6、系统数据文件和信息

对于每个数据文件至少包含3个函数：get（读下一个记录），set，end
1、根据口令文件（/etc/passwd）得到passwd结构(/usr/include/pwd.h)

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.  */  char *pw_dir;                 /* Home directory.  */  char *pw_shell;               /* Shell program.  */};

//根据uid或用户名得到结构struct passwd *getpwuid(uid_t uid);struct passwd *getpwnam(const char *name);//下面是三个函数被上面函数调用//返回口令文件头部void setpwent(void);//返回口令文件的下一个记录项struct passwd *getpwent(void);//关闭文件void endpwent(void);

struct passwd *getpwnam(const char *name){    struct passwd *ptr;    setpwent();    while((ptr = getpwent()) != NULL)    {        if(strcmp(name, ptr->pw_name) == 0)            break;    }    endpwent();    return(ptr);}

2、阴影文件-加密文件的另一个存放文件，阴影文件对应的结构
/etc/shadow 和 /usr/include/shadow.h

struct spwd  {    char *sp_namp;              /* Login name.  */    char *sp_pwdp;              /* Encrypted password.  */    long int sp_lstchg;         /* Date of last change.  */    long int sp_min;            /* Minimum number of days between changes.  */    long int sp_max;            /* Maximum number of days between changes.  */    long int sp_warn;           /* Number of days to warn user to change                                   the password.  */    long int sp_inact;          /* Number of days the account may be                                   inactive.  */    long int sp_expire;         /* Number of days since 1970-01-01 until                                   account expires.  */    unsigned long int sp_flag;  /* Reserved.  */  };

struct spwd *getspanam(const char *name);struct spwd *getspent(void);void setspent(void);void endspend(void);

3、组文件和结构，类似上面
/etc/group 和 /usr/include/grp.h

struct group  {    char *gr_name;              /* Group name.  */    char *gr_passwd;            /* Password.    */    __gid_t gr_gid;             /* Group ID.    */    char **gr_mem;              /* Member list. */  };

struct group *getgrgid(gid_t gid);struct group *getgrnam(const char *name);struct group *getgrent(void);void setgrent(void);void endgrent(void);

4、附属组，一个用户可能属于多个组，当前组和一些附属组

//把进程所属用户的各附属ID写到参数grouplist[]中int getgroups(int gidsetsize, git_t grouplist[]);//为进程所属用户设置附属IDint setgroups(int ngroups, const git_t grouplist[]);

5、登录信息记录，utmp文件记录当前登录到系统的各个用户（var/run/utmp）；wtmp文件跟踪各个登录和注销事件(var/log/wtmp)。有结构与文件对应

6、系统标识

int uname(struct utsname *name);

7、时间和日期
协调世界时：公元1970年1月1日00：00：00这一时间以来经过的秒数。

//返回当前时间和日期（从实时系统时钟获取），也存到calptr指向的地址time_t time(time_t *calptr);//获取特定时钟时间，clock_id可以是CLOCK_REALTIME      实时系统时间CLOCK_MONOTONIC、    不带负跳数的实时系统时间CLOCK_PROCESS_CPUTIME_ID、调用进程的CPU时间CLOCK_THREAD_CPUTIME_UP 调用线程的CPU时间//struct timespec 时间转化为秒和纳秒存放，该函数比上个函数得到时间精度高int clock_gettime(clockid_t clock_id, struct timespec *tsp);//根据不同时钟，转换时间为不同精度int clock_getres(clockid_t clock_id, struct timespec *tsp);//设置特定时钟的时间int clock_settime(clockid_t clock_id, const struct timespec *tsp);

struct timespec  {    __time_t tv_sec;            /* Seconds.  */    long int tv_nsec;           /* Nanoseconds.  */  };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};

//时间转换，分解时间-日历时间struct tm *gmtime(const time_t *calptr);struct tm *localtime(const time_t *calptr);time_t mktime(struct tm *tmptr);和gmtime相反

时间格式定制

//tmptr待处理对象，format格式控制，buf输出到buf，maxsize buf长度size_t strftime(char *restrict buf, size_t maxsize, const char *restrict format, const struct tm *restrict tmptr);//和上面相反char *strptime(char *restrict buf,  const char *restrict format, const struct tm *restrict tmptr);

#include <stdio.h>#include <stdlib.h>#include <time.h>int main(){        time_t t;        struct tm *tmp;        char buf[64];        time(&t);        tmp = localtime(&t);        strftime(buf, 64, "time and date: %r, %d %b %d, %Y ", tmp);        printf("%s\n",buf);        exit(0);}~