chapter 6 系统数据文件和信息

口令文件：/etc/passwd

pwd.h定义的passwd结构：

/* The passwd structure.  */
struct passwd
{
  char *pw_name;        /* Username.  */
  char *pw_passwd;        /* Password.  */
  __uid_t pw_uid;        /* User ID.  */
  __gid_t pw_gid;        /* Group ID.  */
  char *pw_gecos;        /* Real name.这里可以是注释  */
  char *pw_dir;            /* Home directory.  */
  char *pw_shell;        /* Shell program.  */
};

根据username或者user id来获取记录：

#include <sys/type.h>

#include <pwd.h>

struct passwd* getpwuid(uid_t uid);

struct passwd* getpwnam(const char* name);

获取全部口令文件：

#include <sys/type.h>

#include <pwd.h>

struct passwd* getpwent(void);

void setpwent(void);

void endpwent(void);

其中，getpwent依次获取一条记录，知道NULL。setpwent打开归零，endpwent关闭。这个具有通用性，其他文件和这个是类似的。

/etc/shadow

这个文件存放用户名和密码的密文，从而使得passwd文件的密码密文用*代替，防止猜测密码。这样，passwd文件可由各用户读取。

组文件：/etc/group

<grp.h>中定义的group类型：

struct group
  {
    char *gr_name;        /* Group name.    */
    char *gr_passwd;        /* Password.    */
    __gid_t gr_gid;        /* Group ID.    */
    char **gr_mem;        /* Member list.    */
  };
根据组名或者组id获取group对象：

#include <sys/type.h>

#include <grp.h>

struct group* getgrgid(git_t git);

struct group* getgrnam(const char* name);

这种函数返回的是指针，实际内容存放在静态存储区，要复制的话必须把内容考出来，否则容易冲掉。

获取全部组对象：

#include <sys/type.h>

#include <grp.h>

struct group* getgrent(void);

void setgrent(void);

void endgrent(void);

组的特殊性在于一个user 可以属于不同的 group，称为添加组。存取和设置：

#include <sys/type.h>

#include <unistd.h>

int getgroups(int size, gid_t list[]);//把进程所属的user的所有group放在list中，最大个数是size

#include <grp.h>

int setgroups(size_t size, const gid_t* list);//超级用户调用来为进程设置添加组id表。

int initgroups(const char* user, git_t group);//root可以调用，把group加入添加组，同时这个group是/etc/passwd中写的组。

其他数据文件：

对比：

说明     数据文件            头文件         结构            附加的关键字搜索

口令     /etc/passwd       <pwd.h>      passwd        getpwnam,getpwuid

组        /etc/group          <grp.h>        group           getgrnam,getgrgid

主机     /etc/hosts          <netdb.h>    hostent        gethostbyname,gethostbyaddr

网络     /etc/networks     <netdb.h>    netent          getnetbyname,getnetbyaddr

协议     /etc/protocols     <netdb.h>    protoent       getprotobyname,getprotobynumber

服务     /etc/services      <netdb.h>    servent        getservbyname,getservbyport

这些数据文件同样有get*,set*,end*函数，比如主机有：

struct hostent* gethostent(void);

void sethostent(int stayopen);

void endhostent(void);

登录记录文件：

ubuntu下是/var/run/utmp,/var/log/wtmp。

who用的是utmp文件，last用的是wtmp文件。

#include <bits/utmp.h>

/* The structure describing an entry in the user accounting database.  */
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.  */
};

这是ubuntu下的。

系统标识：

这个函数返回与主机和操作系统相关的消息：

#include <sys/utsname.h>

int uname(struct utsname* buf);

/* Structure describing the system and machine.  */
struct utsname
  {
    /* Name of the implementation of the operating system.  */
    char sysname[_UTSNAME_SYSNAME_LENGTH];

    /* Name of this node on the network.  */
    char nodename[_UTSNAME_NODENAME_LENGTH];

    /* Current release level of this implementation.  */
    char release[_UTSNAME_RELEASE_LENGTH];
    /* Current version level of this release.  */
    char version[_UTSNAME_VERSION_LENGTH];

    /* Name of the hardware type the system is running on.  */
    char machine[_UTSNAME_MACHINE_LENGTH];

#if _UTSNAME_DOMAIN_LENGTH - 0
    /* Name of the domain of this node on the network.  */
# ifdef __USE_GNU
    char domainname[_UTSNAME_DOMAIN_LENGTH];
# else
    char __domainname[_UTSNAME_DOMAIN_LENGTH];
# endif
#endif
  };

