Philip Guenther wrote:
On Thu, 24 Sep 2009, Howard Chu wrote:
It seems the dichotomy between libldap and libldap_r is a relic from the
bad old days of dcethreads / cmathreads when linking a threaded library
into an otherwise non-threaded program would cause all sorts of strange
and wonderful failures.
Hmm, my impression from reading the source was that it was more about use
models, where libldap is appropriate when any given LDAP handle would not
be accessed concurrently by multiple threads and libldap_r is appropriate
when a given LDAP handle may be serviced/used concurrently by multiple
threads.
libldap_r was added out of necessity for the latter case, yes. Prior to that
there was basically no consideration for threading at all.
Indeed, my impression from reading the ldap_r bits was that processing of
replies wasn't ideal when threads were asking for specific replies, as
asking for a given reply blocked *all* other ldap_result() calls on that
handle until that specific reply was received.
(To hopefully make clear:
thread1 thread2
ldap_result(h, msg_A)
ldap_result(h, msg_B)
receive msg_B
<still blocked...>
receive msg_A
return from ldap_result
return from ldap_result
)
Not entirely true. Threads only block in ldap_int_select(); it's undefined
which thread will wake up first when select has data ready. The above scenario
may occur sometimes, and does not occur other times. We may consider it a bug
that the behavior is non-deterministic, but that's a separate issue.
libldap_r is still missing some thread-specific features though - we should
wrap all library initialization in a pthread_once() call, and we should be
using thread-specific data for the LDAP* errno value.
Without knowing what usage models this is supposed to address or provide,
I don't see the benefit.
The benefit is decreasing the number of libraries that need to be maintained,
and eliminating the conflict when code linked against one library is
dynamically loaded into a process that was already linked with the other.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
