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

由于历史原因，口令文件是 /etc/passwd，而且是一个 ASCII 文件。

# cat /etc/passwd
root:x:0:0:root:/root:/bin/bash
tarena:x:1000:1000:tarena,,,:/home/tarena:/bin/bash
nobody:x:65534:65534:nobody:/nonexistent:/bin/sh

各字段含义就不讲了，关于这些登录项，请注意下列各点：

# finger -p tarena
Login: tarena                   Name: tarena
Directory: /home/tarena                 Shell: /bin/bash
Last login Tue Aug  2 11:40 2016 (CST) on pts/3 from ubuntu.local
No mail.

POSIX.1 定义了两个获取口令文件项的函数，在给出用户登录名或者数值用户ID后，这两个函数就能查询相关项。

#include <pwd.h>
struct passwd *getpwuid(uid_t uid);
struct passwd *getpwnam(const char *name);  

（1）函数解析

这两个函数都返回一个指向 passwd 结构的指针，该结构已由这两个函数在执行时填入信息。passwd 结构通常是函数内部的静态变量，只要调用任一相关函数，其内容就会被重写。

（2）示例说明

#include <stdio.h>
#include <stdlib.h>
#include <pwd.h>int main (void)
{struct passwd *pwd;if ((pwd = getpwuid (1000)) == NULL)perror ("fail to getpwuid"), exit (1);printf("%s,%s,%ld,%ld,%s,%s,%s\n",pwd->pw_name, pwd->pw_passwd,pwd->pw_uid, pwd->pw_gid, pwd->pw_gecos, pwd->pw_dir, pwd->pw_shell);  if ((pwd = getpwnam ("tarena")) == NULL)perror ("fail to getpwnam"), exit (1);printf("%s,%s,%ld,%ld,%s,%s,%s\n",pwd->pw_name, pwd->pw_passwd,pwd->pw_uid, pwd->pw_gid, pwd->pw_gecos, pwd->pw_dir, pwd->pw_shell);  return 0;
}
输出结果：
tarena,x,1000,1000,tarena,,,,/home/tarena,/bin/bash
tarena,x,1000,1000,tarena,,,,/home/tarena,/bin/bash

#include <pwd.h>
struct passwd* getpwent(void);
void setpwent(void);
void endpwent(void); 

（1）函数解析

用于查看 登录名和用户 ID。

#include <pwd.h>
#include <sys/types.h>
#include <stdio.h>int main (void)
{struct passwd *user;setpwent ();while ((user = getpwent ()) != NULL){if (strcmp ("tarena", user->pw_name) == 0)break;}printf ("%s:%d:%d:%s:%s:%s\n",user->pw_name,user->pw_uid,user->pw_gid,user->pw_gecos,user->pw_dir,user->pw_shell);endpwent ();return 0;
}
输出结果：
tarena:1000:1000:tarena,,,:/home/tarena:/bin/bash

（3）示例总结

在函数开始处调用 setpwent 是自我保护性的措施，以便确保如果调用者在此之前已经调用 getpwent 打开了有关文件情况下，反绕有关文件使他们定位到文件开始处。getpwnam 和 getpwuid 完成后不应使有关文件仍处于打开状态。所以应调用 endpwent 关闭它们。

#include<pwd.h>
#include<sys/types.h>
#include <stdio.h>
int main (void)
{struct passwd *user;int i;for(i = 0; i < 4; i++) {user = getpwent ();printf ("%s :%d :%d :%s:%s:%s\n", user->pw_name, user->pw_uid, user->pw_gid,user->pw_gecos, user->pw_dir, user->pw_shell);}
//  setpwent();user = getpwent ();printf ("=============================\n");printf ("%s :%d :%d :%s:%s:%s\n", user->pw_name, user->pw_uid, user->pw_gid,user->pw_gecos, user->pw_dir, user->pw_shell);endpwent();return 0;
}
输出结果：
root :0 :0 :root:/root:/bin/bash
daemon :1 :1 :daemon:/usr/sbin:/bin/sh
bin :2 :2 :bin:/bin:/bin/sh
sys :3 :3 :sys:/dev:/bin/sh
=============================
sync :4 :65534 :sync:/bin:/bin/sync

二、阴影口令

加密口令是经单向加密算法处理过的用户口令副本。因为此算法是单向的，所以不能从加密口令猜测到原来的口令。

/* Structure of the password file.  */
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 changethe password.  */long int sp_inact;     /* Number of days the account may beinactive.  */long int sp_expire;        /* Number of days since 1970-01-01 untilaccount expires.  */unsigned long int sp_flag;  /* Reserved.  */};

