Freeing memory allocated in the inferior

Freeing memory allocated in the inferior

[Matthew Malcomson]

Hello,

I've just been reading the code, and was hoping someone could help me 
understand something.

It seems that memory allocated in the inferior (with 
`value_allocate_space_in_inferior()` is never freed, even when the gdb 
process exits/detaches.

Is this true (i.e. have I missed something)?
I found an old comment on an old bug that implies this is correct 
https://sourceware.org/bugzilla/show_bug.cgi?id=16236#c1, and the 
pointer appears to remain valid in the inferior over a lot of allocation 
after detaching, but neither of those are particularly conclusive.

If so, why is this?

I can see it would be difficult to know when to free them during 
execution, because the user may simply remember the pointer address the 
variable is stored and attempt to use it, but what about storing the 
allocations on a data structure somewhere, and freeing them when gdb is 
closing/detaching?
I doubt many users are remembering pointer addresses after having 
detached and attached again to a process, but I guess there could be an 
extra `set` parameter introduced if this were the case.

Re: Freeing memory allocated in the inferior

[Simon Marchi]

On 2017-02-20 05:11, Matthew Malcomson wrote:
> Hello,
> 
> I've just been reading the code, and was hoping someone could help me
> understand something.
> 
> It seems that memory allocated in the inferior (with
> `value_allocate_space_in_inferior()` is never freed, even when the gdb
> process exits/detaches.
> 
> Is this true (i.e. have I missed something)?
> I found an old comment on an old bug that implies this is correct
> https://sourceware.org/bugzilla/show_bug.cgi?id=16236#c1, and the
> pointer appears to remain valid in the inferior over a lot of
> allocation after detaching, but neither of those are particularly
> conclusive.
> 
> If so, why is this?
> 
> I can see it would be difficult to know when to free them during
> execution, because the user may simply remember the pointer address
> the variable is stored and attempt to use it, but what about storing
> the allocations on a data structure somewhere, and freeing them when
> gdb is closing/detaching?
> I doubt many users are remembering pointer addresses after having
> detached and attached again to a process, but I guess there could be
> an extra `set` parameter introduced if this were the case.

The memory could be needed even after gdb exits/detaches.  Consider the 
case where you use gdb to insert a string in some data structure and 
detach from the inferior to let it run freely.  If gdb frees the memory 
containing the string, the data structure will contain an invalid 
pointer.