Re: pthreads and c++ destructor

Hi Victor,

You're violating one of the fundamental concepts of threaded
programming here. Any time you modify static data within a thread and that
data could be accessed from another thread you will need to use some sort
of locking routine to guarantee that only one of the threads has access to
the static data at any given time.

For now, lets ignore the 1/2 second sleep. At the end of the loop
you create a thread which calls the ReadArray function. Then at the
beginning of the loop you call the ModifyArray function. It is likely that
ReadArray is still running in it's thread, and you could end up modifying
the array while you are trying to read it (thus the bogus values).

Even the 1/2 second sleep is no guarantee. If the system is busy it
is possible that the ModifyArray thread may not have finished by the time
the ReadArray function runs.

If you use locking (I think the pthread lock function is something
like pthread_mutex_lock) you should be able to avoid these problems and
avoid the use of the sleep routine altogether. Also keep in mind that it's
possible that you'll have several (more than two) threads running at once,
so if you care about the order of reads and writes you'll need to add a
little more logic to your functions to enforce ordering.

Thanks,

Aaron Albertson


c400-l-bounces+albertaa=us.ibm.com@xxxxxxxxxxxx wrote on 10/05/2010
09:04:51 AM:

From: Victor Gonzalez <victorpy@xxxxxxxxxxx>
To: <c400-l@xxxxxxxxxxxx>
Date: 10/05/2010 09:09 AM
Subject: Re: [C400-L] pthreads and c++ destructor
Sent by: c400-l-bounces+albertaa=us.ibm.com@xxxxxxxxxxxx


here i post a working code, if anyone could compile it and try. In
my previous mail i forgot to put the delete line in the Read function

#define _MULTI_THREADED
#include <iostream>
#include <pthread.h>
#include <unistd.h>
#include <stdio.h>

using namespace std;


class MyClass
{

char **myCharArray;

public:

void ModifyArray()

{
myCharArray = new char*[10];

for(int i = 0; i < 10; i++)
{
myCharArray[i] = new char[20];
sprintf(myCharArray[i], "%s%i", "mod ", i);

}




}

void ReadArray()
{
for(int i = 0; i < 10; i++)
{
cout << "element " << i << " "<<myCharArray[i]<<endl;
delete [] myCharArray[i];
}

}

};

MyClass *g_myclass;

void *ModClass(void *parm)
{
//mutex lock not shown
g_myclass->ModifyArray();

return NULL;
}

void *ReadClass(void *parm)
{
//mutex lock not shown
g_myclass->ReadArray();

return NULL;
}


int main()
{

int i = 0, rc;
pthread_t thread1, thread2;

g_myclass = new MyClass();

while(i < 1000)
{

//thread api not used, just pseudo-code
//1)
//createThread(ModClass);
rc = pthread_create(&thread1, NULL, ModClass, NULL);

//sleep(1)
usleep(500000);
//2)
rc = pthread_create(&thread2, NULL, ReadClass, NULL);

i++;

}

}

From: victorpy@xxxxxxxxxxx
To: c400-l@xxxxxxxxxxxx
Date: Tue, 5 Oct 2010 13:28:07 +0000
Subject: [C400-L] pthreads and c++ destructor


Hello to all,

I think im having a problem with threads and c++ destructor. I'll

explain the problem

Supose i have a Class myclass that is global for all threads. A

thread is created modify some attributes of the class then
terminates. All the threads that modifies the class are detached.

After some time i try to access the modified attribute of the

class, but this no longer have the value i put in the first thread.
i.e. the value is gone or is trash.

Let me put a practical example

Class MyClass
{

char **myCharArray;

void ModifyArray()

{
myCharArray = new char*[10];

for(int i = 0; i < 10; i++)
myCharArray[i] = new char[20];
}

char ReadArray()
{
for(int i = 0; i < 10; i++)
cout << "element " << i << " "<<myCharArray[i]<<endl;

}

};

void *ModClass(void *parm)
{
//mutex lock not shown
g_myclass->ModifyArray();
}

void *ReadClass(void *parm)

{

//mutex lock not shown

g_myclass->ReadArray();

}

MyClass *g_myclass

int main()
{

int i = 0

g_myclass = new MyClass();

while(i < 10)
{

//thread api not used, just pseudo-code
//1)
createThread(ModClass);

sleep(10)
//2)
createThread(ReadClass);

i++;

}

}

1) In the first thread the array is modified using new operator,

after that the thread is detached.

2) After some period of time the array is accessed for read, but

in some cases after various iteration, the element in the array is gone.

Could it be that the modification of the array with new in the

thread is deleted after the execution of the thread?. I think it
could be that because i read about thread releasing resources after

detaching

Any help will be welcome

Thanks

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