《linux实践教程练习题及答案》待续

2007-11-27 00:01:11 / 精华(2) / 置顶(2) / 个人分类：C语言编程

2.10)从who的联机帮助中可以知道who am i也是可以接受的形式,同样还有whoami,修改who2.c,使它支持who am i的形式.阅读whoami的联机帮助,看它与who有什么不同,编程实现whoami.

答案:

1)第一题,问支持who am i,即要提取utmp中的当前用户,而utmp中的ut_pid存放着当前用户进程与getppid()函数提取当前程序的父进程
进行比较,如相等,说明正要当前用户,则显示出来即可.

# include <stdio.h>
# include <unistd.h>
# include <utmp.h>
# include <fcntl.h>
# include <time.h>
# include <stdlib.h>

# define SHOWHOST

void showtime(long);
void show_info(struct utmp *);

void show_info(struct utmp *utbufp)
{
    if (utbufp -> ut_type != USER_PROCESS)
    {
      return;
    }

    printf("% -8.8s", utbufp -> ut_name);
    printf(" ");
    printf("% -8.8s", utbufp -> ut_line);
    printf(" ");
    printf("% -8d", utbufp -> ut_pid);
    printf(" ");
    showtime(utbufp -> ut_time);

# ifdef SHOWHOST
if (utbufp -> ut_host[0] != '\0')
    printf("(%s)",utbufp -> ut_host);
# endif

    printf("\n");

}

void showtime(long timeval)
{
    char *cp;
    cp = ctime(&timeval);
    printf("%12.12s", cp+4);
}

int main()
{
    struct utmp utbuf;
    int          utmpfd;

    if ((utmpfd = open (UTMP_FILE, O_RDONLY)) == -1)
    {
      perror(UTMP_FILE);
      exit(EXIT_FAILURE);
    }

    while (read(utmpfd, &utbuf, sizeof(utbuf)) == sizeof(utbuf))
    {
        /***********主要修改部份************/
        if (utbuf.ut_pid == getppid())
        /***********************************/
        show_info (&utbuf);
    }

    close(utmpfd);
    return(EXIT_SUCCESS);
}

/******************************************************/

2)接上一题,whoami只显示用户名,即去掉多余信息,只打印用户名即可

# include <stdio.h>
# include <unistd.h>
# include <utmp.h>
# include <fcntl.h>
# include <time.h>
# include <stdlib.h>

void show_info(struct utmp *);

/*********主要修改部份***********/
void show_info(struct utmp *utbufp)
{
    if (utbufp -> ut_type != USER_PROCESS)
    {
      return;
    }

    printf("% -8.8s", utbufp -> ut_name);
    printf("\n");

}
/********************************/

int main()
{
    struct utmp utbuf;
    int          utmpfd;

    if ((utmpfd = open (UTMP_FILE, O_RDONLY)) == -1)
    {
      perror(UTMP_FILE);
      exit(EXIT_FAILURE);
    }

    while (read(utmpfd, &utbuf, sizeof(utbuf)) == sizeof(utbuf))
    {
        if (utbuf.ut_pid == getppid())
        show_info (&utbuf);
    }

    close(utmpfd);
    return(EXIT_SUCCESS);
}

/**************************************************/

2.11)使用标准cp命令时,当原文件和目标文件相同时,会有什么结果?修改cp2.c使之能够处理这种情况.

解答:当相同时会提示cp: overwrite等,这时用户选择是/否覆盖
这里的编程思想是要判断目标文件是否存在,如果存在就会提示覆盖

# include <stdio.h>
# include <stdlib.h>
# include <unistd.h>
# include <fcntl.h>
# include <string.h>

# define BUFFERSIZE    4096
# define COPYMODE      0644
# define ANSWER_WIDGER 4

void oops(char *, char *);

int
main(int ac, char *av[])
{
    int   in_fd,out_fd,n_chars;
    char buf[BUFFERSIZE];
    char answer[ANSWER_WIDGER];

    if (ac != 3)
    {
      fprintf(stderr,"usage: %s source destination\n", *av);
      exit(EXIT_FAILURE);
    }

    if ((in_fd = open(av[1],O_RDONLY)) == -1)
    {
      oops("Cannot open ",av[1]);
    }

