Why does Sleep() stop all created threads? I want to create a thread but keep the Main function in sleep until the thread finishes.
bool _finished = false;
void testcount(void *p){
int i = 0;
while(i<=30){
i++;
std::cout<<i<<"\n";
Sleep(1000);
}
_finished = true;
}
BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved){
HANDLE test = NULL;
test = CreateThread(NULL, NULL, (LPTHREAD_START_ROUTINE)testcount, NULL, NULL, NULL);
if(test)
std::cout<<"test thread created";
CloseHandle(test);
while(!_finished)
Sleep(1000);
return true;
}
I am trying it now like this, but the program just never finishes, because while with Sleep stops the thread. I don't want to return anything on Main while thread not finished. Any solution?
Calls to DllMain are serialised by Win32.
All new threads start by calling DllMain (with thread attach flag), and the call the method passed to CreateThread.
Therefore your thread is waiting to call DllMain, which cannot happen until your first thread leaves DllMain.
As commenter John Sheridan notes Raymond Chen's blog post from 2007 is a really good explanation.
PS. for correct C/C++ library initialisation you should be using _beginthread or _beginthreadex rather than CreateThread directly.
The new thread is blocked because your main thread does not leave DllMain, as described in Richard's answer.
Your code also contains a data race and has undefined behavior even after this deadlock is fixed. The new thread writes to _finished, the main thread reads from _finished, concurrently. You could try to use std::atomic<bool> instead of bool to fix this, assuming availability of C++11, or you could use Win32 primitives for thread synchronization.
Changes for standard solution using std::atomic<bool>:
#include <atomic>
std::atomic<bool> finished_{false};
// Rest remains the same
Related
I am trying to create and execute a thread using Windows CreateThread API. am seeing that running the program gives non-deterministic behavior. I am seeing from the output of the program that sometimes the thread method "my" gets executed and sometimes not. What might be the reason for this? The program is very simple as shown below. Also what should dwThreadID be initialized with. Should it be 0 or any integer value?
PS:Compiling in Visual Studio.
#include "stdafx.h"
#include <iostream>
#include <windows.h>
using namespace std;
// DWORD WINAPI
DWORD WINAPI my (LPVOID lpParam ) {
for (int i = 0; i < 5;i++){
cout << "I am ";
}
return 0;
}
int main()
{
DWORD dwThreadID = 0;
DWORD dwThrdParam = 0;
HANDLE h = NULL;
h = CreateThread(
NULL, // default security
0, // default stack size
my, // name of the thread function
0, // no thread parameters
0, // default startup flags
&dwThreadID);
if (h == NULL){
cout <<"It is null";
}
cout << " BBBB" << endl ;
CloseHandle(h);
cout << "BBBB ";
return 0;
}
I am seeing from the output of the program that sometimes the thread
method "my" gets executed and sometimes not. What might be the reason
for this?
Probably your main thread exits before your second thread is even executed. Place breakpoint after call to CreateThread.
Or even more cleaner wait, just wait for second thread using WaitForSingleObject.
This effectively make your main thread waiting until "my" thread is done.
If CreateThread fails for some reason, you can call GetLastError right after CreateThread, it will give you ultimate answer.
Also what should dwThreadID be initialized with.
Quick look into MSDN yields following:
A pointer to a variable that receives the thread identifier.
It is indeterminate whether or not your thread executes. The main thread creates it and then terminates the program. The following may happen:
The thread may execute before the main thread terminates.
The program may terminate before the thread executes.
The thread may partially execute and be terminated when the program terminates.
You would need to add synchronization in order to be sure that the thread reaches completion. Pass the thread handle to WaitForSingleObject.
Your program does not check for errors. CreateThread returns NULL when it fails. If that happens, and only if that happens, call GetLastError for information as to why the function failed. If you add a call to WaitForSingleObject then you should check its return value for errors as described by the documentation.
What should dwThreadID be initialized with?
It does not need to be initialized because CreateThread guarantees it will be assigned if CreateThread succeeds.
Firstly do not use CreateThread it is intended for library providers. Use beginthreadex or std::thread
The likely behavior, is that the thread always runs, but it's output is being overwritten. This is because there is no synchronisation.
