Making sense of TSD

[Jason Evans]

On Apr 11, 2012, at 12:08 PM, Mike Hommey wrote:

> On Wed, Apr 11, 2012 at 12:05:05PM -0700, Jason Evans wrote:
>> On Apr 11, 2012, at 10:22 AM, Mike Hommey wrote:
>>> I'm trying to make sense of the tsd implementation without __thread,
>>> which would seem to fit the tls model on windows. But I don't see
>>> why the wrapper struct is needed at all. The first thing that
>>> bothers me is that the isstatic == true case is set to abort on
>>> opt_abort, yet is supported. And it doesn't seem very obvious why
>>> initialized is needed.
>> 
>> The wrapper struct is needed with pthreads TSD because
>> pthread_[gs]etspecific() only operate on (void *) pointers.  Thus it's
>> impossible to store more than a pointer-sized item with pthreads TSD
>> unless that pointer refers to an allocated structure.  I don't
>> remember the details of Windows's TLS/TSD API; if it can store any
>> type/structure, then no wrapper struct is needed.
> 
> Well, the void* pointer could just point to whatever structure you want
> to store, instead of having a struct containing two bools which
> usefulness I'm doubtful about, and a pointer to that allocated structure.
> At least it seems so. That was my question.

The utility of isstatic could be argued, but the initialized flag is critical to how cleanup handlers work.  Yes, these two flags could be separate TSD keys (with all the requisite bool<-->(void *) casting), but the actual data being stored has to be allocated anyway, so this is IMO a cleaner solution.  If Windows supports arbitrary types in a similar fashion to __thread, then the wrapper structure certainly isn't needed, since the flags can be separate keys.

Jason