java读取utmp文件的问题：报错

潜水很久了，第一次发帖，OSchina是个好地方

这几天做一个VPN的管理系统，遇到了这样一个题目，就是要读取/var/run/utmp这个文件，获取登入信息。

utmp是一个二进制文件，who命令就读取了这个文件，参考源代码后，发现C下读取utmp文件特别简单

struct utmp current_record;//一个缓冲区
int utmpfd; //文件描述符
int reclen = sizeof(current_record);

if ((utmpfd = open(UTMP_FILE, O_RDONLY)) == -1) {
        perror(UTMP_FILE);
        return 1;
}
while (read(utmpfd, &current_record, reclen) == reclen)
        show_info(&current_record); //显示utmp的内容
close(utmpfd);

其中 utmp 结构体的定义如下

struct utmp
{
  short int ut_type;            /* Type of login.  */
  pid_t ut_pid;                 /* Process ID of login process.  */
  char ut_line[UT_LINESIZE];    /* Devicename.  */
  char ut_id[4];                /* Inittab ID.  */
  char ut_user[UT_NAMESIZE];    /* Username.  */
  char ut_host[UT_HOSTSIZE];    /* Hostname for remote login.  */
  struct exit_status ut_exit;   /* Exit status of a process marked
                                   as DEAD_PROCESS.  */
/* The ut_session and ut_tv fields must be the same size when compiled
   32- and 64-bit.  This allows data files and shared memory to be
   shared between 32- and 64-bit applications.  */
#if __WORDSIZE == 64 && defined __WORDSIZE_COMPAT32
  int32_t ut_session;           /* Session ID, used for windowing.  */
  struct
  {
    int32_t tv_sec;             /* Seconds.  */
    int32_t tv_usec;            /* Microseconds.  */
  } ut_tv;                      /* Time entry was made.  */
#else
  long int ut_session;          /* Session ID, used for windowing.  */
  struct timeval ut_tv;         /* Time entry was made.  */
#endif

  int32_t ut_addr_v6[4];        /* Internet address of remote host.  */
  char __unused[20];            /* Reserved for future use.  */
};
在java下，我想使用randomaccessfile来读取这个文件，

先定义utmp

public class utmp {
 static final int UTMPSIZE = 384;
 static final int UT_LINESIZE = 32;
 static final int UT_NAMESIZE = 32;
 static final int UT_HOSTSIZE = 256;

 short ut_type; /* Type of login. */
 int ut_pid; /* Process ID of login process. */
 byte ut_line[]; /* Devicename. */
 byte ut_id[]; /* Inittab ID. */
 byte ut_user[]; /* Username. */
 byte ut_host[]; /* Hostname for remote login. */

 class exit_status /* Exit status of a process marked as DEAD_PROCESS. */
 {
  short e_termination; /* Process termination status. */
  short e_exit; /* Process exit status. */

  exit_status() {
   e_termination = -1;
  }
 }

 exit_status ut_exit;
 int ut_session; /* Session ID, used for windowing. */

 class timeval /* Time entry was made. */
 {
  int tv_sec; /* Seconds. */
  int tv_usec; /* Microseconds. */
 }

 timeval ut_tv;
 int ut_addr_v6[]; /* Internet address of remote host. */
 byte __unused[]; /* Reserved for future use. */

 utmp() {
  ut_line = new byte[UT_LINESIZE];
  ut_id = new byte[4];
  ut_user = new byte[UT_NAMESIZE];
  ut_host = new byte[UT_HOSTSIZE];
  ut_addr_v6 = new int[4];
  __unused = new byte[20];
 }
}

然后考虑c的内存对齐问题，在读取的时候

for (int i = 0; i < in.length() / utmp.UTMPSIZE; i++) {
   utmp tem = new utmp();
   int loc=0;
   
   in.seek(loc+44);
   tem.ut_type = in.readShort();
//System.out.println(in.getFilePointer());

   in.seek(loc+48);
   tem.ut_pid = in.readInt();
//System.out.println(in.getFilePointer());
   
   in.seek(loc+52);
   for (int j = 0; j < utmp.UT_LINESIZE; j++)
    tem.ut_line[j] = in.readByte();
   
   in.seek(loc+84);
   for (int j = 0; j < 4; j++)
    tem.ut_id[j] = in.readByte();
   
   in.seek(loc+88);
   for (int j = 0; j < utmp.UT_NAMESIZE; j++)
    tem.ut_user[j] = in.readByte();
   
   in.seek(loc+120);
   for (int j = 0; j < utmp.UT_HOSTSIZE; j++)
    tem.ut_host[j] = in.readByte();

没到一个位置就使用seek函数调整指针（对齐偏移printf("pid^%d^\n",(unsigned int)&utbufp->ut_pid-(unsigned int)&utbufp)通过上面这个printf获得）

但是最终结果令人失望，读取失败，求大大帮忙指点下哪里有问题啊