Check the return code and GetLastError
The value of dwThreadid does not matter
In my Windows service application I may resort to calling TerminateThread API on some of the threads in my process. (Note that I do so only as a last resort measure when a thread fails to quit in a "normal fashion" using signaling mechanism and thread synchronization techniques.) What I've noticed in the event log submitted by a customer is that very rarely TerminateThread may throw the STATUS_INVALID_THREAD exception, which happens only when that API is called on a thread belonging to a threadpool.
Since I know for sure that none of my threads are started from a threadpool, the thread that my call to TerminateThread attempts to close must be coming from another process. This could happen only due to a race condition where my thread handle is closed first and then is passed again to the TerminateThread API while the OS reuses it for some other thread in another process.
So my question is, since my service is running with sufficiently high privileges (as localService) can TerminateThread API in this situation inadvertently terminate some thread belonging to another process? And if yes, how can I prevent this (apart from finding the race-condition, that I'm doing now)?
Let's let the docs speak for themselves:
http://msdn.microsoft.com/en-us/library/windows/desktop/ms686717(v=vs.85).aspx
Do not use TerminateThread():
TerminateThread is a dangerous function that should only be used in the most extreme cases. You should call TerminateThread only if you know exactly what the target thread is doing, and you control all of the code that the target thread could possibly be running at the time of the termination. For example, TerminateThread can result in the following problems:
[...]
You can terminate any thread, as long as you have a handle with sufficient privileges:
A thread cannot protect itself against TerminateThread, other than by controlling access to its handles. The thread handle returned by the CreateThread and CreateProcess functions has THREAD_TERMINATE access, so any caller holding one of these handles can terminate your thread.
Note that I do so only as a last resort measure when a thread fails to quit in a "normal fashion" using signaling mechanism and thread synchronization techniques.
That is a case where you cannot call TerminateThread. You can only call TerminateThread if you have precise control over the thread you are terminating and its full cooperation. If the thread fails to quit in the normal fashion, then you have lost control over the thread, the very opposite of the required conditions under which you can call TerminateThread.
If a process has lost control over one of its threads, the process cannot be saved. This is a fundamental property of threads -- they do not provide isolation of anything but flow control.
If you must do this you can do it like this.
All you need to start is the thread handle.
You call the first function with thread handle as input using the "undocumented" NtQueryInformationThread() function with the ThreadQuerySetWin32StartAddress argument, you get the StartAddress of the thread. More reading # NTInternals.
It calls NTQueryInformationThread by function address after getting the address via GetProcAddress. Then it calls it with the ThreadQuerySetWin32StartAddress argument, getting the StartAddress of the thread.
Then you call the second function which loops through all the threads via CreateToolHelp32Snapshot and compares against the supplied StartAddress. It calls TerminateThread once it finds it.
enum THREADINFOCLASS
{
ThreadQuerySetWin32StartAddress = 9,
};
typedef NTSTATUS(__stdcall * f_NtQueryInformationThread)(HANDLE, THREADINFOCLASS, void*, ULONG_PTR, ULONG_PTR*);
ULONG_PTR GetThreadStartAddress(HANDLE hThread)
{
auto NtQueryInformationThread = reinterpret_cast<f_NtQueryInformationThread>(GetProcAddress(GetModuleHandleA("ntdll.dll"), "NtQueryInformationThread"));
if (!NtQueryInformationThread)
return 0;
ULONG_PTR ulStartAddress = 0;
NTSTATUS Ret = NtQueryInformationThread(hThread, ThreadQuerySetWin32StartAddress, &ulStartAddress, sizeof(ULONG_PTR), nullptr);
if (NT_FAIL(Ret))
return 0;
return ulStartAddress;
}
bool TerminateThreadByStartaddress(ULONG_PTR StartAddress)
{
HANDLE hSnap = CreateToolhelp32Snapshot(TH32CS_SNAPTHREAD, 0);
if (hSnap == INVALID_HANDLE_VALUE)
return false;
THREADENTRY32 TE32 = { 0 };
TE32.dwSize = sizeof(THREADENTRY32);
BOOL Ret = Thread32First(hSnap, &TE32);
while (Ret)
{
HANDLE hTempThread = OpenThread(THREAD_ALL_ACCESS, FALSE, TE32.th32ThreadID);
if (!hTempThread)
{
Ret = Thread32Next(hSnap, &TE32);
continue;
}
if (StartAddress == GetThreadStartAddress(hTempThread))
{
TerminateThread(hTempThread, 0);
CloseHandle(hTempThread);
CloseHandle(hSnap);
return true;
}
CloseHandle(hTempThread);
Ret = Thread32Next(hSnap, &TE32);
}
CloseHandle(hSnap);
return false;
}
Credits to my friend Broihon, I didn't write this code but have used it before.
