《Intro to Computer Systems》(csapp)LAB5(shlab)

异常控制流

了解基本的LinuxC编程，除了《csapp》的第八章以外，也推荐看看《Linux/Unix编程实践教程》以及《Unix环境高级编程》。本次lab对进程和信号、控制流一定要充分理解。

记录一些老是搞错的问题

kill(pid, sig) pid>0，如果父进程和子进程同在一个组，那么SIGINT、SIGTSTP子进程由于调用execve被内存信号屏蔽被替换，会直接挂起或者终止。因此在fork之后使用setpgid(0, 0)给子进程重新分组。正如文档所解释的。

int pid = wait(NULL)阻塞调用进程,返回值为回收的终止的进程。
int pid = waitpid(-1 , NULL, 0)相当于wait(NULL)
int pid = waitpid(-1, NULL, WNOHANG)，非阻塞，如果没有回收到进程则返回0，否则返回收到的进程号，出错返回-1。
int pid = waitpid(-1, &state, WNOHANG | WUNTRACED)，非阻塞异常检测。使用WUNTRACED参数，如果子进程异常返回，状态也会记录在state中。怎么用state，man 2 wait查查手册。

LAB

看到很多博客说本次LAB难度不大，个人感觉如果不参考《CSAPP》第8章而自己写的话很容易因为对系统接口的不熟悉等原因而产生BUG。独立完成本次实验的话要仔细查阅手册，以及注意细节。在《Linux/Unix编程实践教程》里也简单实现过一个shell（不过那个lab侧重于有限状态机实现分支控制）。本次lab根据给测试样例一点一点完善shell。

分析

这里简单分析一下已经提供的代码框架。

• 执行我们写的shell时，根据参数进行对应的操作，比如-v、-h、-p参数。
• 默认关闭了SIGINT、SIGTSTP、SIGCHLD、SIGQUIT信号，shell屏蔽了这些信号，模拟一个shell只能被Ctrl + C关闭。
• 定义了一个作业结构体jobs数组，存储当前shell中有哪些作业，他们处于，shell刚运行时初始化该数组。
• 通过eval方法执行在模拟的shell中输入的内容。通过提供的parseline函数对输入通过空格进行分割成多个字符串。（parseline方法中对于\的处理逻辑很怪，效果也和真实的shell差距很大，我这里没弄没明白它到底如何处理\字符的）。
• 还提供了一些处理job作业的函数，后面用到再提。
• 之后便是我们需要完成的功能。

实现

本打算逐步补充代码，写着写着发现在前台后台运行以及jobs处理时会发现很多BUG。有时候可能需要因为后面的样例而修改前面的代码，于是还是很多地方借鉴了书上的内容。因此博客中只贴出整体代码分析。

• 如下图，文档中说的这一处我为了提升性能从而使用了sigsuspend (const sigset_t *__set)，为了免于传递sigset_t的困扰我没有使用waitfg。(听王贺说子进程返回，阻塞wait和SIGCHLD哪个先触发是一个未定义行为。)
  

代码

/* 
 * tsh - A tiny shell program with job control
 * 
 * <Put your name and login ID here>
 */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <errno.h>

/* Misc manifest constants */
#define MAXLINE    1024   /* max line size */
#define MAXARGS     128   /* max args on a command line */
#define MAXJOBS      16   /* max jobs at any point in time */
#define MAXJID    1<<16   /* max job ID */

/* Job states */
#define UNDEF 0 /* undefined */
#define FG 1    /* running in foreground */
#define BG 2    /* running in background */
#define ST 3    /* stopped */

/* 
 * Jobs states: FG (foreground), BG (background), ST (stopped)
 * Job state transitions and enabling actions:
 *     FG -> ST  : ctrl-z
 *     ST -> FG  : fg command
 *     ST -> BG  : bg command
 *     BG -> FG  : fg command
 * At most 1 job can be in the FG state.
 */

/* Global variables */
extern char **environ;      /* defined in libc */
char prompt[] = "tsh> ";    /* command line prompt (DO NOT CHANGE) */
int verbose = 0;            /* if true, print additional output */
int nextjid = 1;            /* next job ID to allocate */
char sbuf[MAXLINE];         /* for composing sprintf messages */

struct job_t {              /* The job struct */
    pid_t pid;              /* job PID */
    int jid;                /* job ID [1, 2, ...] */
    int state;              /* UNDEF, BG, FG, or ST */
    char cmdline[MAXLINE];  /* command line */
};
struct job_t jobs[MAXJOBS]; /* The job list */
/* End global variables */


/* 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);
pid_t Fork();

