[patch/rfa] allow unwinding "past main" for dummy frames

Randolph Chung randolph@tausq.org
Mon Dec 6 04:27:00 GMT 2004

> Could you explain to me how inside_main_func is returning true for a
> dummy frame?
> The code is there for inside_entry_func because we used to use the
> actual entry point as a location for the dummy frame.

it's very twisted :)

this is what it says in hppa-hpux-tdep.c:

     On HPUX, functions in the main executable and in libraries can be located
     in different spaces.  In order for us to be able to select the right
     space for the function call, we need to go through an instruction seqeunce
     to select the right space for the target function, call it, and then
     restore the space on return.

     There are two helper routines that can be used for this task -- if
     an application is linked with gcc, it will contain a __gcc_plt_call
     helper function.  __gcc_plt_call, when passed the entry point of an
     import stub, will do the necessary space setting/restoration for the
     target function.

so, in a function called by gdb, we have the following frames on the

#0 callee
#1 __gcc_plt_call
#2 <dummy frame>
#3 current function when user called "callee" from gdb (main in this case)

so, what happens is that we call __gcc_plt_call using a stack trampoline
which tells __gcc_plt_call to return to "current function". after
__gcc_plt_call calls the callee function, it looks up the return address
("current function") and restores the space registers to the correct
value for that address. ergo, <dummy frame> actually has a frame pc that
belongs to "current function", and if that's main, dummy frame will
trigger the inside_main_func check.

does this make sense?

Randolph Chung
Debian GNU/Linux Developer, hppa/ia64 ports

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