csapp实验，一个简单的shell. Lab Assignment L5: Writing Your Own Unix Shell

实验指导书 http://csapp.cs.cmu.edu/3e/shlab.pdf

该知道的在实验指导书都有了，以下是感觉这个实验重要的地方

• 清楚前台和后台的概念，这是shell创造的概念，有外部命令时我们直接去运行的，只不过如果是前台命令会去等待执行完毕，而后台不管。所以信号处理函数都只是对shell而言，按下ctrl+z和ctrl+c都是发给shell的。（这里有一个进程组的概念，信号默认是发给前台进程组的，所以现在发的就是我们自己写的shell了，为了不让外部命令创建的子进程接收到这些信号，要在创建子进程时设置单独的进程组号）
• 要清楚什么时候要阻塞什么信号，为什么要阻塞。像创建子进程时阻塞SIGCHLD是因为可能子进程运行完了，父进程才往jobs里add一个job，为了防止这种情况，要阻塞这个信号，add完再取消阻塞。但不能阻塞全部信号，因为可能收到SIGTSTP或SIGINT。
• 而在SIGCHLD的处理函数中，我阻塞了所有信号，现在看来似乎没有必要，因为另外两个信号处理函数没有对全局的jobs进行操作。不过在这个程序里也没所谓了。信号处理函数最好写得简单一点，waitpid写的地方也少一点，不然就会很乱。下面列下书上说的保守的好习惯吧
• 处理程序尽可能简单。处理程序可能只是简单地设置全局标志并立即返回，所有与接收信号相关的处理都由主程序执行，主程序周期性检查并重置这个标志。
• 在处理程序中只调用异步信号安全函数。这里的异步信号安全函数指的是：要么函数可重入（如至访问局部变量），要么不能被信号处理程序后中断。
• 保存和恢复errno。防止干扰errno
• 阻塞所有信号。保护对共享全局数据结构的访问。像我们这里只有一个处理函数对jobs进行访问，其实可以补阻塞所有信号的。不过这也是一个好习惯吧，毕竟如果哪一天改了，却忘了这里有个隐患呢？
• 用volatile声明全局变量，volatile声明的会告诉编译器补要缓存这个变量，总是从内从读取。
• 使用sig_atomic_t标志，这可以保证对标量的读写是不可中断的（原子的）。
• 哦，还有就是正确使用waitpid的参数，waitpid是默认阻塞的。摄者WNOHANG可以立即返回，设置WUNTRACED可以检查已终止和被停止的子进程，这在前台等待那里有用。
• 还有就是shell在发信号到前台是发到前台整个进程组的，所以kill的时候记得要负数。
• 信号处理函数和收到信号前正在运行的函数是并发的，如果把waitpid这种相对耗时的操作不放在信号处理函数，会不符合实验指导书要求。而且似乎把waitpid放在信号处理函数也更合理。

因为电脑是Windows的，所以敲代码在sublime，然后放到CLion上看看有没有warning，没有就上传到云服务器的Linux下跑，出错的话都是先去思考，想不到才去gdb调试，gdb调试没有IDE那么爽，到断点不能同时看到几个变量的值（也许是不会用），只能一个一个去p。感觉按键盘也没有点鼠标那么快捷。不过还是要多点熟悉Linux下调试吧。打算重装个图形化虚拟机，然后用CLion调试。

