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; }