C语言实现一个Shell（ShellLab）

本文章就是在做ShellLab的源码以及对于内容的讲解，因为我自身比较菜

因此其中的解释对于小白来说还是比较的友好的，其中思路代码等都来自于郭郭
建议读的顺序是sigint sigstop -> sigchld_hanlder -> waitfg -> builtin_command -> do_fg -> eval

做之前需要通读一遍csapp第八章的内容，尤其是waitpid中的各种参数、sigprocmask进行信号屏蔽的例子要看懂，信号中断的处理。。。共勉，一起加油！

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 


#define MAXLINE    1024   
#define MAXARGS     128   
#define MAXJOBS      16   
#define MAXJID    1<<16   


#define UNDEF 0 
#define FG 1    
#define BG 2    
#define ST 3    




extern char **environ;      
char prompt[] = "tsh> ";    
int verbose = 0;            
int nextjid = 1;            
char sbuf[MAXLINE];         

struct job_t {              
    pid_t pid;              
    int jid;                
    int state;              
    char cmdline[MAXLINE];  
};
struct job_t jobs[MAXJOBS]; 






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);


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);


int main(int argc, char **argv) 
{
    char c;
    char cmdline[MAXLINE];
    int emit_prompt = 1; 

    
    dup2(1, 2);

    
    while ((c = getopt(argc, argv, "hvp")) != EOF) {
        switch (c) {
        case 'h':             
            usage();
	    break;
        case 'v':             
            verbose = 1;
	    break;
        case 'p':             
            emit_prompt = 0;  
	    break;
	default:
            usage();
	}
    }

    

    
    Signal(SIGINT,  sigint_handler);   
    Signal(SIGTSTP, sigtstp_handler);  
    Signal(SIGCHLD, sigchld_handler);  

    
    Signal(SIGQUIT, sigquit_handler); 

    
    initjobs(jobs);

    
    while (1) {

	
	if (emit_prompt) {
	    printf("%s", prompt);
	    fflush(stdout);
	}
	if ((fgets(cmdline, MAXLINE, stdin) == NULL) && ferror(stdin))
	    app_error("fgets error");
	if (feof(stdin)) { 
	    fflush(stdout);
	    exit(0);
	}

	
	eval(cmdline);              
	fflush(stdout);             
	fflush(stdout);
    } 

    exit(0); 
}
  

void eval(char *cmdline) 
{

    //在原书中P543中，我们通过mask解决了子进程早于父进程执行从而出错的情况
    //我们在子进程中解掉block 因为子进程同样继承了父进程的mask，但是如果子进程再fork的话，它的子进程不应该被block，因此子进程中应该解除block
    char * argv[MAXARGS];
    int bg;     //judge if is bg
    pid_t  pid;
    sigset_t mask;
    bg = parseline(cmdline, argv);

    if (!builtin_cmd(argv)) {

        //掩码不在函数外面加是因为：我们上面函数如果是系统调用就直接执行就好了，不需要进行阻塞
        //函数能执行到这里就代表不是系统调用了，那么我们就是应该进行阻塞，因为存在有sigchld_handler 在外面

        sigemptyset(&mask);
        sigaddset(&mask, SIGCHLD);          //阻塞回收子进程
        sigprocmask(SIG_BLOCK, &mask, NULL);

        if ((pid = fork()) < 0)
            unix_error("forking error");
        else if (pid == 0) {        //shell 需要创建一个新的子进程执行我们锁输入的命令
            sigprocmask(SIG_UNBLOCK, &mask, NULL);              //上面讲述的，解除掉子进程中的block

            


            setpgid(0, 0);                                      //创建新的进程组（进程组的ID就是目前进程的ID），不然如果前台有一个kill就将父进程和子进程都杀掉了
            if (execvp(argv[0], argv) < 0) {                //执行文件 argv[0], 后面的就作为参数进行传递
                printf("%s command not foundn", argv[0]);
                exit(1);
            }
        }else {             //parent process add job
            if (!bg) {
                addjob(jobs, pid, FG, cmdline);
            }else {
                addjob(jobs, pid, BG, cmdline);
            }

            sigprocmask(SIG_UNBLOCK, &mask, NULL);
            //上面的都是被mask进行包住了，比如说更改共享变量的时候我们就相当于上了一把锁一样
            if (!bg) {
                waitfg(pid);
            }else {
                printf("[%d] (%d) %s", pid2jid(pid), pid, cmdline);
            }

