[libdispatch-dev] Block not released after dispatch_source_set_event_handler

[Allan Odgaard]

Hi Daniel.

Thanks for the reply — as I was seeing the leak in code that did use the event handler, I spent some more time investigating, and it seems my issue is with linking C++ object files with -fobjc-link-runtime and setting -mmacosx-version-min=10.7.

I am not sure if this is a bug or I am violating some contract by linking with the obj-c run-time for non-obj-c code. Presumably it has to do with the 10.7 ARC-lite compatibility code that clang adds (though my code was pure C++ so did not use ARC).

Kind regards Allan


On Feb 25, 2013, at 6:28 AM, "Daniel A. Steffen" <dsteffen at apple.com> wrote:

> 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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>