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Update notes on "Shared memory emulation folder" with current behaviour.

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Ion Gaztañaga
2021-02-21 00:41:54 +01:00
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1 changed files with 7 additions and 13 deletions
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@@ -6637,21 +6637,15 @@ want [*Boost.Interprocess] to initialize the COM library with another model, def
Shared memory (`shared_memory_object`) is implemented in Windows using memory mapped files, placed in a
shared directory in the shared documents folder (`SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders\Common AppData`).
This directory name is the last bootup time obtained via COM calls (if `BOOST_INTERPROCESS_BOOTSTAMP_IS_LASTBOOTUPTIME`) defined
or searching the system log for a startup event (the default implementation), so that each bootup shared memory is created in a new
folder obtaining kernel persistence shared memory.
This directory name is the last bootup time obtained via Registry values of the Session Manager. This behaviour is the default
since Boost 1.74.
If using `BOOST_INTERPROCESS_BOOTSTAMP_IS_LASTBOOTUPTIME`, due to COM implementation related errors,
in Boost 1.48 & Boost 1.49 the bootup-time folder was dumped and files
were directly created in shared documents folder, reverting to filesystem persistence shared memory. Boost 1.50 fixed those issues
and recovered bootup time directory and kernel persistence. If you need to reproduce Boost 1.48 & Boost 1.49 behaviour to communicate
with applications compiled with that version, comment `#define BOOST_INTERPROCESS_HAS_KERNEL_BOOTTIME` directive
in the Windows configuration part of `boost/interprocess/detail/workaround.hpp`.
Old [*Boost.Interprocess] versions (until Boost 1.48 ) used COM calls (via macro `BOOST_INTERPROCESS_BOOTSTAMP_IS_LASTBOOTUPTIME`)
but that timestamp was unreliable in Virtual Machines and time adjustements. Next versions until (Boost 1.74) used EventLog startup event
as timestamp, but several users discovered this event could be absent in the Event Log if the system was up for a long time. You
can force this Event Log based timestamp defining BOOST_INTERPROCESS_BOOTSTAMP_IS_EVENTLOG_BASED.
If using the default implementation, (`BOOST_INTERPROCESS_BOOTSTAMP_IS_LASTBOOTUPTIME` undefined) and the Startup Event is not
found, this might be due to some buggy software that floods or erases the event log.
In any error case (shared documents folder is not defined or bootup time could not be obtained, the library throws an error. You still
In any error case (shared documents folder is not defined or bootup time could not be obtained), the library throws an error. You still
can use [*Boost.Interprocess] defining your own directory as the shared directory. When your shared directory is a compile-time constant,
define `BOOST_INTERPROCESS_SHARED_DIR_PATH` when using the library and that path will be used to place shared memory files. When you have
to determine the shared directory at runtime, define `BOOST_INTERPROCESS_SHARED_DIR_FUNC` and implement the function