[CentOS] Determine next UID number

John Kennedy skebi69 at gmail.com
Thu Oct 14 08:43:07 EDT 2010


On Wed, Oct 13, 2010 at 17:16, Spiro Harvey <spiro at knossos.net.nz> wrote:

> John Kennedy <skebi69 at gmail.com> wrote:
> > This also does not tell me how useradd knows that on this system at
> > this time the highest UID assigned to a user is 20015.
>
> From the source's mouth (this is from useradd.c in the shadow-utils
> package):
>
> /*
>  * find_new_uid - find the next available UID
>  *
>  *      find_new_uid() locates the next highest unused UID in the password
>  *      file, or checks the given user ID against the existing ones for
>  *      uniqueness.
>  */
> static void find_new_uid (void)
> {
>        const struct passwd *pwd;
>        uid_t uid_min, uid_max;
>
>        uid_min = getdef_unum ("UID_MIN", 1000);
>        uid_max = getdef_unum ("UID_MAX", 60000);
>
>        /*
>         * Start with some UID value if the user didn't provide us with
>         * one already.
>         */
>        if (!uflg)
>                user_id = uid_min;
>
>        /*
>         * Search the entire password file, either looking for this
>         * UID (if the user specified one with -u) or looking for the
>         * largest unused value.
>         */
> #ifdef NO_GETPWENT
>        pw_rewind ();
>        while ((pwd = pw_next ())) {
> #else                           /* using getpwent() we can check against
> NIS users etc. */
>        setpwent ();
>        while ((pwd = getpwent ())) {
> #endif
>                if (strcmp (user_name, pwd->pw_name) == 0) {
>                        fprintf (stderr, _("%s: name %s is not unique\n"),
>                                 Prog, user_name);
> #ifdef WITH_AUDIT
>                        audit_logger (AUDIT_USER_CHAUTHTOK, Prog, "adding
> user",
>                                      user_name, user_id, 0);
> #endif
>                        exit (E_NAME_IN_USE);
>                }
>                if (uflg && user_id == pwd->pw_uid) {
>                        fprintf (stderr, _("%s: UID %u is not unique\n"),
>                                 Prog, (unsigned int) user_id);
> #ifdef WITH_AUDIT
>                        audit_logger (AUDIT_USER_CHAUTHTOK, Prog, "adding
> user",
>                                      user_name, user_id, 0);
> #endif
>                        exit (E_UID_IN_USE);
>                }
>                if (!uflg && pwd->pw_uid >= user_id) {
>                        if (pwd->pw_uid > uid_max)
>                                continue;
>                        user_id = pwd->pw_uid + 1;
>                }
>        }
>
>        /*
>         * If a user with UID equal to UID_MAX exists, the above algorithm
>         * will give us UID_MAX+1 even if not unique. Search for the first
>         * free UID starting with UID_MIN (it's O(n*n) but can be avoided
>         * by not having users with UID equal to UID_MAX).  --marekm
>         */
>        if (!uflg && user_id == uid_max + 1) {
>                for (user_id = uid_min; user_id < uid_max; user_id++) {
> #ifdef NO_GETPWENT
>                        pw_rewind ();
>                        while ((pwd = pw_next ())
>                               && pwd->pw_uid != user_id);
>                        if (!pwd)
>                                break;
> #else
>                        if (!getpwuid (user_id))
>                                break;
> #endif
>                }
>                if (user_id == uid_max) {
>                        fprintf (stderr, _("%s: can't get unique UID\n"),
> Prog);
>                        fail_exit (E_UID_IN_USE);
>                }
>        }
> }
>
>
>
> --
> Spiro Harvey                  Knossos Networks Ltd
> 021-295-1923                  www.knossos.net.nz
>
> _______________________________________________
> CentOS mailing list
> CentOS at centos.org
> http://lists.centos.org/mailman/listinfo/centos
>
>
This looks like what I am talking about. Interesting to see that the program
literally does what the code above does (as much as I can tell. I am not a
coder)

Thanks Spiro.
John

-- 
 John Kennedy
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