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user: Mention file descriptor changes

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Sebastian Huber 2020-09-01 07:03:49 +02:00
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user/migration/v4_11-to-v5.rst
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@ -69,6 +69,25 @@ The directive :c:func:`rtems_clock_get` was removed. See section
`Transition Advice for the Removed rtems_clock_get()` in the
`RTEMS Classic API Guide` for alternatives.
File Descriptors
----------------
In RTEMS 5.1, the list of free file descriptors has a LIFO ordering in contrast
to previous versions where it was a FIFO. This means if an application
regularly opens and closes files (or sockets) it sees the whole range of file
descriptors. The reason for this change was to increase the time before file
descriptors are reused to more likely catch a file descriptor use after close.
This change may surface application issues. If the configured file descriptor
maximum (``CONFIGURE_MAXIMUM_FILE_DESCRIPTORS``) is greater than the
``FD_SETSIZE`` defined by Newlib to 64, then calls to ``select()`` are undefined
behaviour and may corrupt the thread stack. In particular, ``FD_SET()`` may
result in an out of bounds access. It is possible to define a custom
``FD_SETSIZE``. The application must ensure that the custom ``FD_SETSIZE`` is
defined before ``<sys/select.h>`` is included in all modules used by the
application, for example via a global compiler command line define. This
applies also to all third-party libraries used by the application.
Networking
----------