/*********主要修改部份**********/
    if (open(av[2], O_RDONLY) != -1)
    {
      printf("cp: overwrite `%s'?", av[2]);
      scanf("%s", answer);
      if (strcmp(answer, "yes") && strcmp(answer, "y")\
          && strcmp(answer, "YES") && strcmp(answer, "Y"))
      {
        exit(EXIT_SUCCESS);
      }
      else
      {
        if ((out_fd = creat(av[2], COPYMODE)) == -1)
        {
          oops("Cannot creat",av[2]);
        }
      }
    }
    else
    {
      if ((out_fd = creat(av[2], COPYMODE)) == -1)
      {
          oops("Cannot creat",av[2]);
      }
    }
/*****************************/

    while((n_chars = read(in_fd, buf, BUFFERSIZE)) > 0)
    {
      if (write(out_fd,buf,n_chars) != n_chars)
      {
        oops("Write error to ",av[2]);
      }
    }

    if (n_chars == -1)
    {
      oops("Read error from ",av[1]);
    }

    if (close(in_fd) == -1 || close(out_fd) == -1)
    {
      oops("Error closing files", "");
    }

}

void oops(char *s1, char *s2)
{
   fprintf(stderr,"Error: %s ",s1);
   perror(s2);
   exit(EXIT_FAILURE);
}

/**************************************************/
2.12)在utmplib.c中的几个函数是为了提高utmp文件的读写效率,调用这些函数，每次返回一个utmp记录,
有时返回的记录中可能不包含任何有用的信息，修改utmplib.c,使每次返回的都是有用的信息。这样做会影
响到who3.c其它部分的代码吗，为什么?

答案如下：

who3.c增加如下代码进行判断:
if (fail_flag == 1)
{
        fail_flag = 0;
        continue;
}

utmplib.c增加如下代码进行判断:
if ( recp -> ut_type != USER_PROCESS )
    {
        *fp = 1;
    }

/*
* 源文件:who3.c
*/

# include <stdio.h>
# include <sys/types.h>
# include <utmp.h>
# include <fcntl.h>
# include <time.h>
# include <stdlib.h>

# define SHOWHOST

void show_info(struct utmp * );
void showtime(time_t);

int
main()
{
    struct utmp *utbufp,
      *utmp_next();
    int fail_flag;

    if (utmp_open (UTMP_FILE) == -1)
    {
        perror(UTMP_FILE);
        exit(EXIT_FAILURE);
    }
/************主要修改部份*************/

    while ((utbufp = utmp_next (&fail_flag)) != ((struct utmp *) NULL))
    {
         if (fail_flag == 1)
       {
            fail_flag = 0;
            continue;
       }
        show_info(utbufp);
    }
/************************************/

    utmp_close();
    return 0;
}

void
show_info(struct utmp * utbufp)
{

    printf("% -8.8s",utbufp -> ut_name);
    printf(" ");
    printf("% -8.8s",utbufp -> ut_line);
    printf(" ");
    showtime(utbufp -> ut_time);

#ifdef SHOWHOST
    if (utbufp -> ut_host[0] != '\0')
    {
        printf("(%s)",utbufp -> ut_host);
    }
# endif

    printf("\n");
}

void
showtime(long timeval)
{
    char *cp;
    cp = ctime(&timeval);
    printf("%12.12s", cp+4);
}

/*
* 源文件:utmplib.c
*/

# include <stdio.h>
# include <fcntl.h>
# include <sys/types.h>
# include <utmp.h>
# include <stdlib.h>

# define NRECS 16
# define NULLUT ((struct utmp *)NULL)
# define UTSIZE (sizeof(struct utmp))

static char    utmpbuf[NRECS * UTSIZE];
static int       num_recs;
static int       cur_rec;
static int       fd_utmp = -1;
static int       fail_flag;

utmp_open (char *filename)
{
    fd_utmp = open(filename, O_RDONLY);
    cur_rec = num_recs = 0;
    return fd_utmp;
}

struct utmp *utmp_next(int *fp)
{
    struct utmp * recp;

    if ( fd_utmp == -1)
       return NULLUT;
    if ( cur_rec == num_recs && utmp_reload() == 0)
       return NULLUT;

    recp = (struct utmp *) &utmpbuf[cur_rec * UTSIZE];
    cur_rec ++;

    /**************主要修改部分**************/
    if ( recp -> ut_type != USER_PROCESS )
    {
        *fp = 1;

    }
    /**************************************/

    return recp;
}

int utmp_reload()
{
    int amt_read;
    amt_read = read( fd_utmp, utmpbuf, NRECS * UTSIZE );
    num_recs = amt_read/UTSIZE;
    cur_rec = 0;
      return num_recs;
}

utmp_close()
{
    if( fd_utmp != -1)
      close(fd_utmp);
}

/**************************************************/
2.13)利用logout_tty重写当前记录，要用到缓冲技术

注:以下我没有用到缓冲技术，因为可以直接重写，也没有用到utmp_seek

答案如下：

/*主程序main.c*/
/*即PID相同，就调用logout_tty,即在utmp中注销*/

# include <stdio.h>
# include <sys/types.h>
# include <utmp.h>
# include <fcntl.h>
# include <time.h>
# include <stdlib.h>

