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thread::id
specificationSection: 33.4.3.2 [thread.thread.id] Status: NAD Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
View all other issues in [thread.thread.id].
View all issues with NAD status.
Discussion:
Addresses US-184
It is unclear when a thread::id
ceases to be meaningful.
The sentence "The library may reuse the value of a
thread::id
of a terminated thread that can no longer be
joined." implies that some terminated threads can be
joined. It says nothing about detached threads.
[ Resolution proposed by ballot comment: ]
Require a unique
thread::id
for every thread that is (1) detached and not terminated or (2) has an associatedstd::thread
object.
[ 2010-11-22 Howard Hinnant observes ]
A thread can either be running or terminated. Additionally a thread can be joined, detached, or neither. These combine into the five possible states shown in this table:
Running | Terminated | |
---|---|---|
Neither joined nor detached | shall not reuse id | shall not reuse id |
detached | shall not reuse id | may reuse id |
joined | impossible state | may reuse id |
Only if a thread is neither joined nor detached can it be joined. Or said differently, if a thread has already been joined or detached, then it can not be joined. The sentence:
The library may reuse the value of a
thread::id
of a terminated thread that can no longer be joined.
precisely defines the two states shown in the above table where a thread::id may be reused.
The following program illustrates all of the possibilities:
#include <mutex> #include <thread> #include <iostream> #include <chrono> std::mutex mut; void f() { std::lock_guard<std::mutex> _(mut); std::cout << "f id = " << std::this_thread::get_id() << " terminating\n"; } void g() { std::lock_guard<std::mutex> _(mut); std::cout << "g id = " << std::this_thread::get_id() << " terminating\n"; } int main() { std::cout << "main id = " << std::this_thread::get_id() << "\n"; std::thread t1(f); std::thread(g).detach(); std::this_thread::sleep_for(std::chrono::seconds(1)); std::cout << "g's thread::id can be reused here because g has terminated and is detached.\n"; std::cout << "f's thread::id can't be reused here because f has terminated but is still joinable.\n"; std::cout << "f id = " << t1.get_id() << "\n"; t1.join(); std::cout << "f's thread::id can be reused here because f has terminated and is joined.\n"; std::cout << "f id = " << t1.get_id() << "\n"; } main id = 0x7fff71197ca0 f id = 0x100381000 terminating g id = 0x100581000 terminating g's thread::id can be reused here because g has terminated and is detached. f's thread::id can't be reused here because f has terminated but is still joinable. f id = 0x100381000 f's thread::id can be reused here because f has terminated and is joined. f id = 0x0
[2011-02-11 Reflector discussion]
Moved to Tentatively NAD after 5 votes.
Proposed resolution: