passing one function to different threads

Question

0.00/5 (No votes)

See more:

I have to create an application where I'll have to make multiple threads. SoI thought to try making one function and passing it to different threads. Initially I've created two threads and have declared one function to be passed to both of them. All I am trying to do is to pass different integers to those threads and display it in the thread function,here is my code:

DWORD WINAPI Name(LPVOID lpParam)
           {
         int *Ptr=(int*)lpParam;

         for(int j=0;j<2;j++)
              {
              cout<<"Thread"<<endl;
              cout<<*Ptr<<endl;
              }

            return 0;
           }
            int main()
            {


            int a=10,b=15,c=25;
           HANDLE thread1,thread2;
           DWORD threadID,threadID2;
           thread2= CreateThread(NULL,0,Name,LPVOID(a),0,&threadID2);
           thread1= CreateThread(NULL,0,Name,LPVOID(b),0,&threadID);


         for(int i=0;i<5;i++)
          {
           cout<<"Main Thread"<<endl;
          }

          if(thread1==NULL)
            {
           cout<<"Couldn't Create Thread:("<<endl;
           exit(0);
           }
             if(thread2==NULL)
            {
           cout<<"Couldn't Create Thread:("<<endl;
           exit(0);
            }
                  return 0;
             }

but this code is not running properly,i.e compliles fine,starts fine but afterwards gives a debugging error. Could someone let let me know of my mistake and how I could correct it coz being able to utilize one function for multiple threads will be really helpful for me.

Posted 2-May-13 0:30am

Aayman Khalid

Add a Solution

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Jochen Arndt · Answer 1 · 2013-05-02T00:43:00

Solution 1

You are passing the thread parameters by value by casting int to LPVOID. That is the correct way because the variables are local.

But in your thread function, you are accessing them as pointers. You must do it there in a similar way by just casting the LPVOID to int:

C++

int iParam = (int)lpParam;
for(int j=0;j<2;j++)
{
    cout<<"Thread"<<endl;
    cout<<iParam<<endl;
}

Posted 2-May-13 0:43am

Jochen Arndt

Comments

Aayman Khalid 2-May-13 6:53am

Thank you so much:)I guess I haven't been able to understand dereferencing properly,coz uptil now I used to assign values to a pointer like this"int *P=x".
Could you explain to me how this happened,I will be really thankful...

Jochen Arndt 2-May-13 7:26am

LPVOID is often (and here) used to provide a parameter of unspecified type. It is up to you how to use it and ensure that you get the correct value on the 'receiving' side.

But the solution is simple: Cast to those type that has been passed (the type itself or a pointer when passing a pointer) and ensure that the casted type fits. Fitting is no problem when using pointers or types with smaller or equal size (like int, unsigned, char).

But when using pointers, you must ensure that the variable itself still exists when it is accessed. This is very important with threads. Such variables should be global or class members (with class members, the class must exists while the thread is running).

Examples:

// Using static variables
static int iParam1 = 1;
static char str1[128] = "test";
CreateThread(NULL, 0, Name, (LPVOID)&iParam1, 0, &ID);
CreateThread(NULL, 0, Name, (LPVOID)str1, 0, &ID);

// Using class members
// In the header file int m_iParam2, LPSTR m_lpszStr;
m_iParam2 = 2;
m_lpszStr = new char[128];
strcpy(m_lpszStr, "Test");
CreateThread(NULL, 0, Name, (LPVOID)&iParam2, 0, &ID);
CreateThread(NULL, 0, Name, (LPVOID)m_lpszStr, 0, &ID);
// Ensure that thread has terminated before deleting the string
// and the class still exists.

In both above cases, access the parameters by casting to int* and LPSTR (or LPCSTR if it is not changed):
// lpParam is ptr to int
int *pParam = (int *)lpParam;
// get value from above pointer
int iParam1 = *pParam;
// get value from passed parameter
int iParam2 = *((int *)lpParam);

LPSTR lpszStr = (LPSTR)lpParam;
// When not changing the string, const should be used
LPCSTR lpszStrC = (LPCSTR)lpParam;

I hope this answers your questions.

Aayman Khalid 2-May-13 7:46am

One more thing: does type casting return some value(for different data types or any value at all),e.g if two threads are utilising one function but the argument of one thread is an integer and that of the other function is a string,then I have to take a descision based on the type casting,so is it possible???

Jochen Arndt 2-May-13 8:04am

Type casting tells the compiler that he should treat the value on the right side as another type so that it can be assigned to the variable on the left side. Without casting, the compiler throws an error.

In special cases casting changes the value (e.g. when casting a double to an int where the fractional digits are cut off or cutting of high bits when the left side has less bits than the right). But with pointers and integers, the value itself is unchanged but used in a different way.

In your original code, you passed the integers by value (10 and 15) and casted them to pointers to int. That means that you had two pointers with value 10 and 15. When then accessing the data of these pointers (printing them with cout), the memory at the addresses 10 and 15 is read.

Knowing this, it should be clear that you can't pass different types to one function this way. The common way to pass different types when only one parameter is available, is the usage of structures containing a member that identifies the type of the passed data and other members that contain the data.

Aayman Khalid 2-May-13 6:55am

and also,will it work if I pass srings or character arrays to it???

Aayman Khalid 2-May-13 7:49am

coz I've tried to do it the other way,i.e separately declaring one pointer for string and other for an integer in the function and it doesn't work.

Jochen Arndt 2-May-13 8:09am

See my other comment. You can't pass two different types of variables to the thread function without knowing which type to use. In such case it may be better to have two different thread functions.

pasztorpisti · Answer 2 · 2013-05-02T00:48:00

Solution 2

C++

LPVOID(a) -> &a
LPVOID(b) -> &b

Remove the unused c variable.
Before return 0; in your main function you should call:

C++

WaitForSingleObject(thread1, INFINITE);
WaitForSingleObject(thread2, INFINITE);

To prevent your program exiting before the threads finish.

Posted 2-May-13 0:48am

pasztorpisti

Comments

Aayman Khalid 2-May-13 6:55am

Thank you,pasztorpisti:)

pasztorpisti 2-May-13 7:02am

You are welcome!

Aayman Khalid 2-May-13 7:12am

And could you guide me what should I do if want to pass a string or an array to the thread please...

pasztorpisti 2-May-13 7:17am

There are many ways but as you are a beginner I would recommend the following: Allocate memory for your data dynamically with new, pass the stuff as a pointer to your thread and delete the allocated storage by the thread. Example:
int* p = new int[10];
You simply pass p as the thread parameter and in your thread function you cast it back to int* and then use it. When you are done you execute delete [] p; in your thread function. You can pass a string similarly as a string is nothing more than an array of characters.

Aayman Khalid 2-May-13 7:34am

Thanks a lot:)