/*
 * 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[MAXLINE];
    char buf[MAXLINE];
    int bg;
    pid_t pid;
    sigset_t mask, prev, mask_all;
    
    sigfillset(&mask_all);
    sigemptyset(&mask);
    sigaddset(&mask, SIGCHLD);

    strcpy(buf, cmdline);
    bg = parseline(buf, argv);
    if (argv[0] == NULL)    //Ignore empty lines
        return;
    //屏蔽SYNCHLD
    if (!builtin_cmd(argv)) {
        sigprocmask(SIG_BLOCK, &mask, &prev);
        if ((pid = Fork()) == 0) {  //child
            setpgid(0, 0);
            sigprocmask(SIG_BLOCK, &prev, NULL);
            if (execve(argv[0], argv, environ) < 0) {
                printf("%s: Command not found.\n", argv[0]);
                exit(0);
            }
        }
        //添加进jobs
        if (!bg)
            addjob(jobs, pid, FG, cmdline);
        else
            addjob(jobs, pid, BG, cmdline);
        if (!bg) {    // if fb, block
            while (fgpid(jobs) != 0)
                sigsuspend(&prev);  //or sleep
        }
        else
            printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);
        sigprocmask(SIG_SETMASK, &prev, 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") == 0)   // quit cmd
        exit(0);
    else if (strcmp(argv[0], "jobs") == 0) {
        listjobs(jobs);
        return 1;
    }else if(strcmp(argv[0], "bg") == 0 || strcmp(argv[0], "fg") == 0){
		do_bgfg(argv);
		return 1;
	}
    if (!strcmp(argv[0], "&"))
        return 1;
    return 0;     /* not a builtin command */
}

/* 
 * do_bgfg - Execute the builtin bg and fg commands
 */
void do_bgfg(char **argv) 
{
    pid_t pid;
    int jid;
    struct job_t* job;

    if (argv[1] == NULL) {
        printf("%s command requires PID or %%jobid argument\n", argv[0]);
        return;
    }
    if(argv[1][0] != '%' && (argv[1][0] < '0' || argv[1][0] > '9')) {
        printf("%s: argument must be a PID or %%jobid\n", argv[0]);
        return;
    }

    if(argv[1][0]=='%'){    //job
        jid = atoi((*(argv + 1)) + 1);
        job = getjobjid(jobs, jid);
        if(job == NULL){
            printf("%s: No such job\n", argv[1]);
            return;
        }
        pid = job->pid;
    }else{
        pid = atoi(*(argv + 1));
        job = getjobpid(jobs, pid);
        if(job == NULL){
            printf("(%d): No such process\n", pid);
            return;
        }
    }

    /* 防止jobs竞争 */
    sigset_t mask, prev;
    sigemptyset(&mask);
    sigaddset(&mask, SIGCHLD);
    sigprocmask(SIG_BLOCK, &mask, &prev);
    kill(-pid, SIGCONT);
    if(!strcmp(argv[0],"bg")) {
        job->state = BG;
        printf("[%d] (%d) %s", job->jid, job->pid, job->cmdline);
    } else {
        job->state = FG;
        while (fgpid(jobs) != 0)
            sigsuspend(&prev);  //or sleep
    }
    sigprocmask(SIG_SETMASK, &prev, NULL);
}

/* 
 * waitfg - Block until process pid is no longer the foreground process
 */
void waitfg(pid_t pid)
{
    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) 
{
    pid_t pid;
    int state;
    while ((pid = waitpid(-1, &state, WNOHANG | WUNTRACED)) > 0) {
        if (WIFSIGNALED(state)) {
            printf("Job [%d] (%d) terminated by signal %d\n", pid2jid(pid), pid, WTERMSIG(state));
            deletejob(jobs, pid);
        }
        else if (WIFSTOPPED(state)) {   // ctrl + z
            struct job_t* job = getjobpid(jobs, pid);
            job->state = ST;
            printf("Job [%d] (%d) stopped by signal %d\n", pid2jid(pid), pid, WSTOPSIG(state));
        } else {    //terminated  normally
            deletejob(jobs, pid);
        }
    }
}

/* 
 * 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 pid;
    if ((pid = fgpid(jobs)) != 0)
        kill(-pid, 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) 
{
    pid_t pid;
    if ((pid = fgpid(jobs)) != 0) {
        kill(-pid, 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);
}

pid_t Fork() {
    pid_t pid;
    if ((pid = fork()) < 0)
        unix_error("Fork error");
    return pid;
}

总结

耗时1天半终于完成该lab，经常由于一个字符常量或者函数返回值整错了debug半天。感觉本次lab难点在于异常流的理解，和jobs竞争的考虑。可以参考《CSAPP》第八章理清楚，还有就是各种函数系统调用不熟悉容易参数写错，造成畏手畏脚不敢写。