Use undocumented NTSYSAPI NTSTATUS NTAPI NtTerminateThread(IN HANDLE ThreadHandle, IN NTSTATUS ExitStatus); from http://undocumented.ntinternals.net where ThreadHandle is result from OpenThread MS function and ExitStatus set to whatever you want.
let's pretend there are no libraries that provide semaphores for C++. I wrote this:
#include <vector>
#include <Windows.h>
class Semaphore {
HANDLE mutexS; // provides mutex in semaphore rutines
std::vector<HANDLE> queue; // provides FIFO queue for blocked threads
int value; // semaphore's value
public:
Semaphore(int init=1);
~Semaphore();
void wait();
void signal();
};
Semaphore::Semaphore(int init) {
value = init;
queue = std::vector<HANDLE>();
mutexS = CreateMutex(0,0,0);
}
Semaphore::~Semaphore() {
CloseHandle(mutexS);
}
void Semaphore::signal() {
WaitForSingleObject(mutexS, INFINITE);
if (++value <= 0) {
HANDLE someOldThread = queue.front();
ResumeThread(someOldThread);
queue.erase(queue.begin());
CloseHandle(someOldThread);
}
ReleaseMutex(mutexS);
}
I would like to know why this implementation of wait() doesn't work:
void Semaphore::wait() {
WaitForSingleObject(mutexS, INFINITE);
if (--value < 0) {
HANDLE thisThread = GetCurrentThread();
queue.push_back(thisThread);
ReleaseMutex(mutexS);
SuspendThread(thisThread );
}
else
ReleaseMutex(mutexS);
}
And this one works:
void Semaphore::wait() {
WaitForSingleObject(mutexS, INFINITE);
if (--value < 0) {
HANDLE thisThread = GetCurrentThread();
HANDLE alsoThisThread;
DuplicateHandle(GetCurrentProcess(), thisThread, GetCurrentProcess(), &alsoThisThread, 0, 0, DUPLICATE_SAME_ACCESS);
queue.push_back(alsoThisThread);
ReleaseMutex(mutexS);
SuspendThread(alsoThisThread);
}
else
ReleaseMutex(mutexS);
}
What exactly happens in each case? I've been banging my head over it for a lot of time now. The first implementation of wait, which doesn't work, makes my program block (well, it probably blocks some thread forever). The 2nd implementation works like a charm. What gives ? Why do I need to duplicate thread handles and block the duplicate ?
MSDN helps a lot here ;)
GetCurrentThread returns a pseudo-handle which is a constant for "the current thread":
A pseudo handle is a special constant that is interpreted as the current thread handle.
So when you push it in the queue, you are always pushing a constant that says "the current thread", which is obviously not what you want.
To get a real handle, you have to use DuplicateHandle
If hSourceHandle is a pseudo handle returned by GetCurrentProcess or GetCurrentThread, DuplicateHandle converts it to a real handle to a process or thread, respectively.
A final note: I suppose you are implementing this as a "test" right? Because there are several potential problems.. A very good learning exercise would be to dig them out. But you should not use this in production code.
Out of curiosity: if you want to experiment a little more, the "canonical" way of implementing semaphore with mutexes is to use two mutexes: see here
MSDN documentation for GetCurrentThread has the answer (accents are mine):
The return value is a pseudo handle for the current thread.
A pseudo handle is a special constant that is interpreted as the current thread handle. The calling thread can use this handle to specify itself whenever a thread handle is required.
...
The function cannot be used by one thread to create a handle that can be used by other threads to refer to the first thread. The handle is always interpreted as referring to the thread that is using it. A thread can create a "real" handle to itself that can be used by other threads, or inherited by other processes, by specifying the pseudo handle as the source handle in a call to the DuplicateHandle function.
