🚀 go-pugleaf

Keith Bennett (kbennett@cpcug.org) wrote:
: This question concerns the 4th parameter passed to _beginthread().
: This is the void * that can point to anything the programmer wants
: it to.

: I have a function that loops and starts 'n' separate threads (pls see
: example below).  In order to be safe, do I need to guarantee that the
: memory pointed to by that pointer I pass is active and dedicated
: exclusively to that thread for the thread's lifetime?  (Assuming I
: don't *want* any interference from other threads.)

: For example, I assume that the code below will not work properly because
: the contents pointed to by &n may change before the newly started thread
: can read it:


: void _Optlink my_thread_func( void *args)
: {
: 	int the_number_passed = * (int *) args;  /* ... */
: }


: void my_thread_starting_func()
: {
: 	for( int n = 0; n < 4; n++)
: 		_beginthread( my_thread_func, 0, 32 * 1024, &n);
: }


: But how about if I change this to:

: void my_thread_starting_func()
: {
: 	int n[4];

: 	n[0] = 0; n[1] = 1; n[2] = 2; n[3] = 3;

: 	for( int i = 0; i < 4; i++)
: 		_beginthread( my_thread_func, 0, 32 * 1024, &n[i]);
: }

: What if 'my_thread_starting_func()' finishes before one or more of my
: 'my_thread_func()''s reads the value, and that stack area is already
: overwritten by another function?

: So, am I correct in assuming that in this situation I must absolutely
: guarantee that this memory will be used exclusively by that thread
: for as long as the thread might need it?  And therefore, that I should
: probably use a static int array?

I haven't tried it so I may have this all wrong, but Dorfman and 
Neuberger (Effective Multithreading in OS/2) list a function called 
_threadstore() which is to be used in conjunction with _beginthread and 
_endthread for "per thread data area".  the prototype is

#include <process.h>
void **_threadstore(void);

They are using the CSet++ compiler.

Hope this helps,

Hawley Rising
rising@crl.com

 
Protect the memory you pass to the thread with a semaphore.  The first
thing you do with the thread is copy the structure onto the stack and
then release the semaphore.  Or use the _threadstore call in CSEt++.
 
Bob

In article <33tkbt$ssr@cpcug.org>, kbennett@cpcug.org (Keith Bennett) writes:
>This question concerns the 4th parameter passed to _beginthread().
>This is the void * that can point to anything the programmer wants
>it to.
>
>I have a function that loops and starts 'n' separate threads (pls see
>example below).  In order to be safe, do I need to guarantee that the
>memory pointed to by that pointer I pass is active and dedicated
>exclusively to that thread for the thread's lifetime?  (Assuming I
>don't *want* any interference from other threads.)
>
>For example, I assume that the code below will not work properly because
>the contents pointed to by &n may change before the newly started thread
>can read it:
>
>
>void _Optlink my_thread_func( void *args)
>{
>	int the_number_passed = * (int *) args;  /* ... */
>}
>
>
>void my_thread_starting_func()
>{
>	for( int n = 0; n < 4; n++)
>		_beginthread( my_thread_func, 0, 32 * 1024, &n);
>}
>
>
>But how about if I change this to:
>
>void my_thread_starting_func()
>{
>	int n[4];
>
>	n[0] = 0; n[1] = 1; n[2] = 2; n[3] = 3;
>
>	for( int i = 0; i < 4; i++)
>		_beginthread( my_thread_func, 0, 32 * 1024, &n[i]);
>}
>
>What if 'my_thread_starting_func()' finishes before one or more of my
>'my_thread_func()''s reads the value, and that stack area is already
>overwritten by another function?
>
>So, am I correct in assuming that in this situation I must absolutely
>guarantee that this memory will be used exclusively by that thread
>for as long as the thread might need it?  And therefore, that I should
>probably use a static int array?
>
>Thanks for your help.
>- Keith
>-- 

Use static int table.

void my_thread_starting_func()
{
   static int n[4];

	n[0] = 0; n[1] = 1; n[2] = 2; n[3] = 3;

	for( int i = 0; i < 4; i++)
		_beginthread( my_thread_func, 0, 32 * 1024, &n[i]);   
  
}

Or this method with passing variable n itself (i haven't tested it ).

_beginthread( my_thread_func, 0, 32 * 1024, (void *) n);

And in Thread:

int the_number_passed = (int ) args;