Thread (usually) not doing what I'd expect

[Graham J Lee]

Hi all,

The code below demonstrates the problem I'm seeing with gnustep-base  
1.13.0 on i386/Linux, where NSThread  
+detachNewThreadSelector:toTarget:withObject: is not consistently  
executing the method indicated.  I'm trying to choose my words  
carefully here, because it *does* create a new LWP.  In fact, if I  
set a breakpoint at the [NSThread ...] line, I can step through  
NSThread.m and see that objc_thread_dispatch returns non-NULL[*].   
However, the global variable x never gets set and none of the NSLog()  
lines in the -doStuff method appear.  I'm not so good with debugging  
multiple-threaded apps but if I step past objc_thread_detach then  
look at the threads display in ddd, I only see one thread, although I  
do get a notification of a new LWP from the debugger.

It looks like it might be racy, because if I step past the section  
(lines 551-555 here, I'm on base 1.13.0) in NSThread.m:
  if (objc_thread_detach(@selector(__sendThreadMethod),thread,nil)  
== NULL)
    {
      [NSException raise: NSInternalConsistencyException
                  format: @"Unable to detach thread (unknown error)"];
    } // <--step to here
}
and continue from there, I do get the change in x and the NSLog()  
messages as expected.  Similarly if I execute the objc_thread_detach 
() function myself in the debugger by assigning its return value to a  
convenience variable, I get the expected result.  But never when just  
running the code, debug=yes or not.  The same program does what I  
expect - i.e. prints out all the NSLog()s and sets x to 3 - with the  
Apple Foundation.

[*] I can't step _into_ it, I guess because my objc runtime wasn't  
built with debugging symbols.

Here's the source.

--8<--
> #include <Foundation/Foundation.h>
>
> NSLock *lock;
> int x=0;
>
> @interface AnObject:NSObject
> {
> }
> - (void)doStuff;
> @end
>
> @implementation AnObject
> - (void)doStuff
> {
>   id localPool=[[NSAutoreleasePool alloc] init];
>   NSLog(@"Here we go, acquiring lock...");
>   [lock lock];
>   x=3;
>   NSLog(@"Relinquish lock...");
>   [lock unlock];
>   [localPool release];
> }
> @end
>
> int
> main(int argc, const char *argv[])
> {
>   id pool = [[NSAutoreleasePool alloc] init];
>   AnObject *obj=[[AnObject alloc] init];
>   lock=[[NSLock alloc] init];
>   // Your code here...
>   NSLog(@"Spawning the thread...");
>   [NSThread detachNewThreadSelector:@selector(doStuff) toTarget:obj  
> withObject:nil];
>   NSLog(@"It's been launched.");
>   // The end...
>   while([lock tryLock]==NO);
>   NSLog(@"Teardown: x=%d",x);
>   [obj release];
>   [lock release];
>   [pool release];
>
>   return 0;
> }
--8<--

Cheers,

Graham.


--
Graham J Lee
http://www.thaesofereode.info/