BFD keeps all of its internal structures in obstacks. There is one obstack per open BFD file, into which the current state is stored. When a BFD is closed, the obstack is deleted, and so everything which has been allocated by BFD for the closing file is thrown away.
BFD does not free anything created by an application, but pointers into
bfd structures become invalid on a bfd_close; for example,
after a bfd_close the vector passed to
bfd_canonicalize_symtab is still around, since it has been
allocated by the application, but the data that it pointed to are
lost.
The general rule is to not close a BFD until all operations dependent
upon data from the BFD have been completed, or all the data from within
the file has been copied. To help with the management of memory, there
is a function (bfd_alloc_size) which returns the number of bytes
in obstacks associated with the supplied BFD. This could be used to
select the greediest open BFD, close it to reclaim the memory, perform
some operation and reopen the BFD again, to get a fresh copy of the data
structures.