Hi,
I apologize if this is not an appropriate forum to be posting this.
I am looking into guacamole's use of pthread_mutexattr_setpshare.
I am trying to port/compile guacamole 1.1.0 on OpenBSD. The only
(significant) errors I am getting pertain to the use of
pthread_mutexattr_setpshared(foo, PTHREAD_PROCESS_SHARED).
If I comment out the 4 lines of code invoking
pthread_mutexattr_setpshare in the source tree, I can it to compile with
gcc on OpenBSD 6.6 (GENERIC.MP) )
I am an experienced C programmer, but I've never looked in threading in
C before so it has required quite a bit of reading. I haven't been able
to find a comprehensive "architecture" document for pthreads, just
various man pages, some with sample code.
For example:
https://linux.die.net/man/3/pthread_mutexattr_init
In the above documentation, it states:
...the possibility that an application may allocate the
synchronization objects from this section in memory that is accessed
by multiple processes (and therefore, by threads of multiple
processes).
It appears that the purpose of the guacamole's
pthread_mutexattr_setpshared(foo, PTHREAD_PROCESS_SHARED) is to permit
multiple threaded processes to access the resource protected by the MUTEX.
However, from my reading of the guacamole code, the MUTEX is only
protecting malloc'd memory, which as far as I know, isn't a resource
that can be accessed by multiple processes.
It is my newby (to pthreads) interpretation of the code that in all 4
cases, the code is malloc'ing memory that is being protected by the
MUTEX. For example:
src/libguac/pool.c:
...
guac_pool* guac_pool_alloc(int size) {
pthread_mutexattr_t lock_attributes;
guac_pool* pool = malloc(sizeof(guac_pool));
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Allocate memory that will only be accessible by this process
/* If unable to allocate, just return NULL. */
if (pool == NULL)
return NULL;
/* Initialize empty pool */
pool->min_size = size;
pool->active = 0;
pool->__next_value = 0;
pool->__head = NULL;
pool->__tail = NULL;
/* Init lock */
pthread_mutexattr_init(&lock_attributes);
pthread_mutexattr_setpshared(&lock_attributes,
PTHREAD_PROCESS_SHARED);
pthread_mutex_init(&(pool->__lock), &lock_attributes);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
This codes saves the MUTEX in the locally allocated memory
No other process can find this MUTEX in malloc'd memory, so setting it
to PTHREAD_PROCESS_SHARED seems totally irrelevant.
If so, I *think* it's OK to just comment out the
pthread_mutexattr_setpshared code as OpenBSD's pthread MUTEX locking
implementation will work fine within a process, just not multiple
processes.
Is this a correct assessment of the code?
Thanks,
Steve W.
