On Sun, 2007-08-05 at 15:31 -0300, Victor wrote:
> Is it safe to call event_add() from a second thread to
> add more events on that event loop base?
No. Two threads cannot modify the same event_base struct without
explicit locking. You would need to use a mutex to protect the
event_add() call.
On a related note...
I would like to see a threaded version of libevent that would give each
thread it's own current_base variable. Right now, there is a global
variable in event.c:
struct event_base *current_base = NULL;
A threaded library would use thread-specific data instead:
static pthread_key_t *current_base_key;
#define current_base \
((struct event_base *) (pthread_get_specific(current_base_key))
This would allow you to use event_add(), event_dispatch(), and
event_loop() in multiple threads in a natural way. Each thread would get
it's own event loop automatically and not interfere with other threads'
operation.
You could also add internal protection within the event_base_()
functions with mutexes so that two threads would not be able to modify
the same event_base struct simultaneously.
My suggestion is to name this threaded version of the library
`libevent_r.a` and have a separate Makefile target to generate the
threaded version. This second target would add -D_REENTRANT to the
CFLAGS to enable all of the threaded codepaths. You would also need to
link it with the pthreads library by adding -lpthread to the LDFLAGS.
Returning to my previous example, you would modify event.c to look
something like this:
#if _REENTRANT
static pthread_key_t *current_base_key;
# define current_base \
((struct event_base *) (pthread_get_specific(current_base_key))
#else
struct event_base *current_base = NULL;
#endif
This approach allows both the threadsafe and non-threadsafe library to
be built from the same source code.
Niels, does this sound like something you would be willing to consider?
If so, I would be willing to do the work and submit a patch.
Regards,
Mark
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