glibc-2.2.3, glibc-2.3.2: `getlogin' resets `usec' part of the itimer interval

[Ivan Shmakov]

        Please look at the code (suggested by Thierry EXCOFFIER):

$ cat >getlogin-test.c <<\EOF
#include <sys/time.h>

void display_itimer ()
{
   struct itimerval itv;
   getitimer (ITIMER_REAL, &itv);
   printf ("itv.it_interval.tv_usec = %d\n", itv.it_interval.tv_usec) ;
}

int main(int argc,char **argv)
{
   struct itimerval itv;

   itv.it_interval.tv_sec  = 0;
   itv.it_interval.tv_usec = 99999 ;
   itv.it_value = itv.it_interval ;
   setitimer (ITIMER_REAL, &itv, 0) ;

   display_itimer() ;
   getlogin() ;
   display_itimer() ;

   return 0 ;
}
EOF
$ gcc -o getlogin-test getlogin-test.c
$ ./getlogin-test
itv.it_interval.tv_usec = 100000
itv.it_interval.tv_usec = 0    <- there's expected to be the same value!
$ 

        Ie. after call to `getlogin' `usec' part of timer interval is
        reset to zero.  Further investigating of the problem shows
        that there's bug (feature?) in `LOCK_FILE' in
        `glibc-2.3.2/sysdeps/generic/utmp_file.c'.  Firstly, an old
        timer value is saved using:

    54    old_timeout = alarm (0);   \

        Then it is restored with:

    85    if (old_timeout != 0)   \
    86      alarm (old_timeout);   \

        So that `usec' part of the timer is completely lost.  The
        ``right way'' is to use `getitimer'/`setitimer' instead, but
        what should be done with `it_value' part?