Re: out of scope pointers in threads

James Kanze <>
Fri, 28 Nov 2008 11:47:04 -0800 (PST)
uche wrote:

I am trying to send a pointer to the thread below; however,
when the thread gets executed, the pointer goes out of scope .
How do I fix it ?

What is "data"? Is it a data type, or a pointer to a data type?
You seem to use it both ways.

int mywrite(char* id, int number_of_characters, char char_array)
  HANDLE Producer;


  //global_char = char_array;

  //create mutual exlusion for producer process to write into the buffer
  //create producer thread and start the function for inserting character=


  //mywriteTh is the entry point of the producer

  ptr =new data;

OK, we have 'data' as a data type, and 'ptr' is, I suppose a

  ptr->character = char_array;

Again, what is char_array? Where does it come from?

  ptr->id = id;

  Producer = (HANDLE) CreateThread (NULL, 0, mywriteTh,
reinterpret_cast<data> (ptr) , 0, &ThId); // i want to send the
pointer to this thread

Anytime you need a reinterpret_cast for a function argument, you
should be asking yourself questions. If I understand the
interface description of CreateThread at MSDN (Microsoft seems
to go in a lot for obfuscated typenames), you don't need any
cast at all; just pass the pointer. (I'm guessing here that
LPVOID is a void*; where the L comes from, I don't know.)

  return 0;

DWORD WINAPI mywriteTh(data ptr)

And according to the documentation, this function must take a
void* (well, an LPVOID) as well, not a data. (The documentation
shows some __in as well. More obfuscation; I don't think it
means anything.)

 //global_char is available here

 DWORD tId = GetCurrentThreadId();

 data *ptr_data = reinterpret_cast<data *>(ptr); // pointer is goes
out of scope

Again, a reinterpret_cast is a no-no. You need a static_cast of
the void* to the exact type of the pointer that was converted to
void*, above.


As for "going out of scope", of course the pointer goes out of
scope. But you're passing a copy of it to the thread, and the
memory it points to is still there.

James Kanze (GABI Software)
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