        }
    }

}


int parseline(const char *cmdline, char **argv) 
{
    static char array[MAXLINE]; 
    char *buf = array;          
    char *delim;                
    int argc;                   
    int bg;                     

    strcpy(buf, cmdline);
    buf[strlen(buf)-1] = ' ';  
    while (*buf && (*buf == ' ')) 
	buf++;

    
    argc = 0;
    if (*buf == ''') {
	buf++;
	delim = strchr(buf, ''');
    }
    else {
	delim = strchr(buf, ' ');
    }

    while (delim) {
	argv[argc++] = buf;
	*delim = '';
	buf = delim + 1;
	while (*buf && (*buf == ' ')) 
	       buf++;

	if (*buf == ''') {
	    buf++;
	    delim = strchr(buf, ''');
	}
	else {
	    delim = strchr(buf, ' ');
	}
    }
    argv[argc] = NULL;
    
    if (argc == 0)  
	return 1;

    
    if ((bg = (*argv[argc-1] == '&')) != 0) {
	argv[--argc] = NULL;
    }
    return bg;
}


int builtin_cmd(char **argv)        //判断是否是内置的函数quit,bg,&,jobs,  是的话进行处理 & return 1
{
    if (strcmp(argv[0], "quit") == 0) {
        exit(0);
        //return 1;
    }else if (strcmp(argv[0],"jobs") == 0) {
        //call function : listjobs
        listjobs(jobs);
        return 1;
    }else if (strcmp(argv[0], "&") == 0) {
        //试过了，只输入&的话 啥也不是
        //bash: syntax error near unexpected token `&'
        return 1;
    }else if (strcmp(argv[0], "bg") == 0 | strcmp(argv[0], "fg") == 0 ) {
        //看看下一个函数就是do_bgfg
        do_bgfg(argv);
        return 1;
    }
    return 0;     
}


void do_bgfg(char **argv) 
{
    struct  job_t * job;
    pid_t  pid;
    int jid;
    int status;

    char * tmp;
    tmp = argv[1];

    if (tmp == NULL) {
        printf("%s command requires PID or %%jobid argumentn", argv[0]);
        return ;
    }
    if (tmp[0] == '%') {
        //input is a jobid
        jid = atoi(tmp + 1);
        job = getjobjid(jobs, jid);             //通过jid找到那个需要执行的job

        if (job == NULL) {
            printf("%s: No such jobn", tmp);
            return ;
        }else {
            pid = job -> pid;
        }
    }else if (isdigit(atoi(&tmp))) {        //只是数字的话那么就是输入的是进程ID了
        pid = atoi(tmp);
        job = getjobpid(jobs, pid);

        if (job == NULL) {
            printf("(%d): No such processn", pid);
            return ;
        }
    }else {
        printf("%s: argument must be a PID or %%jobidn", argv[0]);
    }

    //现在拿到了需要执行的job
    kill(pid, SIGCONT);     //不是kill，是继续执行的意思，本来就工作就继续，没有工作就开始工作

    //已经处理完job了，下面看一下前台后台的处理
    if (strcmp(argv[0], "fg") == 0) {
        job -> state = FG;          //将任务的状态写成是FG：前台
        waitfg(job -> pid);     //在这个前台任务完成之前，都是一直block的
        
    }else {
        printf("[%d] (%d) %s", job -> jid, job -> pid, job -> cmdline);
        job -> state = BG;
    }

}


void waitfg(pid_t pid)
{
    //就是如果前台的程序是pid的话我们就一直等待着

    //首先判断一下给的pid这个任务是否是存在的
    struct jobs * job = getjobpid(jobs, pid);
    if (job == NULL) return ;           //根本就没有这个程序，直接退出

