multiple threading

[harshitm]

hi,
I am new to multithreading in c++. i found a simple code on a site . i think i understood the essentials of multithreading after reading it but when i try to compile the source code which is :-

#include <windows.h> <WINDOWS.H>

#include <strsafe.h> <STRSAFE.H>

#include <stdio.h><STDIO.H>


#define BUF_SIZE 255

//------------------------------------------

// A function to Display the message

// indicating in which tread we are

//------------------------------------------

void DisplayMessage (HANDLE hScreen,
char *ThreadName, int Data, int Count)
{

STRSAFE_LPSTR msgBuf;
size_t cchStringSize;
DWORD dwChars;

// Print message using thread-safe functions.

StringCchPrintf(msgBuf, BUF_SIZE,
TEXT("Executing iteration %02d of %s"
" having data = %02d \n"),
Count, ThreadName, Data);
StringCchLength(msgBuf, BUF_SIZE, &cchStringSize);
WriteConsole(hScreen, msgBuf, cchStringSize,
&dwChars, NULL);
Sleep(1000);
}

//-------------------------------------------

// A function that represents Thread number 1

//-------------------------------------------

DWORD WINAPI Thread_no_1( LPVOID lpParam )
{

int Data = 0;
int count = 0;
HANDLE hStdout = NULL;

// Get Handle To screen.

// Else how will we print?

if( (hStdout =
GetStdHandle(STD_OUTPUT_HANDLE))
== INVALID_HANDLE_VALUE )
return 1;

// Cast the parameter to the correct

// data type passed by callee i.e main() in our case.

Data = *((int*)lpParam);

for (count = 0; count <= 4; count++ )
{
DisplayMessage (hStdout, "Thread_no_1", Data, count);
}

return 0;
}

//-------------------------------------------

// A function that represents Thread number 2

//-------------------------------------------

DWORD WINAPI Thread_no_2( LPVOID lpParam )
{

int Data = 0;
int count = 0;
HANDLE hStdout = NULL;

// Get Handle To screen. Else how will we print?

if( (hStdout =
GetStdHandle(STD_OUTPUT_HANDLE)) ==
INVALID_HANDLE_VALUE )
return 1;

// Cast the parameter to the correct

// data type passed by callee i.e main() in our case.

Data = *((int*)lpParam);

for (count = 0; count <= 7; count++ )
{
DisplayMessage (hStdout, "Thread_no_2", Data, count);
}

return 0;
}

//-------------------------------------------

// A function that represents Thread number 3

//-------------------------------------------

DWORD WINAPI Thread_no_3( LPVOID lpParam )
{
int Data = 0;
int count = 0;
HANDLE hStdout = NULL;

// Get Handle To screen. Else how will we print?

if( (hStdout =
GetStdHandle(STD_OUTPUT_HANDLE))
== INVALID_HANDLE_VALUE )
return 1;

// Cast the parameter to the correct

// data type passed by callee i.e main() in our case.

Data = *((int*)lpParam);

for (count = 0; count <= 10; count++ )
{
DisplayMessage (hStdout, "Thread_no_3", Data, count);
}

return 0;
}


void main()
{
// Data of Thread 1

int Data_Of_Thread_1 = 1;
// Data of Thread 2

int Data_Of_Thread_2 = 2;
// Data of Thread 3

int Data_Of_Thread_3 = 3;
// variable to hold handle of Thread 1

HANDLE Handle_Of_Thread_1 = 0;
// variable to hold handle of Thread 1

HANDLE Handle_Of_Thread_2 = 0;
// variable to hold handle of Thread 1

HANDLE Handle_Of_Thread_3 = 0;
// Aray to store thread handles

HANDLE Array_Of_Thread_Handles[3];

// Create thread 1.

Handle_Of_Thread_1 = CreateThread( NULL, 0,
Thread_no_1, &Data_Of_Thread_1, 0, NULL);
if ( Handle_Of_Thread_1 == NULL)
ExitProcess(Data_Of_Thread_1);

// Create thread 2.

Handle_Of_Thread_2 = CreateThread( NULL, 0,
Thread_no_2, &Data_Of_Thread_2, 0, NULL);
if ( Handle_Of_Thread_2 == NULL)
ExitProcess(Data_Of_Thread_2);

// Create thread 3.

Handle_Of_Thread_3 = CreateThread( NULL, 0,
Thread_no_3, &Data_Of_Thread_3, 0, NULL);
if ( Handle_Of_Thread_3 == NULL)
ExitProcess(Data_Of_Thread_3);


// Store Thread handles in Array of Thread

// Handles as per the requirement

// of WaitForMultipleObjects()

Array_Of_Thread_Handles[0] = Handle_Of_Thread_1;
Array_Of_Thread_Handles[1] = Handle_Of_Thread_2;
Array_Of_Thread_Handles[2] = Handle_Of_Thread_3;

// Wait until all threads have terminated.

WaitForMultipleObjects( 3,
Array_Of_Thread_Handles, TRUE, INFINITE);

printf("Since All threads executed"
" lets close their handles \n");

// Close all thread handles upon completion.

CloseHandle(Handle_Of_Thread_1);
CloseHandle(Handle_Of_Thread_2);
CloseHandle(Handle_Of_Thread_3);
}

then i get the following error:-

MSVCRTD.lib(crtexew.obj) : error LNK2019: unresolved external symbol _WinMain@16 referenced in function ___tmainCRTStartup
1>C:\Users\jolly\Documents\Visual Studio 2010\Projects\learning windows programming\Debug\learning windows programming.exe : fatal error LNK1120: 1 unresolved externals

i am using visual studio 2010.



What is wrong ???
Please help me.
Thanks in advance for any assistance offered.



@kbw :thanks for your help . But after changing to console I was getting this error message that

msg not initialized ( It is in DisplayMessage function .)
Since i am new I did not knew how to initialize it so i simply gave it 0 then also there were some run time errors.
What to do ??


@Computergeek01


Hi ,
yes i do realize that this is not a good example but i am farely new so i was ready to take up any example just so that it works.

For instance i had read that we should not use CreateThread instead _beginthread etc, but beginthread was not there in visual studio and i did not knew it's equivalent in it .
I would be very thankful if u help me in using MULTITHREADING.

[kbw]

Please use the code format tag to format your code.

You're building the app as a Windows app, you need to change it to a Console app. In recent versions of Visual Studio that's the Linker->System->SubSystem setting.

[Computergeek01]

This is a HORRIBLE example of how to use multi-threading. I know this is marked as completed already because kbw already answered your question but you should know this is NOT how to use Multi-threading. If you respond to this I will elaborate.