阴影口令文件是 /etc/shadow

# cat /etc/shadow
root:$6$MNQKabSO$UcLm09JPH7JdppbRZBrj4XoWUQIWRhqhwdzJ9F2mGlwSXo5V.ylP4.gaReQSw3sZSSuczM1iqnMKBrwnbDlgz/:17131:0:99999:7:::
tarena:$6$kea9L4dJ$PeK8uyZ8MesNW6zG1fMMnJIol4icj0nLKAg7Vq78sLJJhEs1Sr6M/VpxBvy.kGeMVDr2SC/EN8Utx5OSIx8Fs/:16630:0:99999:7:::
nobody:*:15453:0:99999:7:::

1、阴影口令函数

#include <shadow.h>
struct spwd *getspnam(const char *name);
struct spwd *getspent(void);
void setspent(void);
void endspent(void);
返回值：成功返回指针，失败返回 NULL

（1）函数功能

#include <stdio.h>
#include <shadow.h>
#include <stdlib.h>int main (void)
{  struct spwd* spwd;  if ((spwd = getspnam("tarena")) == NULL)  perror("getspnam"), exit (1); printf("%s,%s,%ld,%ld,%ld,%ld,%ld,%ld,%lu\n",spwd->sp_namp,spwd->sp_pwdp,spwd->sp_lstchg,spwd->sp_min,spwd->sp_max,spwd->sp_warn,spwd->sp_inact,spwd->sp_expire,spwd->sp_flag);  setspent();  while ((spwd = getspent()) != NULL){if (strcmp ("tarena", spwd->sp_namp) == 0)break;}printf("%s,%s,%ld,%ld,%ld,%ld,%ld,%ld,%lu\n",spwd->sp_namp,spwd->sp_pwdp,spwd->sp_lstchg,spwd->sp_min,spwd->sp_max,spwd->sp_warn,spwd->sp_inact,spwd->sp_expire,spwd->sp_flag);  endspent();  return 0;
}
输出结果：
tarena,$6$kea9L4dJ$PeK8uyZ8MesNW6zG1fMMnJIol4icj0nLKAg7Vq78sLJJhEs1Sr6M/VpxBvy.kGeMVDr2SC/EN8Utx5OSIx8Fs/,16630,0,99999,7,-1,-1,4294967295
tarena,$6$kea9L4dJ$PeK8uyZ8MesNW6zG1fMMnJIol4icj0nLKAg7Vq78sLJJhEs1Sr6M/VpxBvy.kGeMVDr2SC/EN8Utx5OSIx8Fs/,16630,0,99999,7,-1,-1,4294967295

2、思考

$sudo passwd root
Enter new UNIX password: <--- 新的Root用户
Password: <--- 输入你当前用户davinci的密码
密码
Retype new UNIX password: <--- 重复新的Root用户密码
passwd：已成功更新密码

（1）可以写个程序验证密码是不是 root：

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>  int main(void)
{  char *passwd;  char key[] = "root";  passwd = crypt(key, "$6$MNQKabSO$");  printf("root: %s\n", passwd);  return 0;
}
编译：gcc test -lcrypt
输出结果：
root: $6$MNQKabSO$UcLm09JPH7JdppbRZBrj4XoWUQIWRhqhwdzJ9F2mGlwSXo5V.ylP4.gaReQSw3sZSSuczM1iqnMKBrwnbDlgz/

（2）示例总结

#define _XOPEN_SOURCE       /* See feature_test_macros(7) */
#include <unistd.h>
char *crypt(const char *key, const char *salt);#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <crypt.h>
char *crypt_r(const char *key, const char *salt, struct crypt_data *data);Link with -lcrypt.
返回值：返回一个以 NULL 结尾的密码字符串

注意，编译时需要链接 -lcrypt，否则会出现 undefined reference to `crypt' 错误。

#include <unistd.h>
char *getpass( const char *prompt);

函数功能：

//示例一
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <crypt.h>
int main()
{  char passwd[13];  char *key;  char slat[2];  key = getpass("Input first passward:");  slat[0] = key[0];  slat[1] = key[1];  strcpy(passwd,crypt(key,slat));  key = getpass("Input second passward:");  slat[0] = passwd[0];  slat[1] = passwd[1];  printf("After crypt(),1st passwd:%s\n",passwd);  printf("After crypt(),2st passwd:%s\n",passwd);  return 0;  }
编译：
# gcc test.c -lcrypt输出结果：
Input first passward://root
Input second passward://root
After crypt(),1st passwd:roK20XGbWEsSM
After crypt(),2st passwd:roK20XGbWEsSM

