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Unable to change the values of $ebp and $eip

I have some macros which I want to source within gdb to print the backtrace of all the threads I have in my system. So the macro essentially tries to switch to each thread and prints its backtrace. This it does by extracting the stack pointer which is stored in the thread structure. The back pointer and instruction pointer addresses are computed and then the bactrace is printed.

The threads are linked by a linked list. Access the address of the next thread following the next pointer.

Assuming I am in frame 0 of current thread, the macro does the following :

- Load the core dump file in gdb and then do the following :

- For all threads in the system do this for each thread..

(gdb) set $thr=$thr->next_p
(gdb) set $sp=$thr->obj->registers.stackpointer
(gdb) set $ebp=*(int *)($sp+8)
(gdb) set $ebp= *(int *)($sp + 36)
(gdb) bt

When I start execution of this macro from frame 0 of the first thread, the macro executes correctly
and prints the backtrace of all the threads in the system.

Now I exit gdb and restart it again by loading the core file as done above. I first switch to another
frame - say frame 3. Then I start execution of the macro. When I do that, I am able to extract
the value of $sp correctly. But gdb does not allow me to set $eip and $ebp. The macro fails. For every thread in the system, the 1st thread's backtrace is printed. Basically I get a same backtrace for every thread in the system. When I check $ebp and $eip everytime I extract a new $sp and try to set $ebp and $eip, the values set for $ebp and $eip are what it was for the current thread (the thread that caused a core).

Why does this happen ? Why am I not able to set/change values of $ebp and $eip in the macro after I switch to a non-zero frame (older frame) ? The macro works great when I start execution of it from frame 0.


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