Recognizing the main thread based on its thread descriptor

[Florian Weimer <fweimer at redhat dot com>]

Is there an internal way to tell if the current thread is the main thread?

I need this to allocate thread-local state in a particular way (the main 
thread needs to get a copy of the global variable for backwards 
compatibility).  Currently, this is handled with an initializer for the 
TLS variable and an override during pthread_create.  I want to switch 
this to dynamic initialization.

I can probably come up with a clever hack to express this, but I would 
prefer something explicit.

Thanks,
Florian