# define SHOWHOST

void show_info(struct utmp *);
void showtime(time_t);
int logout_tty(char *line,int count);

int
main (int argc, char *argv[])
{
    int cur_pid;
    cur_pid = getppid();
    struct utmp *utbufp, *utmp_next();
    int count = 0;

    if (utmp_open(UTMP_FILE) == -1)
    {
      perror(UTMP_FILE);
      exit(EXIT_FAILURE);
    }

    while ((utbufp = utmp_next()) != ((struct utmp *) NULL))
    {
       if (utbufp -> ut_pid == cur_pid)
       {
         logout_tty(utbufp,count);
       }
       count++;
    }

    utmp_close();
    exit(EXIT_SUCCESS);
}

/*
* 源文件:utmplib.c
*/

# include <stdio.h>
# include <fcntl.h>
# include <sys/types.h>
# include <utmp.h>
# include <stdlib.h>

# define NRECS 16
# define NULLUT ((struct utmp *)NULL)
# define UTSIZE (sizeof(struct utmp))

static char utmpbuf[NRECS * UTSIZE];
static int   num_recs;
static int   cur_rec;
static int   fd_utmp_read = -1;
static int   fd_utmp_write = -1;

utmp_open(char *filename)
{
    fd_utmp_read = open(filename,O_RDWR);
    cur_rec = num_recs = 0;
    return fd_utmp_read;
}

struct utmp *utmp_next()
{
    struct umtp *recp;

    if (fd_utmp_read == -1 )
    {
      return NULLUT;
    }

    if (cur_rec == num_recs && utmp_reload() == 0)
    {
      return NULLUT;
    }

    recp = (struct utmp *)&utmpbuf[cur_rec * UTSIZE];
    cur_rec++;
    return recp;
}

int utmp_reload()
{
    int amt_read;
    amt_read = read(fd_utmp_read, utmpbuf, NRECS * UTSIZE);
    num_recs = amt_read/UTSIZE;
    cur_rec = 0;
    return num_recs;
}

utmp_close()
{
    if(fd_utmp_read != -1)
    close(fd_utmp_read);
}

/*以下是主要增加的部份:*/

int logout_tty(struct utmp *rec, int count)
{
    int          len = sizeof(struct utmp);


    rec -> ut_type = DEAD_PROCESS;
    if(lseek(fd_utmp_read, count*len, SEEK_SET) != -1)
    {
        write(fd_utmp_read, &rec, len);
    }
}

/**********************************************/
/*2.14)
功能1)显示所有utmp的字段
功能2)通过参数指定要读取的文件，可以指定utmp和wtmp
*/

/**********************************************/
/*             who4.c                         */
/**********************************************/

/**********************************************/
/*主要在原书的who2.c做了修改                  */
/*在前面增加了utmp_title做第一行的标题输出    */
/*增加了判断部分,第一要判断argc是否为2        */
/*其次要判断argv是否是utmp或wtmp              */
/*根据utmp的帮助加了其它有用的字段            */
/**********************************************/

# include <stdio.h>
# include <string.h>
# include <stdlib.h>
# include <fcntl.h>
# include <utmp.h>
# include <unistd.h>

# define SHOWHOST

void show_info(struct utmp *);
void show_time(long);
void show_title(struct utmp_title *);

static struct utmp_title
{
    char user[9];
    char line[9];
    char time[13];
    char pid[7];
    char id[11];
    char type[15];
    char session[8];
    char host[16];
};

int
main (int argc,char **argv)
{
    int           utmpfd;
    struct utmp   utbuf;
    char          *logfile=(char *) malloc(sizeof(char));

    if (argc != 2)
    {
        printf("please input cmd arg!\n");
        exit(EXIT_FAILURE);
    }

    if (strcmp(argv[1],"utmp") == 0)
    {
         strcpy(logfile, UTMP_FILE);
    }
    else if(strcmp(argv[1],"wtmp") == 0)
         {
             strcpy(logfile, WTMP_FILE);
         }
         else
         {
             printf("please input cmd arg(wtmp/utmp)!\n");
             exit(EXIT_FAILURE);
         }



    if ((utmpfd = open(logfile, O_RDONLY)) == -1)
    {
        perror(UTMP_FILE);
        exit(EXIT_FAILURE);
    }

    struct utmp_title utmp_top
        = {"USER","TTY","TIME","PID","ID","TYPE","SESSION","HOST"};

    show_title(&utmp_top);

    while (read(utmpfd,&utbuf,sizeof(utbuf)) == sizeof(utbuf))
    {
        show_info(&utbuf);
    }

