On Wed, 2005-09-14 at 09:36 -0400, rh wrote:
> struct c
> {
> struct a ;
> struct b ;
> } ;
>
> Given a pointer to 'a' in 'struct c', as we all know you can
> access / modify 'b' in 'struct c'.
With "in-line" structures as shown - yes, that's possible.
(incredibly ugly, but possible!).
But if struct c was tweaked to be:
struct c
{
struct a *;
struct b *;
}
and all you've got is the *value* of the "struct a" pointer,
then you can't get from there to the "struct b" pointer.
That's the situation with the Net-SNMP suite:
> // snmp_api.c
> struct session_list {
> struct session_list *next;
> netsnmp_session *session; <=======
> netsnmp_transport *transport;
> struct snmp_internal_session *internal;
> };
> and that snmp_open() apparently returns the necessary session
> pointer,
The (internal) "session" pointer, yes - *NOT* the (enclosing)
"session_list" pointer.
Two options:
a) use 'snmp_sess_open()' instead of 'snmp_open()'
(which actually returns an opaque pointer to the
"struct session_list" structure)
b) use 'snmp_sess_pointer()'
to get from the "netsnmp_session" pointer
back to the enclosing "struct session_list"
(again as an opaque pointer)
> > There is already a defined API (snmp_sess_transport) for
> > retrieving the transport structure associated with a particular
> > session, and the socket is one of the top-level fields of this
> > transport structure. It's probably not unreasonable to access
> > this field (and perhaps the transport flags field) directly.
>
> That's exactly what I need !
Note that it's this 'snmp_sess_transport' API works with
the opaque (session_list) pointer - *NOT* the "netsnmp_session"
pointer returned by 'snmp_open()'.
So you probably want something like:
netsnmp_session *sess = snmp_open(...);
netsnmp_transport *t = snmp_sess_transport(
snmp_sess_pointer( sess ));
int sock = t->socket;
Dave
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