Utmp.h

1. Utmp.h • The utmp file records information about who is currently using the system • The file is a sequence of utmp entries, as defined in struct utmp in the utmp.h file • The utmp gives the name of the special file associated with the user’s terminal, user’s login name, and the time of the login time • /usr/include/utmp.h • /var/adm/utmp • $more /usr/include/utmp.h

2. #define UTMP_FILE “var/adm/utmp” • #define WTMP_FILE “var/adm/wtmp” • Wtmp file records all logins and logouts. • #define ut_name ut_user • Struct utmp{ • char ut_user[32]; // login name • char ut_id[14]; • char ut_line[32]; // device name • char ut_type; // type of entry • pid_t ut_pid; // process id • struct exit_status{ • short e_termination; // process termination status • short e_exit; // process exit status • } ut_exit; • time_t ut_time; // time entry was made • char ut_host[64]; // host name • };

3. Writing who • We have three system calls we need to connect to a file, extract chunks of data from a file, and close a file. • #include <stdio.h> • #include<utmp.h> • #include<fcntl.h> • #include<unistd.h> • #define SHOWHOST // include remote machine on output

4. int main() • { • struct utmp current_record; • int utmpfd; • int reclen = sizeof(current_record); • if( (utmpfd = open(UTMP_FILE,O_RDONLY)) == -1){ • perror(UTMP_FILE); • exit(1); • } • while( read(utmpfd, &current_record, reclen) == reclen) • show_info(&current_record); • close(utmpfd); • return 0; • }

5. Show_info() – displays contents of the utmp struct • show_info( struct utmp *utbufp) • { • printf(“%-8.8s”, utbufp->ut_name); • printf(“ “); • printf(“%-8.8s”, utbufp->ut_line); • printf(“ “); • printf(“%10ld”, utbufp->ut_time); • printf(“ “); • #ifdef SHOWHOST • printf(“(%s)”, utbufp->ut_host); • #endif • printf(“ “); • }

6. Blank record • Who command lists username of users logged into the system. The above program lists what it finds in the utmp file. • The utmp file seems to have records for all terminals, even unused ones • Modify the program to skip the entries with blank user names • In utmp.h, there are following definition for ut_type • #define EMPTY 0 • #define RUN_LVL 1 • #define BOOT_TIME 2 • #define OLD_TIME 3 • #define NEW_TIME 4 • #define INIT_PROCESS 5 • #define LOGIN_PROCESS 6 • #define USER_PROCESS 7 • #define DEAD_PROCESS 8

7. Display login time in Human-Readable form • Time_t data type – unix stores times as the number of seconds since midnight, Jan. 1,1970. • Time_t data type is an integer • Ut_time in utmp records represents the login time as the number of seconds since the beginning of the Epoch

8. Making a time_t readable: ctime • ctime() is function that converts a number of seconds since the start of Unix time into a sensible format. • char *ctime( const time_t *timep); • It converts the calendar time into a string in the form • “Wed Jun 30 21:49:08 1993” • We have time_t value in the utmp records. • We want a string looks like • Jun 30 21:49 • Wed Jun 30 21:49:08 1993 • Pick 12 characters from position 4 • Printf(“%12.12s”, cp+4);

9. Rewrite show_info • void show_info(struct utmp *utbufp) • { • char * cp; • if( utbufo->ut_type != USER_PROCESS) • return; • printf(“%-8.8s”, utbufp->tu_name); • printf(“ “); • printf(“%-8.8s”, utbufp->ut_line); • cp = ctime(&utbuf->ut_time); • printf(“12.12s”, cp+4); • #ifdef SHOWHOST • if( utbufp->ut_host[0] != ‘\0’) • printf(“ (%s)”, utbufp->ut_host); • #endif • printf(“\n”); • }