i am trying to implement a user level thread package with preemption between threads.
There is provision for threads relinquishing the processor themselves (ie without preemption): this is peacefully implemented using setjmp() and longjmp().
For preemption, I set a timer using setitimer() and install a signal handler to catch the signal SIGVTALRM this timer generates.
When a thead is executing and the timer expires, i enter the signal handler.I want to save the state the currently interrupted thread had before coming into the signal handler into a structure similar to jmp_buf(which setjmp() and longjmp() use), so that signal handler saves this state(to be resumed later when scheduled) and passes control to another thread.
The important point is that I MUST SAVE THAT STATE OF THIS INTERRUPTED THREAD THAT IT HAD AT THE TIME OF
GETTING INTERRUPTED BY THE SIGNAL __AND__ __NOT__ THE CURRENT STATE ( the state inside the
signal handler).
{{ As i think, the reason for this is :
The implementation of signal handlers in linux is done by creating a seperate stack etc for the signal handler's text
and stack regions, such that at the end of the text region some code is added etc. to make sure that the signal handler
returns by executing the system call sigreturn (this is required so that the kernel can do some posix required
book keeping before continuing with the execution of the interrupted process by a ret_from_sys_call). if i save the context of this interrupted thread in the signal handler and longjmp() to some other thread's saved state, when i later resume the execution of this interrupted thread, i'll in effect be resuming execution in this signal handler and since the signal handler's return address will NOT be valid then : i won't be returning to the place in the program where i got interrupted earlier, rather i will return with the sigreturn system call into the kernel, ...... and everything will turn upside down.
}}
So the problem boils down to how can i find in the signal handler the state of the process when the signal was received so that i can put it in a jmp_buf struct and resume execetion from there.
so much for it.
tell me if u can implement preemptive scheduling of C code in a user level thread in some other way.
thnx
Abhishek
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This archive was generated by hypermail 2b29 : Mon Apr 15 2002 - 22:00:24 EST