Accounting changes

[Diomidis Spinellis]

Robert Watson wrote:
> 
> On Thu, 19 Apr 2007, Diomidis Spinellis wrote:
> 
>> I will work on developing a mechanism for backwards compatibility.
>>
>> Adding a version and length field in each record as you now propose is 
>> I think the way to go.  However, it is not trivial, because 
>> lastcomm(1) often reads the file backwards.  One approach would be to 
>> add a flag in ac_flags indicating a new version, and second copy of 
>> the length field before ac_flags.  Thus, the new structure would be 
>> something like:
> 
> Are there any alignment/padding considerations we should be taking into 
> account with the last two fields to make sure they are consistently 
> aligned with respect to the end of the record as new fields are added to 
> the middle?

Thanks for pointing this out.  True, we can't depend on ac_flag being in 
a fixed offset from the end of the record.  On i386 the situation is:

Old size=48 offsetof ac_flag=44 end-offset=4
New size=60 offsetof ac_flag=58 end-offset=2

and on amd64:

Old size=56 offsetof ac_flag=48 end-offset=8
New size=64 offsetof ac_flag=62 end-offset=2

If we follow the route of allowing the records to be read from both 
ends, we'll have to think of a construct that will portably maintain 
ac_flags at the same offset from the end as in the old struct acct. 
Proposals welcomed!

- dds

PS The proposed layout in my previous email didn't include the changes 
for the fields.  This is the correct layout I used for the above 
calculations:

struct nacct {
     u_int8_t      ac_zero;    /* zero identifies new version */
     u_int8_t      ac_version; /* record version number */
     u_int16_t ac_len;         /* record length */

     char      ac_comm[AC_COMM_LEN];    /* command name */
     float      ac_utime;      /* user time */
     float      ac_stime;      /* system time */
     float      ac_etime;      /* elapsed time */
     time_t     ac_btime;      /* starting time */
     uid_t      ac_uid;        /* user id */
     gid_t      ac_gid;        /* group id */
     float      ac_mem;        /* average memory usage */
     float      ac_io;         /* count of IO blocks */
     __dev_t   ac_tty;         /* controlling tty */

     u_int16_t ac_len2;        /* record length */
     u_int8_t  ac_flag;        /* accounting flags */
};