singsing ~ # cat test.c
#include <stdio.h>
#include <stdlib.h>
#include <pwd.h>

main()
{
  struct passwd *pd;

  if (NULL == (pd = getpwnam("root")))
     perror("getpwnam() error.");
  else
  {
     printf("The user name is: %s\n", pd->pw_name);
     printf("The user id   is: %u\n", pd->pw_uid);
     printf("The group id  is: %u\n", pd->pw_gid);
     printf("The initial directory is:    %s\n", pd->pw_dir);
     printf("The initial user program is: %s\n", pd->pw_shell);
  }

}
singsing ~ # cc test.c -o test
singsing ~ # ./test
The user name is: root
The user id   is: 0
The group id  is: 0
The initial directory is:    /root
The initial user program is: /bin/bash
singsing ~ #