id	summary	reporter	owner	description	type	status	milestone	component	version	severity	resolution	keywords	cc
12494	Cannot expand vector in place using interprocess::allocator and interprocess::rbtree_best_fit	camden.mannett@…	Ion Gaztañaga	"When using reserve() on a {{{interprocess::vector}}} (synomyn for {{{container::vector}}}) inside a {{{interprocess::basic_managed_mapped_file}}} (using the default {{{interprocess::rbtree_best_fit}}} memory algorithm), it cannot in place expand forward.

I noticed this when getting help on [https://stackoverflow.com/questions/39780787/boostmanaged-mapped-file-cannot-allocate-all-of-grown-space this] SO question.  Using the code in that question, put a breakpoint at \boost\interprocess\mem_algo\rbtree_best_fit.hpp:964, and then trace the call stack back to {{{vector::priv_reserve_no_capacity(size_type new_cap, version_2)}}}:

{{{
pointer reuse = 0;
pointer const ret(this->m_holder.allocation_command(allocate_new | expand_fwd | expand_bwd, new_cap, real_cap = new_cap, reuse));
}}}

The {{{reuse}}} pointer is set to null, resumably so it can be overridden by the allocator.  The pointer is passed up the stack to {{{interprocess::allocator::allocation_command(boost::interprocess::allocation_type command, size_type limit_size, size_type &prefer_in_recvd_out_size, pointer &reuse)}}} where the raw pointer is retrieved:

{{{
value_type *reuse_raw = ipcdetail::to_raw_pointer(reuse);
pointer const p = mp_mngr->allocation_command(command, limit_size, prefer_in_recvd_out_size, reuse_raw);
}}}

{{{reuse_raw}}} continues to be passed up the stack to where our breakpoint is:

{{{
//Expand in place
prefer_in_recvd_out_size = preferred_size;
if(reuse_ptr && (command & (boost::interprocess::expand_fwd | boost::interprocess::expand_bwd))){
  void *ret = priv_expand_both_sides
     (command, limit_size, prefer_in_recvd_out_size, reuse_ptr, true, backwards_multiple);
  if(ret)
     return ret;
}
}}}

{{{reuse_ptr}}} is our raw pointer and is as null as it was when created, causing the expand in place block to be skipped.  In short there appears to be no way for a vector to be in place expanded when used inside a memory mapped file."	Bugs	closed	To Be Determined	interprocess	Boost 1.60.0	Problem	invalid