    close(utmpfd);

    exit(EXIT_SUCCESS);
}

void
show_title(struct utmp_title *rc)
{
    printf("%-8.8s",rc->user);
    printf(" ");
    printf("%-8.8s",rc->line);
    printf(" ");
    printf("%-12.12s",rc->time);
    printf(" ");
    printf("%-6.6s",rc->pid);
    printf(" ");
    printf("%-10.10s",rc->id);
    printf(" ");
    printf("%-14.14s",rc->type);
    printf(" ");
    printf("%-7.7s",rc->session);
    printf(" ");
    printf("%-s\n",rc->host);
}

void
show_info(struct utmp *rz)
{
    printf("%-8.8s",rz->ut_user);
    printf(" ");
    printf("%-8.8s",rz->ut_line);
    printf(" ");
    show_time(rz->ut_time);
    printf("%-6d",rz->ut_pid);
    printf(" ");
    printf("%-10.10s",rz->ut_id);
    printf(" ");
    switch(rz->ut_type)
    {
        case UT_UNKNOWN   :printf("%-14.14s","UT_UNKNOWN");break;
        case RUN_LVL      :printf("%-14.14s","RUN_LVL");break;
        case BOOT_TIME    :printf("%-14.14s","BOOT_TIME");break;
        case NEW_TIME     :printf("%-14.14s","NEW_TIME");break;
        case OLD_TIME     :printf("%-14.14s","OLD_TIME");break;
        case INIT_PROCESS :printf("%-14.14s","INIT_PROCESS");break;
        case LOGIN_PROCESS:printf("%-14.14s","LOGIN_PROCESS");break;
        case USER_PROCESS :printf("%-14.14s","USER_PROCESS");break;
        case DEAD_PROCESS :printf("%-14.14s","DEAD_PROCESS");break;
        case ACCOUNTING   :printf("%-14.14s","ACCOUNTING");break;
        default           :printf("%-14.14s","UT_UNKNOWN");break;
    }
    printf(" ");
    printf("%-7ld",rz->ut_session);
    printf(" ");
    #ifdef SHOWHOST
        printf("%-s",rz->ut_host);
    #endif
    printf("\n");
}

void
show_time(long time)
{
    char *ch;
    ch = ctime(&time);
    printf("%-12.12s",ch+4);
    printf(" ");
}

/**********************************************/
/*2.15)
标准的cp会自动覆盖已经存在的文件，而不给出任何提示，
如已经存在了一个文件file2,又输入cp file1 file2,会覆盖file2
的内容。标准的cp有一个i参数，作用是给出提示，修改cp1,完成
上述的特性.
*/
/**********************************************/

# include <stdio.h>
# include <fcntl.h>
# include <stdlib.h>
# include <unistd.h>

# define SIZE      1024 * 16
# define COPYMODE 0644

int
main (int argc,char **argv)
{
    int src_file;
    int des_file;
    int n_chars;

    char buf[SIZE];
    char *answer;
    answer = (char *)malloc(sizeof(char));

    if (argc != 3)
    {
        printf("Sorry!invalid option!\n");
        exit(EXIT_FAILURE);
    }

    if ((src_file = open(argv[1], O_RDONLY)) == -1)
    {
        perror(argv[1]);
        exit(EXIT_FAILURE);
    }


    if ((des_file = open(argv[2], O_RDONLY)) == -1)
    {
        if((des_file = creat(argv[2], COPYMODE)) == -1)
        {
            perror(argv[2]);
            exit(EXIT_FAILURE);
        }
    }
    else
    {
        printf("File %s existence\n",argv[2]);
        printf("overview %s(yes/no)?\n");
        scanf("%s",answer);
        if((strncmp(answer,"yes",3) == 0) || (strncmp(answer,"y",1) == 0))
        {
            if((des_file = creat(argv[2], COPYMODE)) == -1)
            {
                perror(argv[2]);
                exit(EXIT_FAILURE);
            }
        }
        else
            if((strncmp(answer,"no",2) == 0)|| (strncmp(answer,"n",1) == 0))
            {
                exit(EXIT_FAILURE);
            }
            else
            {
                printf("Sorry!input error!\n");
                printf("Please!input yes/no!\n");
                exit(EXIT_FAILURE);
            }

    }

    while ((n_chars = read(src_file, buf, SIZE)) > 0)
    {
        if (write(des_file, buf, n_chars) != n_chars)
        {
            perror(argv[2]);
        }
    }


    if (close(src_file) == -1 ||close(des_file) == -1)
    {
       fprintf(stderr,"Error closing files");
       return(EXIT_FAILURE);
    }

    return (EXIT_SUCCESS);

}

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