std::vector does not release memory in a thread

Question

If I create a thread with _beginthreadex, and in the thread I used std::vector<WCHAR> that consumes 200MB of memory - when the thread ends, the memory is not released. Even after CloseHandle, the memory is not released.

Here is a working example:

#include <windows.h>
#include <process.h>
#include <vector>
using namespace std;

unsigned __stdcall   Thread_RestartComputer(void* pComputerName)
{
    std::vector<WCHAR> asdf(100000000);
    _endthreadex(0);
    return 0;
}
int main()
{
    UINT threadID = 0;
    HANDLE hThread = (HANDLE)_beginthreadex(NULL, 0, &Thread_RestartComputer, 0, CREATE_SUSPENDED, &threadID);
    ResumeThread(hThread);
    WaitForSingleObject(hThread, INFINITE);
    CloseHandle(hThread);
}

I thought that std::vector<WCHAR> released the memory when it went out of scope?

How do you measure the memory? Are you sure it's not a false positive? — Some programmer dude, Mar 22 '22 at 16:35
https://learn.microsoft.com/en-us/cpp/c-runtime-library/reference/endthread-endthreadex?view=msvc-170 explicitly says that you must not call `CloseHandle` when using `_beginthreadex`. — Iziminza, Mar 22 '22 at 16:39
Your `_endthreadex` call kills thread before destructor has a chance to run. — freakish, Mar 22 '22 at 16:40
Even if `std::vector` (and its allocator) releases memory, there is no guarantee that the implementation of `operator new()` will release it to the host system. Using OS specific functions for managing threads (like `_beginthreadex()`) which have no relationship to the implementation of `std::vector` (or any other memory management done by the C++ standard library) is even less likely to release memory. — Peter, Mar 22 '22 at 16:40
I suggest you use [`std::thread`](https://en.cppreference.com/w/cpp/thread/thread) instead. — Some programmer dude, Mar 22 '22 at 17:04
No, instead of the Windows low-level threading functions. `std::thread t(ThreadFunction, possible, arguments, to, the, thread, function); t.join();` — Some programmer dude, Mar 22 '22 at 17:10
@JeffR -- The implementers of the C++ 11 standard compiler you're using for Windows already has worked out the details that you are getting wrong. The `std::thread` and relevant functions and classes do everything you are trying to do, only correctly. Unless there is a compelling reason to do so (like you're using a C++98 compiler), go with the standard threading functions, and leave the hard work of figuring out how to implement `std::thread` to the compiler experts. — PaulMcKenzie, Mar 22 '22 at 17:34

score 7 · Accepted Answer · answered Mar 22 '22 at 16:39

7

C++ doesn't know that calling _endthreadex makes the thread go away. So it doesn't call the destructors of local variables like asdf before it calls _endthreadex.

Solution: Don't do that. return 0; ends the thread and calls the destructor.

answered Mar 22 '22 at 16:39

user253751

57,427
7
48
90

But the MS docs say to call `_endthreadex` if you call `_beginthreadex`, or did I misread them? – JeffR Mar 22 '22 at 16:43
2

@JeffR "You can call _endthread or _endthreadex explicitly to terminate a thread; however, _endthread or _endthreadex is called automatically when the thread returns from the routine passed as a parameter to _beginthread or _beginthreadex" – user253751 Mar 22 '22 at 16:46

Iziminza · Answer 2 · 2022-03-22T16:47:31.740

5

As you can see in the documentation for _endthreadex, _endthreadex causes C++ destructors pending in the thread not to be called.

So just remove the call to _endthreadex and you should be fine.

The reasons for this behaviour are explained in this answer.

edited Mar 22 '22 at 16:47

answered Mar 22 '22 at 16:42

Iziminza

374
3
8

2

The sentence "Terminating a thread with a call to `endthread` or `_endthreadex` helps ensure proper recovery of resources allocated for the thread." is severely misleading, as it rather does the opposite in your case! – Iziminza Mar 22 '22 at 16:48
Yes, it is very misleading! Thanks. – JeffR Mar 22 '22 at 17:24

std::vector does not release memory in a thread

2 Answers2