三、组文件

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

# cat /etc/group
root:x:0:
tarena:x:1000:

#include <grp.h>
struct group *getgrgid(gid_t gid);
struct group *getgrnam(const char *name);
//两个函数返回值：如果成功返回指针，出错则返回NULL。  struct group *getgrent(void);  //如果成功返回指针，出错或者到达文件结尾则返回NULL
void setgrent(void);
void endgrent(void);  

#include <sys/types.h>
#include <unistd.h>
int getgroups(int size, gid_t list[]);#include <grp.h>
int setgroups(size_t size, const gid_t *list);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
setgroups(): _BSD_SOURCE#include <sys/types.h>
#include <grp.h>
int initgroups(const char *user, gid_t group);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
initgroups(): _BSD_SOURCE

六、系统标识

#include <sys/utsname.h>
int uname(struct utsname * name);
返回值：成功返回非负值，失败返回 -1

（1）参数解析

/* 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_GNUchar domainname[_UTSNAME_DOMAIN_LENGTH];
# elsechar __domainname[_UTSNAME_DOMAIN_LENGTH];
# endif
#endif};

（2）示例说明

#include <stdio.h>
#include <sys/utsname.h>
#include <stdlib.h>int main (void)
{struct utsname uts;if (uname (&uts) == -1)perror ("fail to uname"), exit (1);elseprintf ("%s %s %s %s %s\n", uts.sysname, uts.nodename, uts.release, uts.version, uts.machine);return 0;
}
输出结果：
Linux ubuntu 3.2.0-23-generic-pae #36-Ubuntu SMP Tue Apr 10 22:19:09 UTC 2012 i686

（3）Linux 下的 uname 命令

-a或--all：显示全部的信息；
-m或--machine：显示电脑类型；
-n或-nodename：显示在网络上的主机名称；
-r或--release：显示操作系统的发行编号；
-s或--sysname：显示操作系统名称；
-v：显示操作系统的版本；
-p或--processor：输出处理器类型或"unknown"；
-i或--hardware-platform：输出硬件平台或"unknown"；
-o或--operating-system：输出操作系统名称；
--help：显示帮助；
--version：显示版本信息。

示例：

# uname -a
Linux ubuntu 3.2.0-23-generic-pae #36-Ubuntu SMP Tue Apr 10 22:19:09 UTC 2012 i686 i686 i386 GNU/Linux

2、gethostname 函数

#include <unistd.h>
int gethostname(char * name, int namelen);
返回：若成功则为0，若出错则为-1

（1）函数解析

HOST_NAME_MAX - _SC_HOST_NAME_MAX
Max length of a hostname, not including the terminating null byte, as returned by gethostname(2). Must not be less than _POSIX_HOST_NAME_MAX (255).

# grep "_UTSNAME_LENGTH" * -rn
i386-linux-gnu/sys/utsname.h:33:# define _UTSNAME_SYSNAME_LENGTH _UTSNAME_LENGTH
i386-linux-gnu/sys/utsname.h:36:# define _UTSNAME_NODENAME_LENGTH _UTSNAME_LENGTH
i386-linux-gnu/sys/utsname.h:39:# define _UTSNAME_RELEASE_LENGTH _UTSNAME_LENGTH
i386-linux-gnu/sys/utsname.h:42:# define _UTSNAME_VERSION_LENGTH _UTSNAME_LENGTH
i386-linux-gnu/sys/utsname.h:45:# define _UTSNAME_MACHINE_LENGTH _UTSNAME_LENGTH
i386-linux-gnu/sys/utsname.h:77:# define SYS_NMLN  _UTSNAME_LENGTH
i386-linux-gnu/bits/utsname.h:24:#define _UTSNAME_LENGTH 65
i386-linux-gnu/bits/utsname.h:29:#define _UTSNAME_DOMAIN_LENGTH _UTSNAME_LENGTH

（2）示例说明

#include <stdio.h>
#include <unistd.h>int main (void)
{char name[65];gethostname(name, sizeof(name));printf("hostname = %s\n", name);return 0;
}
输出结果：
hostname = ubuntu

-v：详细信息模式；
-a：显示主机别名；
-d：显示DNS域名；
-f：显示FQDN名称；
-i：显示主机的ip地址；
-s：显示短主机名称，在第一个点处截断；
-y：显示NIS域名。

# hostname
ubuntu