后的TRACE得糜，發(fā)現(xiàn)要求要實(shí)現(xiàn)FG,BG了
那么思路是首先要在BUILTIN_CMD 里把這個(gè)命令加上，隨后實(shí)現(xiàn)DO_FGBG

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在實(shí)現(xiàn)DO_FGBG晰洒，就是按照這個(gè)命令的功能朝抖，首先解析參數(shù)，拿到第二個(gè)看有沒有%谍珊，如果有治宣，從JID拿到PID。然后就是發(fā)送SIG_CONG的命令砌滞，讓程序做起來（如果STOP的話侮邀，如果沒STOP這個(gè)命令會(huì)被忽略掉（冪等））
最后別忘記要模仿EVAL的對(duì)前后端進(jìn)程的處理，前端要調(diào)用WAITFG進(jìn)行阻塞贝润，后端要打印绊茧。
大概代碼

/* 
 * 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;
}

到這里TRACE9和10就過了。TRACE 11打掘，12华畏，13是另外一個(gè)SIGINT和SIGSTOP的CASE，我們之前的代碼似乎也能過胧卤，不需要作啥修改唯绍。

STEP 5

根據(jù)TRACE 14拼岳，需要對(duì)FG,BG 做一些ERROR HANDLING枝誊，就是各種JID,PID不合法的CORNER CASE 做輸出處理。

/* 
 * 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;
}

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TRACE 15 也贈(zèng)送過了惜纸。之前代碼寫的可以哈

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TRACE 16 失敗了叶撒。發(fā)現(xiàn)我們的SHELL 沒法停止。

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看一下TRACE 16在干嘛

STEP 6

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原來是程序里面在即調(diào)了SIG_INT 和 SIG_STOP耐版。因?yàn)檫@個(gè)程序用代碼發(fā)的祠够，我們無法用HANDLER機(jī)制去捕捉到。并且因?yàn)槭荢TOP狀態(tài)粪牲，所以我們的SIGCHLD HANDLER里面是會(huì)忽略的古瓤。那么就沒有HANDLER去DELETE JOB 或者去修改進(jìn)程的狀態(tài)從FG變?yōu)镾T，那么WAIT FG就會(huì)無限等待。

思考了一下落君，這下WUNTRACED需要登場(chǎng)了穿香。這個(gè)可以監(jiān)聽到STOP的子進(jìn)程的情況，這下問題就變成绎速，我們必須用不同分支去隔離開STOP還是TERMINATE的皮获。

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這個(gè)時(shí)候發(fā)現(xiàn)SIG INT 會(huì)堵住。

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看來還需要加個(gè)判斷纹冤，因?yàn)槲覀兊拇a只判斷了正常退出洒宝，沒有判斷SIG_INT的退出。

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最后發(fā)現(xiàn)有2句話沒打印萌京，就補(bǔ)一下雁歌，

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但是再做REGRESSION TEST的時(shí)候，發(fā)現(xiàn)枫夺，這樣的話SIG_INT（SIG_STOP）的提示語言 會(huì)被打印2次将宪，一次是SIG_INT（SIG_STOP） HANDLER里，一次在SIG_CHLD HANDLER里橡庞。

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但是我們可以看到JID较坛，一次是2，一次是0扒最。我們可以用JID是否為0來過濾丑勤。當(dāng)然我們也可以去掉SIG_INT HANDLER里的那次打印。同時(shí)要把DELETE也移動(dòng)過去吧趣，因?yàn)槲覀円_保法竞，先打印后DELETE JOB，不然JID會(huì)變成0.

最終代碼

/* 
 * 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 */


/* 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;
}