CITS2002 Systems Programming  
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Another example - accessing a system's password entries

On a stand-alone Linux system, one not dependent on a network-based server to provide its user information, some local user information is (historically) stored in the textfile /etc/passwd, with each user's information stored one-per-line with fields separated by colons.

Rather than expecting every user program to parse this information correctly, Linux systems host standard XOPEN header files and libraries to conveniently provide the information.

We can iterate through this information with the help of the getpwent() function, which returns a pointer to a 'struct passwd' structure: 

#include  <stdio.h>
#define __USE_XOPEN_EXTENDED
#include <sys/types.h>
#include <pwd.h>

#if 0
The header file <pwd.h> declares:

   struct passwd {
       char   *pw_name;       /* username */
       char   *pw_passwd;     /* user password */
       uid_t   pw_uid;        /* user ID */
       gid_t   pw_gid;        /* group ID */
       char   *pw_gecos;      /* user information */
       char   *pw_dir;        /* home directory */
       char   *pw_shell;      /* shell program */
   };

   struct passwd *getpwent(void);
#endif

int main(int argc, char *argv[])
{
    struct passwd *pwd;

    while((pwd = getpwent()) != NULL) {
        printf("%20s\t%-30s\t%s\n",
                pwd->pw_name, pwd->pw_dir, pwd->pw_shell);
    }
    return 0;
}

 

CITS2002 Systems Programming, Lecture 16, p2, 19th September 2023.