Question

I’m trying to learn how create new threads and run them. I need to pass a few variables into the function that is run on the new thread but I can’t find out how to actually pass anything to that new function/thread.

I’m following http://www.devarticles.com/c/a/Cplusplus/Multithreading-in-C/1/ but it only goes through how to pass a single parameter and nothing else.

Side question, do threads work the exact same way as functions do except just on a different thread or is it a little more complicated than just that?

Thanks,

-Faken

Answer 1

The underlying OS allows to pass only one parameter to a thread CreateThread:

HANDLE WINAPI CreateThread(
  __in_opt   LPSECURITY_ATTRIBUTES lpThreadAttributes,
  __in       SIZE_T dwStackSize,
  __in       LPTHREAD_START_ROUTINE lpStartAddress,
  __in_opt   LPVOID lpParameter,
  __in       DWORD dwCreationFlags,
  __out_opt  LPDWORD lpThreadId
);

Accordingly the CRT thread create function allows also only one parameter, despite the name being arglist:

uintptr_t _beginthreadex( 
   void *security,
   unsigned stack_size,
   unsigned ( *start_address )( void * ),
   void *arglist,
   unsigned initflag,
   unsigned *thrdaddr 
);

Given these restrictions the usual convention is to pass a pointer to a structure/class with all the arguments. Usually, with C++, one creates a static function that will be the thread handler and passes an instance as the argument:

class Foo
{
  int _someState;
  int _otherState;
  char _moreState[256];

  unsigned DoWork();
  static unsigned ThreadHandler(void*);

public:
  void StartThread();
}

void Foo::StartThread()
{
   _beginthreadex(..., Foo::ThreadHandler, this, ...);
}

unsigned Foo::ThreadHandler(void* arglist)
{
   Foo* pFoo = (Foo*) arglist;
   return pFoo->DoWork();
}

unsigned Foo::DoWork()
{
  // do here all the thread work
}

This is a fairly common idiom and in effect it allows you to pass as much state (=arguments) as needed to the thread.