I am having a strange behavior: WaitForSingleObject seems to crash when I return 0 from my thread, but if I call "ExitThread(0)" then it does not.
void waitForThread(DWORD &threadId)
{
HANDLE hThread = OpenThread(SYNCHRONIZE,
FALSE,
threadId);
if (hThread == NULL) return;
WaitForSingleObject(hThread, INFINITE); // Crashes here (not even returning)
CloseHandle(hThread);
}
According to the documentation:
ExitThread is the preferred method of exiting a thread in C code. However, in C++ code, the thread is exited before any destructors can be called or any other automatic cleanup can be performed. Therefore, in C++ code, you should return from your thread function.
This does not make sense to me. I would think that "return 0" in my function with the signature:
DWORD WINAPI foo(LPVOID arg);
would be fine. For completeness, the thread is created using CreateThread, as such:
DWORD threadId;
HANDLE pth = CreateThread(NULL, // Default security attributes
0, // Default stack size
foo, // Thread name
arg, // Argument to thread
0, // Default creation flag
&threadId); // Returns thread ID
Does anyone know why the return statement would crash "WaitForSingleObject" please? I have put print statement before and after WaitForSingleObject, and one when the thread exists, and the behavior I see is: "Before WaitForSingleObject", "Thread finishes", Crash. Does anyone understand this behavior please?
Read the problem carefully first.
There is a worker thread which gets spawned from a CreateInstance of CTest class. Here is the prototype of the class. hThread is the handle to thread and hEventShutdown is the event used to shutdown thread when program exits. WaitForShutdown is the public function which is used to signal hEventShutdown and wait on handle to thread till thread exits gracefully. WaitForShutdown is invoked from Exit of Application.
//pseudocode
CTest
{
public:
CTest* CreateInstance();
static threadproc(void *pv);
void WaitForShutdown();
public:
HANDLE hThread;
HANDLE hEventShutdown;
}
void CTest::CTest* CreateInstance()
{
// spawn a thread, pass 'this' pointer to thread , use beginthreadex
hThread = beginthreadex ( threadproc, this );
}
unsigned int CTest::threadproc( void *pv)
{
Ctest *ptest = (Ctest*)pv;
do
{
HANDLES hArray[2] = { pv->hEventShutdown, someotherhandle }
dwResult = Waitformultipleobjects( hArrary , 2);
if ( dwResult == WAIT_OBJECT_0)
delete pTest; // since it is allocated dynamically ( This is required due to some known reasons in my code)
if(dwResult == WAIT_OBJECT_0 + 1)
Doprocessing(); //DoProcessing when other thread signal someotherhandle
}while (1)
void CTest::WaitForShutdown()
{
SetEvent ( hEventShutdown);
WaitForSingleObject ( hThread,INFINITE);
}
void CTest::~CTest()
{
Closehandle(hThread)
Closehandle(hEventShutdown);
}
Now if you look at the code carefully, you will find that event is signaled from WaitForShutdown function, thread comes out of WaitForMultipleOjbects and deletes pointer of CTest. It means destructor of CTest is invoked which will obviously close thread handle ( hThread). But WaitForSingleObject from WaitForShutdown is actually waiting on thread handle. So here behavior will be undefined ( I think so, you can correct me if I am wrong). Another problem is destructor of Ctest is invoked when WaitForSingleObject is waiting on its member hThread which is not correct. I can not remove delete pTest from thread since it has to be there due to some reasons.
How will you suggest the solution to the above ?
Couple of Solution which I can think of :
I can keep thread handle in another map but I dont want to do it.
I can copy thread handle to some local variable before WaitForSingleObject in WaitForShutdown and will wait on it. Don;t know is it right ? you tell me.
Or I will use Duplicatehandle API to get reference of existing thread handle before WaitForSingleObject and wait on it. Dont know is it correct. Dont know if will the duplicate handle be alive after CloseHandle on original.
I will keep thread ID, get thread handle from thread ID and keep waiting on thread handle in WaitForShutdown. This looks more elegant but I do not know is there any way to get handle from thread id.
Correct me.
Your feedback appreciated.
The simplest way to handle this would be to simply delete the thread from WaitForShutdown, after WaitForSingleObject returns. This ensures all the handles you need - and more importantly, the object itself - remain alive to the end.
I have run this piece as is. Seems it works, and it doesn't crash.
Strangely that we can call CloseHandle(hthread), before we go out of WaitforSingleObject(hThread,INFINITE).
Of course, "academic" way to join thread is firstly WaitForSingleObject(hThread,INFINITE) than CloseHandle(hThread). So that is my suggestion - do in this way.
I don't have to add anymore.