Unix Prog: User Identification

1. getlogin

It is possible that single user id in passwd file can map to different entries with different login name. getlogin system call can return the current login name.

Definition:

 ubuntu@ip-172-31-23-227:~$ less /usr/include/unistd.h  
 ......  
 /* Return the login name of the user.  
   
   This function is a possible cancellation point and therefore not  
   marked with __THROW. */  
 extern char *getlogin (void);  
 ......  

2. Example:

record.c:

 #include<stdio.h>  
 #include<stdlib.h>  
 #include<unistd.h>  
 #include<pwd.h>  
   
 int main(int argc, char* argv[])  
 {  
  char *login;  
   
  // Retrieve and output the login name  
  if((login = getlogin()) == NULL) {  
   printf("getlogin error!\n");  
   exit(1);  
  }  
   
  printf("%s\n", login);  
   
  // Retrieve the passwd record based on login name  
  struct passwd *pw;  
  if((pw = getpwnam(login)) == NULL) {  
   printf("getpwnam error!\n");  
   exit(2);  
  }  
   
  printf("Username: %s, Password: %s, User ID: %d, Group ID: %d, \n",  
      pw->pw_name, pw->pw_passwd, pw->pw_uid, pw->pw_gid);  
  printf("realname: %s, home dir: %s, shell: %s\n", pw->pw_gecos, pw->pw_dir, pw->pw_shell);  
   
  exit(0);  
 }  

shell:
After getting the current login name with "getlogin()", then we use getpwnam to retrieve the password record from passwd file based on retrieved login name.

 ubuntu@ip-172-31-23-227:~$ ./record.out  
 ubuntu  
 Username: ubuntu, Password: x, User ID: 1000, Group ID: 1000,  
 realname: Ubuntu, home dir: /home/ubuntu, shell: /bin/bash