获取主机：

系统时间：

这个是系统时间函数，其中虚线受TZ影响。

#include <time.h>

/* Used by other time functions.  */
struct tm
{
  int tm_sec;            /* Seconds.    [0-60] (1 leap second) */
  int tm_min;            /* Minutes.    [0-59] */
  int tm_hour;            /* Hours.    [0-23] */
  int tm_mday;            /* Day.        [1-31] */
  int tm_mon;            /* Month.    [0-11] */
  int tm_year;            /* Year    - 1900.  */
  int tm_wday;            /* Day of week.    [0-6] */
  int tm_yday;            /* Days in year.[0-365]    */
  int tm_isdst;            /* DST.        [-1/0/1]*/

#ifdef    __USE_BSD
  long int tm_gmtoff;        /* Seconds east of UTC.  */
  __const char *tm_zone;    /* Timezone abbreviation.  */
#else
  long int __tm_gmtoff;        /* Seconds east of UTC.  */
  __const char *__tm_zone;    /* Timezone abbreviation.  */
#endif
};

format如下：

       %a     The abbreviated weekday name according to the current locale.

       %A     The full weekday name according to the current locale.

       %b     The abbreviated month name according to the current locale.

       %B     The full month name according to the current locale.

       %c     The preferred date and time representation for the current locale.

       %C     The century number (year/100) as a 2-digit integer. (SU)

       %d     The day of the month as a decimal number (range 01 to 31).

       %D     Equivalent to %m/%d/%y.  (Yecch — for Americans only.   Americans  should  note  that  in  other  countries
              %d/%m/%y  is  rather  common.  This means that in international context this format is ambiguous and should
              not be used.) (SU)

       %e     Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space. (SU)

       %E     Modifier: use alternative format, see below. (SU)

       %F     Equivalent to %Y-%m-%d (the ISO 8601 date format). (C99)

       %G     The ISO 8601 week-based year (see NOTES) with century as a decimal number.  The 4-digit year  corresponding
              to  the  ISO  week  number (see %V).  This has the same format and value as %Y, except that if the ISO week
              number belongs to the previous or next year, that year is used instead. (TZ)

       %g     Like %G, but without century, that is, with a 2-digit year (00-99). (TZ)

       %h     Equivalent to %b.  (SU)

       %H     The hour as a decimal number using a 24-hour clock (range 00 to 23).

       %I     The hour as a decimal number using a 12-hour clock (range 01 to 12).

       %j     The day of the year as a decimal number (range 001 to 366).

       %k     The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank.   (See
              also %H.)  (TZ)

       %l     The  hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank.  (See
              also %I.)  (TZ)

       %m     The month as a decimal number (range 01 to 12).

       %M     The minute as a decimal number (range 00 to 59).

       %n     A newline character. (SU)

       %O     Modifier: use alternative format, see below. (SU)

       %p     Either "AM" or "PM" according to the given time value, or the corresponding strings for the current locale.
              Noon is treated as "PM" and midnight as "AM".

       %P     Like %p but in lowercase: "am" or "pm" or a corresponding string for the current locale. (GNU)

       %r     The time in a.m. or p.m. notation.  In the POSIX locale this is equivalent to %I:%M:%S %p.  (SU)

       %R     The time in 24-hour notation (%H:%M). (SU) For a version including the seconds, see %T below.

       %s     The number of seconds since the Epoch, that is, since 1970-01-01 00:00:00 UTC. (TZ)

       %S     The  second as a decimal number (range 00 to 60).  (The range is up to 60 to allow for occasional leap sec‐
              onds.)

       %t     A tab character. (SU)

       %T     The time in 24-hour notation (%H:%M:%S). (SU)

       %u     The day of the week as a decimal, range 1 to 7, Monday being 1.  See also %w.  (SU)

       %U     The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday  as
              the first day of week 01.  See also %V and %W.

       %V     The  ISO 8601 week number (see NOTES) of the current year as a decimal number, range 01 to 53, where week 1
              is the first week that has at least 4 days in the new year.  See also %U and %W.  (SU)

       %w     The day of the week as a decimal, range 0 to 6, Sunday being 0.  See also %u.

       %W     The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday  as
              the first day of week 01.

       %x     The preferred date representation for the current locale without the time.

       %X     The preferred time representation for the current locale without the date.

       %y     The year as a decimal number without a century (range 00 to 99).

       %Y     The year as a decimal number including the century.

       %z     The   time-zone   as   hour   offset   from   GMT.    Required  to  emit  RFC 822-conformant  dates  (using
              "%a, %d %b %Y %H:%M:%S %z"). (GNU)

       %Z     The timezone or name or abbreviation.

       %+     The date and time in date(1) format. (TZ) (Not supported in glibc2.)

       %%     A literal '%' character.