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segmentation fault

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Agni

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segmentation fault

Dec 18th, 2008

This code here is giving segmentation fault. Any clues why? also any general tips for writing good multi-threaded code are welcome.

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 c++ Syntax (Toggle Plain Text) 
#include <windows.h>
#include <process.h>
#include <iostream>
 
using namespace std;
 
unsigned __stdcall Display(void* p)
{
	Sleep(500);
	cout << "Display" << endl;
	cout << *((int*)p) << endl;
	return 0;
}
int main()
{
	unsigned taddr = NULL;
	unsigned createFlag = 0;
 
	for(int i=0;i<5;i++)
	{
		if (_beginthreadex(NULL,0,Display,&i,createFlag,&taddr))
		{
			cout << "success" << endl;
		}
		else
			cout << "failed" << endl;
	}
 
	cout << "main's execution is over" << endl;
	ExitThread(0);
}
#include <windows.h>
#include <process.h>
#include <iostream>

using namespace std;

unsigned __stdcall Display(void* p)
{
	Sleep(500);
	cout << "Display" << endl;
	cout << *((int*)p) << endl;
	return 0;
}
int main()
{
	unsigned taddr = NULL;
	unsigned createFlag = 0;

	for(int i=0;i<5;i++)
	{
		if (_beginthreadex(NULL,0,Display,&i,createFlag,&taddr))
		{
			cout << "success" << endl;
		}
		else
			cout << "failed" << endl;
	}

	cout << "main's execution is over" << endl;
	ExitThread(0);
}

"A tree laden with fruits always bends low"

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Join Date: Aug 2005

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Ancient Dragon Ancient Dragon is offline

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Still Learning

Re: segmentation fault

Dec 18th, 2008

>>ExitThread(0);
delete that line in main()

The most important thing in the Olympic Games is not to win but to take part, just as the most important thing in life is not the triumph but the struggle. The essential thing is not to have conquered but to have fought well.
-Pierre de Coubertin, The Olympic Creed Inspired by Bishop Ethelbert

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Join Date: Dec 2007

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Agni

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Re: segmentation fault

Dec 18th, 2008

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Originally Posted by Ancient Dragon

>>ExitThread(0);
delete that line in main()

But if i do that the threads never execute as 'main' terminates and all the threads die. Any reason why you think it might help?

I found a possible solution to the problem, since the threads would sleep in 'Display' and by the time they wake up, main exits hence 'i' doesn't exist anymore and when i try to print the value it crashes. Actually i executed it once without the 'sleep' and it went through fine. Though i think it might still crash some other time when main exits before any of the threads is left incomplete.

"A tree laden with fruits always bends low"

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Join Date: Sep 2008

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unbeatable0 unbeatable0 is offline

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Re: segmentation fault

Dec 18th, 2008

It doesn't matter that 'i' doesn't exist when main() ends and the thread is using it, simply because when main() ends the whole program shuts down, including the threads.

Last edited by unbeatable0; Dec 18th, 2008 at 7:23 am. Reason: Have written 'than' instead of 'that'

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Join Date: Dec 2007

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Agni

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Re: segmentation fault

Dec 18th, 2008

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Originally Posted by unbeatable0

It doesn't matter than 'i' doesn't exist when main() ends and the thread is using it, simply because when main() ends the whole program shuts down, including the threads.

The threads wont shut down because I'm using 'ExitThread', if I'm not wrong its the equivalent of pthread_exit for Unix.

"A tree laden with fruits always bends low"

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Join Date: Aug 2005

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Ancient Dragon Ancient Dragon is offline

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Still Learning

Re: segmentation fault

Dec 18th, 2008

ExitThread() belongs inside a thread, not in main().

•

ExitThread is the preferred method of exiting a thread in C code.

There are other methods available to keep main() alive until the threads terminate on their own.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
#include <windows.h>
#include <process.h>
#include <iostream>
 
using namespace std;
unsigned __stdcall Display(void* p)
{
	Sleep(500);
	cout << "Display" << endl;
	cout << *((int*)p) << endl;
	return 0;
}
int main()
{
	unsigned taddr = NULL;
	unsigned createFlag = 0;
    HANDLE handles[5] = {0};
 	for(int i=0;i<5;i++)
	{
		if (handles[i] = (HANDLE)_beginthreadex(NULL,0,Display,&i,createFlag,&taddr))
		{
			cout << "success" << endl;
		}
		else
			cout << "failed" << endl;
	}
    WaitForMultipleObjects(5, handles, TRUE, INFINITE);
 
	cout << "main's execution is over" << endl;
}
#include <windows.h>
#include <process.h>
#include <iostream>

using namespace std;
unsigned __stdcall Display(void* p)
{
	Sleep(500);
	cout << "Display" << endl;
	cout << *((int*)p) << endl;
	return 0;
}
int main()
{
	unsigned taddr = NULL;
	unsigned createFlag = 0;
    HANDLE handles[5] = {0};
 	for(int i=0;i<5;i++)
	{
		if (handles[i] = (HANDLE)_beginthreadex(NULL,0,Display,&i,createFlag,&taddr))
		{
			cout << "success" << endl;
		}
		else
			cout << "failed" << endl;
	}
    WaitForMultipleObjects(5, handles, TRUE, INFINITE);

	cout << "main's execution is over" << endl;
}

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Salem

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Void main'ers are DOOMed

Re: segmentation fault

Dec 18th, 2008

> Any clues why?
Because you passed a POINTER to i.
The main() thread exits, the i variable goes out of scope and another thread is looking down the barrel of a segfault.

> also any general tips for writing good multi-threaded code are welcome.
Well the first step is to make sure that whatever parameter you pass has a lifetime which exceeds the thread creator.

Using dynamic memory, and then making the thread the owner of that memory is one way to achieve this.
This also solves the other problem of helping to ensure that only one thread has access to the memory passed into it.

If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
--
If your code lacks code tags, you will be IGNORED