    


    while(fgpid(jobs) == pid) {     //spin ..... until pid is not foreground

    }

}






//为什么处理信号的时候不需要进行block呢？

//因为信号是不会进行计数的，如果本身存在了一个信号的话，你来多少信号我都只知道：现在存在一个信号执行，并不关心有多少个
//第二个同类型的信号是不会打断上一个正在处理的信号的：-）
void sigchld_handler(int sig) 
{
    
    int olderrno = errno;
    pid_t pid;
    int status;
    
    while (waitpid(-1, &status, WNOHANG | WUNTRACED) > 0) {
        if (WIFEXITED(status)) {
            //通过正常的exit进行的退出
            deletejob(jobs, pid);
        }else if (WIFSIGNALED(status)) {
            //通过被信号杀掉
            int jid = pid2jid(pid);
            printf("Job [%d] (%d) terminated by signal %dn", jid, pid, WTERMSIG(status));          //返回导致被信号杀掉的pid，这两个函数是搭配使用的
            deletejob(jobs, pid);
        }else {
            //子进程已经是stop了
            struct job_t * job = getjobpid(jobs, pid);            //获得指向已经退出子进程的结构体的指针，我们更改它的状态
            job -> state = ST;                  //修改进程的状态为stop

            int jid = pid2jid(pid);
            printf("Job [%d] (%d) Stopped by signal %dn", jid, pid, WSTOPSIG(status));
        }
    }

    errno = olderrno;
}


void sigint_handler(int sig) 
{
    
    pid_t pid = fgpid(jobs);
    if (pid == 0)           //前台没有进程，直接return
        return;
    kill(-pid, sig);
}


void sigtstp_handler(int sig) 
{
    
    pid_t pid = fgpid(jobs);
    if (pid == 0)           //前台没有进程，直接return
        return;
    kill(-pid, sig);
}






void clearjob(struct job_t *job) {
    job->pid = 0;
    job->jid = 0;
    job->state = UNDEF;
    job->cmdline[0] = '';
}


void initjobs(struct job_t *jobs) {
    int i;

    for (i = 0; i < MAXJOBS; i++)
	clearjob(&jobs[i]);
}


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;
}


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) {         //如果这个存放job的结构还没有被占用，就分配给当前的进程
	    jobs[i].pid = pid;
	    jobs[i].state = state;
	    jobs[i].jid = nextjid++;        //golbal argument : nextjid 是下一个可以分配的jobid， 如果用完了就置为1
	    if (nextjid > MAXJOBS)
		nextjid = 1;
	    strcpy(jobs[i].cmdline, cmdline);
  	    if(verbose){            //是否打印其他的信息
	        printf("Added job [%d] %d %sn", jobs[i].jid, jobs[i].pid, jobs[i].cmdline);
            }
            return 1;
	}
    }
    printf("Tried to create too many jobsn");
    return 0;
}


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;
}


pid_t fgpid(struct job_t *jobs) {   //因为前台任务只有一个，因此返回的只有一个值
    int i;

    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].state == FG)
	    return jobs[i].pid;
    return 0;
}


struct job_t *getjobpid(struct job_t *jobs, pid_t pid) {        //通过pid找到任务列表中的任务（返回的是结构体指针）
    int i;

    if (pid < 1)
	return NULL;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].pid == pid)
	    return &jobs[i];
    return NULL;
}


struct job_t *getjobjid(struct job_t *jobs, int jid)        //通过jid找到任务，返回的是任务的结构体指针
{
    int i;

    if (jid < 1)
	return NULL;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].jid == jid)
	    return &jobs[i];
    return NULL;
}


int pid2jid(pid_t pid)  //通过pid找到jobid
{
    int i;

    if (pid < 1)
	return 0;
    for (i = 0; i < MAXJOBS; i++)
	if (jobs[i].pid == pid) {
            return jobs[i].jid;
        }
    return 0;
}


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);
	}
    }
}






void usage(void) 
{
    printf("Usage: shell [-hvp]n");
    printf("   -h   print this messagen");
    printf("   -v   print additional diagnostic informationn");
    printf("   -p   do not emit a command promptn");
    exit(1);
}


void unix_error(char *msg)
{
    fprintf(stdout, "%s: %sn", msg, strerror(errno));
    exit(1);
}


void app_error(char *msg)
{
    fprintf(stdout, "%sn", msg);
    exit(1);
}


handler_t *Signal(int signum, handler_t *handler) 
{
    struct sigaction action, old_action;

    action.sa_handler = handler;  
    sigemptyset(&action.sa_mask); 
    action.sa_flags = SA_RESTART; 

    if (sigaction(signum, &action, &old_action) < 0)
	unix_error("Signal error");
    return (old_action.sa_handler);
}


void sigquit_handler(int sig) 
{
    printf("Terminating after receipt of SIGQUIT signaln");
    exit(1);
}