把代码放到sublime上会发现好看很多哈，感觉比在CLion上还舒服，而且跳转也方便不少。
在sublime下

在CLion下

/*  * tsh - A tiny shell program with job control *  * <wrc> */#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 *///是否正在等待的前台完成volatile sig_atomic_t waiting =0;volatile sig_atomic_t waitingPid =-1;//正在等待的前台的pid，仅当waiting为1时有效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, sigset_t *set);void sigchld_handler(int sig);void sigtstp_handler(int sig);void sigint_handler(int sig);//包装函数void Sigfillset(sigset_t *);void Sigemptyset(sigset_t *);void Sigaddset(sigset_t *, int);void Sigprocmask(int how, const sigset_t *set, sigset_t *oldsest);/* 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 + 1];    int bg = parseline(cmdline, (char **) &argv);    if (!builtin_cmd((char **) &argv)) {        pid_t pid;        sigset_t mask_one, prev_one;        //其实好像不需要阻塞全部信号        //Sigfillset(&mask_all);        Sigemptyset(&mask_one);        Sigaddset(&mask_one, SIGCHLD);        //这里要显示阻塞CHILD信号,防止子进程提早执行完但父进程还未addjob        Sigprocmask(SIG_BLOCK, &mask_one, &prev_one);        if ((pid = fork()) < 0) {            unix_error("Fork error");        }        if (pid == 0) {//Child runs here            Sigprocmask(SIG_SETMASK, &prev_one, NULL);//unblock 子进程的SIGCHLD            setpgid(pid, 0);            if (execve(argv[0], argv, environ) < 0) {                printf("%s: Command not found\n", argv[0]);                exit(1);            }        }        //Sigprocmask(SIG_BLOCK, &mask_all, NULL);        addjob(jobs, pid, ((bg == 1) ? BG : FG), cmdline);        //Sigprocmask(SIG_SETMASK, &mask_one, NULL);        //这里是不是有点多此一举？应该不是，毕竟ctrl+z和ctrl+c的信号不应该在这里阻塞的，不然就死锁了        if (!bg) {            waiting=1;            waitingPid=pid;            waitfg(pid, &prev_one);        } else {            printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);        }        Sigprocmask(SIG_SETMASK, &prev_one, NULL);    }    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")) {        //是不是还需要回收子进程？        while (waitpid(-1, NULL, 0) > 0);        exit(0);    }    if (!strcmp(argv[0], "jobs")) {        listjobs(jobs);        return 1;    }    if (!strncmp(argv[0], "bg", 2) || !strncmp(argv[0], "fg", 2)) {        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) {    int bOrf, isJob, id;    char *temp;    if (!strncmp(argv[0], "bg", 2)) {        bOrf = 0;    } else {        bOrf = 1;    }    if(argv[1]==NULL){        if(!bOrf){            printf("bg command requires PID or %%jobid argument\n");        }else{            printf("fg command requires PID or %%jobid argument\n");        }        return;    }    struct job_t *job;    if (!strncmp(argv[1], "%%", 1)) {        isJob = 1;        temp = argv[1]+1;        id = atoi(temp);        job = getjobjid(jobs, id);    } else {        isJob = 0;        temp = argv[1];        id = atoi(temp);        job = getjobpid(jobs, id);    }    if(id==0){        if(bOrf){            printf("fg: argument must be a PID or %%jobid\n");        }else{            printf("bg: argument must be a PID or %%jobid\n");        }        return;    }    if (job == NULL) {        if (!isJob) {            printf("(%d): No such process\n",id);        } else {            printf("%%%d: No such job\n",id);        }        return;    } else {        if (0 == bOrf) {//bg            job->state = BG;            kill(-job->pid, SIGCONT);            printf("[%d] (%d) %s", job->jid, job->pid, job->cmdline);        } else {//fg            //waitfg            job->state = FG;            sigset_t mask, prev;            Sigemptyset(&mask);            Sigaddset(&mask, SIGCHLD);            //这里要显示阻塞CHILD信号,防止子进程唤醒，执行完，回收完了，父进程才去判断，就会一直等            Sigprocmask(SIG_BLOCK, &mask, &prev);            waiting=1;            waitingPid=job->pid;            kill(-job->pid, SIGCONT);            waitfg(job->pid, &prev);            Sigprocmask(SIG_SETMASK, &prev, NULL);        }    }    return;}/* * waitfg - Block until process pid is no longer the foreground process */void waitfg(pid_t pid, sigset_t *set) {    while (waiting==1) {        sigsuspend(set);    }    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 temp = errno;    pid_t cpid;    int status;    sigset_t mask_all, prev_one;    Sigfillset(&mask_all);    while ((cpid = waitpid(-1, &status, WUNTRACED | WNOHANG)) > 0) {        //这里应该判断是挂起还是终止，根据不同来更新或删除job        Sigprocmask(SIG_BLOCK, &mask_all, &prev_one);        if(cpid==waitingPid)            waiting=0;        if (WIFEXITED(status)) {            deletejob(jobs, cpid);        } else if (WIFSIGNALED(status)) {            printf("Job [%d] (%d) terminated by signal %d\n", pid2jid(cpid), cpid, WTERMSIG(status));            deletejob(jobs, cpid);        } else if (WIFSTOPPED(status)) {            struct job_t *fjob = getjobpid(jobs, cpid);            fjob->state = ST;            printf("Job [%d] (%d) stopped by signal 20\n", pid2jid(cpid), cpid);        }        Sigprocmask(SIG_SETMASK, &prev_one, NULL);    }    errno = temp;    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 temp = errno;    pid_t fpid;    fpid = fgpid(jobs);    if (fpid > 0) {        kill(-fpid, SIGINT);//向整个进程组发    }    errno = temp;    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) {    pid_t fpid;    fpid = fgpid(jobs);    if (fpid > 0) {        kill(-fpid, SIGTSTP);//向整个进程组发    }    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);}void Sigfillset(sigset_t *set) {    if (sigfillset(set) < 0) {        unix_error("sigfillset error");    }}void Sigemptyset(sigset_t *set) {    if (sigemptyset(set) < 0) {        unix_error("sigemptyset error");    }}void Sigaddset(sigset_t *set, int num) {    if (sigaddset(set, num) < 0) {        unix_error("sigaddset error");    }}void Sigprocmask(int how, const sigset_t *set, sigset_t *oldsest) {    if (sigprocmask(how, set, oldsest) < 0) {        unix_error("sigprocmask error");    }}