Tom Phoenix wrote:
> On 3/13/07, Dharshana Eswaran <[EMAIL PROTECTED]> wrote:
>
>> I was going thro the topic "Accessing packed data structures" in the Perl
>> Complete Reference Book. I came across this example:
>>
>> struct utmp {
>> char ut_user[8]; /* User login name */
>> char ut_id[4]; /* /etc/inittab id */
>> char ut_line[12]; /* device name */
>> short ut_pid; /* process ID */
>> short ut_type; /* type of entry */
>> struct exit_status ut_exit; /* The exit status of a process */
>> /* marked as DEAD_PROCESS. */
>> time_t ut_time; /* time entry was made */
>> };
>>
>> For the above structure:
>>
>> The pack template given was "a8a4a12ssssl".
>>
>> I am, somehow, not able to understand how to generate the pack
>> template from
>> the structure's data types.
>
> The pieces match up this way:
>
> char ut_user[8]; /* User login name: a8 */
> char ut_id[4]; /* /etc/inittab id: a4 */
> char ut_line[12]; /* device name: a12 */
> short ut_pid; /* process ID: s */
> short ut_type; /* type of entry: s */
> struct exit_status ut_exit; /* The exit status of a process */
> /* marked as DEAD_PROCESS: s s */
> time_t ut_time; /* time entry was made: l */
>
> A char array becomes an a42, with the number being the length of the
> array. A short becomes s, and a long becomes l. (You do need to know
> something about how C stores data in memory. Note that exit_status and
> ut_exit are two variables, even though they are declared on the same
> line; that's why there are two s's in the comment.)
No, exit_status is an identifier and ut_exit is the variable. In C it is
declared something like:
struct exit_status {
short one;
short two;
};
And is used something like:
struct exit_status variable_1, variable_2;
See the C Reference Manual at:
http://cm.bell-labs.com/cm/cs/who/dmr/cman.pdf
John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall
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