[libdispatch-dev] Block not released after dispatch_source_set_event_handler

[Daniel A. Steffen]

Hi Allan,

dispatch sources are created in a suspended state and must be resumed after configuration, see the dispatch_source_create(3) manpage and the SUSPENSION section of dispatch_object(3) for details.

All setters of source configuration are asynchronous in nature and only take effect when the source is not suspended.
Once dispatch_resume(source) is called, the handler block retained by dispatch_source_set_event_handler() will be released when the subsequent dispatch_source_set_event_handler_f() takes effect.

Daniel 

On Feb 23, 2013, at 8:04, Allan Odgaard <lists+libdispatch at simplit.com> wrote:

> I am seeing an issue where the block given to dispatch_source_set_event_handler appear not to be released when a new handler is set.
> 
> If I build and run the code below:
> 
>   xcrun clang++ -std=c++11 -stdlib=libc++ test.cc
> 
> It outputs the ‘create’ line, but never any ‘destroy’.
> 
> If I don’t call dispatch_source_set_event_handler then I see the expected ‘destroy’. Alternatively I can call an extra Block_release(), which also results in the shared pointer being destroyed.
> 
> This is on Mac OS X 10.8.2 Build 12C60 using Apple LLVM version 4.2 (clang-425.0.24) (based on LLVM 3.2svn).
> 
> ----------8<----------
> 
> #include <dispatch/dispatch.h>
> #include <Block.h>
> #include <stdio.h>
> #include <memory>
> 
> void setup_dispatch_source ()
> {
>   struct record_t
>   {
>      record_t ()  { fprintf(stderr, "%p: create\n", this); }
>      ~record_t () { fprintf(stderr, "%p: destroy\n", this); }
>   };
> 
>   std::shared_ptr<record_t> record(new record_t);
>   auto block = Block_copy(^(){ fprintf(stderr, "%p\n", record.get()); });
> 
>   dispatch_source_t source = dispatch_source_create(DISPATCH_SOURCE_TYPE_VNODE, open("/tmp", O_EVTONLY, 0), DISPATCH_VNODE_REVOKE, dispatch_get_main_queue());
>   dispatch_source_set_event_handler(source, block);
>   dispatch_source_set_event_handler_f(source, NULL);
> 
>   Block_release(block);
> }
> 
> int main (int argc, char const* argv[])
> {
>   setup_dispatch_source();
>   return 0;
> }
> 
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