User-Level Native Thread Primitives (GThread) Library White Paper (G06.26+)

User-Level Native Thread Primitives (GThread Library) 03/31/2005
Hewlett-Packard Company--540065-001 Page 43 of 46
3.6.3. Thread Switching (Context Manipulation)
Changes to thread switching are divided into two types; changes associated with switching swapped
threads, and changes associated with switching static threads.
3.6.3.1. Changes Associated with Swapped Threads
For swapped threads, the TNS or TNS/R native clients must perform the four following steps to
switch threads:
1) The context for the current thread must be captured (GTHREAD_SETJMP_).
2) The contents of the runtime stack(s) for the current thread must be copied to its save area
(manually done by the client using pointers and sizes obtained directly from the thread
control block). This step may also include dynamic resizing of the save area if the client code
is designed to handle overflow of that area.
3) The contents of the save area for the target thread must be copied to the runtime stack(s)
(manually done by the client using pointers and sizes obtained directly from the thread
control block). This step can only be done after some GTHREAD ‘thread switching
preparation’ is performed (the step must be preceded by a call to
GTHREAD_SWAPIN_SETUP_).
4) Control is switched to the target thread (GTHREAD_LONGJMP_).
The changes introduced by the TNS/E platform do not affect the externals of
GTHREAD_SETJMP_ or GTHREAD_LONGJMP_, so client code to implement steps 1 and 4 is
unaffected.
Steps 2 and 3, however, where the client saves and restores runtime stack information require
client changes for TNS/E.
On TNS/R, the client must access elements in the thread control block directly in order to
perform the stack copying. These elements implied a runtime architecture consisting of a single
stack. TNS/E architecture introduces the concept of a runtime environment consisting of two
stacks, so client code would minimally need to change to take the two stack architecture into
account. The client code could be further complicated if it needed to execute on both TNS/R and
TNS/E platforms.
Four interfaces are exported to hide the various platform stack implementations and present the
client with the concept of a single ‘logical stack’ that needs saving on context switches.
The first two interfaces replace the manual stack copying currently performed by clients for steps
2 and 3, respectively:
GTH_STACK_SAVE_( GThread_CB cb )
GTH_STACK_RESTORE_( GThread_CB cb )