How to perform in different thread

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Hi!
I would like to create a separate thread for WriteDataToPort. How to create and pass cstring as parameter to it.
Below is my code snippet.

C++

CString ss=_T("");
while(!feof(fpRead))
{
	size_t count = fread(file_buffer, sizeof(char), l_size, fpRead);
	for(int i=0; i< count; i++)			
	{
		ss = file_buffer[i];
		m_CommPort->WriteDataToPort(ss.GetBuffer(ss.GetLength()));
	}
}

void CCommPort::WriteDataToPort(TCHAR* f_WriteData)
{
	WaitForSingleObject(m_hWriteCompleteEvent,INFINITE);	
	ResetEvent(m_hWriteCompleteEvent);
	m_bKeyboardWrite =TRUE;		
	memset(m_szWriteBuff, 0, sizeof(m_szWriteBuff));
	tcscpy(m_szWriteBuff, f_WriteData);	
	SetEvent(m_hWriteEvent);
}

i have tried in this way, but getting exception. Please do the needful.
hThread = CreateThread(NULL,0,WriteDataToPortThread,LPVOID)ss.GetBuffer(ss.GetLength()),0,NULL);

DWORD __stdcall CMainFrame::WriteDataToPortThread(LPVOID lpParam)
{
    CString* ss = (CString*)lpParam;
    ((CMainFrame*)AfxGetMainWnd())->m_CommPort->WriteDataToPort(ss->GetBuffer(ss->GetLength()));
    return 0;
}

Thanks much
Sam.

Posted 4-Feb-13 16:51pm

Mr Sam

Updated 4-Feb-13 17:22pm

Sandeep Mewara

v2

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H.Brydon 4-Feb-13 23:38pm

Your CreateThread() statement has invalid syntax. Perhaps fix that up...

Without you doing that, I am guessing that you want to pass "const char*" data through the LPVOID pointer, and the types on both casts should probably be "const char*" (or "const char*const" or perhaps "char*")...

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Sergey Alexandrovich Kryukov · Answer 1 · 2013-02-04T17:38:00

Please see: http://www.computersciencelab.com/MultithreadingTut1.htm[^].

Locate the section "Creating Threads Under Windows", and the code sample using _beginthread. Here is the idea: last parameter arglist is the void pointer. It is used to pass any data to the thread entry point function passed as the first parameter start_address. When you do that, start_address will be started as it is called with the arglist pointer passed as its only argument. In your implementation of start_address, you should cast this void pointer to the pointer to the data type you used to pass as the arglist when you called _beginthread.

Please see:
http://msdn.microsoft.com/en-us/library/aa246693%28v=vs.60%29.aspx[^].

See also my past answer for some more ideas: Starting a thread from a thread within a class[^].

—SA

michaelmel · Answer 2 · 2013-02-04T17:54:00

1) Allocate a string (object of CString class) - but not on stack. You can make it a global or a member of the application class, or even create it dynamically.
2) Create a new thread using AfxBeginThread and pass it the address of your string object, e.g.

AfxBeginThread(YourThreadFunction, &g_MyString, THREAD_PRIORITY_NORMAL);

3) inside your thread function (e.g. YourThreadFunction), cast LPVOID p to CString * in the beginning and then you can use it as a pointer to string in your thread's loop:

C++

INT YourThreadFunction(LPVOID p)
{
     CString *pStr = (CString *)p;
     while (!gCancel)
     {
          // your loop
     }
     return 0;
}

It is of course crude as, but you get the idea. All the usual pre-cautions about simultaneous access, reallocation and so on will still apply.

I normally create a thread interface class that contains everything that is needed to pass data between 2 threads - which could contain strings amongst other things.

How to perform in different thread

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