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Traversing the thread list

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This code example lists the running threads for a specified process. First, the ListProcessThreads function takes a snapshot of the currently executing threads in the system. To do that, it uses the CreateToolhelp32Snapshot function, and then it walks through the list recorded in the snapshot using the Thread32First and Thread32Next functions. The parameter for ListProcessThreads is the process identifier of the process whose threads are to be listed.

To follow along with the code example, use Visual Studio to create a new project from the C++ Console App project template, and add the code below to it.

#include <windows.h>
#include <tlhelp32.h>
#include <tchar.h>

//  Forward declarations:
BOOL ListProcessThreads( DWORD dwOwnerPID );
void printError( TCHAR const* msg );

int main( void )
{
  ListProcessThreads(GetCurrentProcessId() );
  return 0;
}

BOOL ListProcessThreads( DWORD dwOwnerPID ) 
{ 
  HANDLE hThreadSnap = INVALID_HANDLE_VALUE; 
  THREADENTRY32 te32; 
 
  // Take a snapshot of all running threads  
  hThreadSnap = CreateToolhelp32Snapshot( TH32CS_SNAPTHREAD, 0 ); 
  if( hThreadSnap == INVALID_HANDLE_VALUE ) 
    return( FALSE ); 
 
  // Fill in the size of the structure before using it. 
  te32.dwSize = sizeof(THREADENTRY32 ); 
 
  // Retrieve information about the first thread,
  // and exit if unsuccessful
  if( !Thread32First( hThreadSnap, &te32 ) ) 
  {
    printError( TEXT("Thread32First") );  // Show cause of failure
    CloseHandle( hThreadSnap );     // Must clean up the snapshot object!
    return( FALSE );
  }

  // Now walk the thread list of the system,
  // and display information about each thread
  // associated with the specified process
  do 
  { 
    if( te32.th32OwnerProcessID == dwOwnerPID )
    {
      _tprintf( TEXT("\n     THREAD ID      = 0x%08X"), te32.th32ThreadID ); 
      _tprintf( TEXT("\n     base priority  = %d"), te32.tpBasePri ); 
      _tprintf( TEXT("\n     delta priority = %d"), te32.tpDeltaPri ); 
    }
  } while( Thread32Next(hThreadSnap, &te32 ) );

  _tprintf( TEXT("\n"));

//  Don't forget to clean up the snapshot object.
  CloseHandle( hThreadSnap );
  return( TRUE );
}

void printError( TCHAR const* msg )
{
  DWORD eNum;
  TCHAR sysMsg[256];
  TCHAR* p;

  eNum = GetLastError( );
  FormatMessage( FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
         NULL, eNum,
         MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), // Default language
         sysMsg, 256, NULL );

  // Trim the end of the line and terminate it with a null
  p = sysMsg;
  while( ( *p > 31 ) || ( *p == 9 ) )
    ++p;
  do { *p-- = 0; } while( ( p >= sysMsg ) &&
                          ( ( *p == '.' ) || ( *p < 33 ) ) );

  // Display the message
  _tprintf( TEXT("\n  WARNING: %s failed with error %d (%s)"), msg, eNum, sysMsg );
}