Time a user spent online using wtmp

Question

While using wtmp, i was able to get the tty of the user login, the time of login and more information. however, I want to be able to know how much time the user spent online. Is there a way to get the logout time ?

Thanks,

#define WTMP "/var/log/wtmp"
#define K               1024

int main()
{
 struct utmp w;
 FILE *fd;
 time_t t;
 struct tm *timeinfo;
 char buffer[K];

 fd = fopen(WTMP, "r");

 if(fd == NULL) {
    fprintf(stderr, "cannot open %s\n", WTMP);
    exit(0);
 }

 while(fread(&w, sizeof(struct utmp), 1, fd) == 1) {
    if(w.ut_type == USER_PROCESS) {
        if( strcmp(w.ut_user, "user1") == 0) {

        /* get time in seconds, convert to struct tm, make printable formate*/
        t = w.ut_tv.tv_sec;
        timeinfo = localtime (&t);

        strftime (buffer, K, "%a %b %e %R (EST)", timeinfo);
        printf("time: %s  Login tty: %s\n", buffer, w.ut_line);

        }
    }
 } 
 fclose(fd);
}

Answer 1

From the source code of the "last" command, which you can download from http://ftp.de.debian.org/debian/pool/main/s/sysvinit/sysvinit_2.88dsf.orig.tar.gz (once you have the download, unpack it and check sysvinit-2.88dsf/src/last.c):

    case USER_PROCESS:
        /*
         *  This was a login - show the first matching
         *  logout record and delete all records with
         *  the same ut_line.
         */
        c = 0;
        for (p = utmplist; p; p = next) {
            next = p->next;
            if (strncmp(p->ut.ut_line, ut.ut_line,
                UT_LINESIZE) == 0) {
                /* Show it */
                if (c == 0) {
                    quit = list(&ut, p->ut.ut_time,
                        R_NORMAL);
                    c = 1;
                }
                if (p->next) p->next->prev = p->prev;
                if (p->prev)
                    p->prev->next = p->next;
                else
                    utmplist = p->next;
                free(p);
            }
        }
        /*
         *  Not found? Then crashed, down, still
         *  logged in, or missing logout record.
         */
        if (c == 0) {
            if (lastboot == 0) {
                c = R_NOW;
                /* Is process still alive? */
                if (ut.ut_pid > 0 &&
                    kill(ut.ut_pid, 0) != 0 &&
                    errno == ESRCH)
                    c = R_PHANTOM;
            } else
                c = whydown;
            quit = list(&ut, lastboot, c);
        }