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.. SPDX-License-Identifier: CC-BY-SA-4.0
=============
ID Allocation
=============
:Author: Matthew Wilcox
Overview
========
A common problem to solve is allocating identifiers (IDs); generally
small numbers which identify a thing. Examples include file descriptors,
process IDs, packet identifiers in networking protocols, SCSI tags
and device instance numbers. The IDR and the IDA provide a reasonable
solution to the problem to avoid everybody inventing their own. The IDR
provides the ability to map an ID to a pointer, while the IDA provides
only ID allocation, and as a result is much more memory-efficient.
IDR usage
=========
Start by initialising an IDR, either with :c:func:`DEFINE_IDR`
for statically allocated IDRs or :c:func:`idr_init` for dynamically
allocated IDRs.
You can call :c:func:`idr_alloc` to allocate an unused ID. Look up
the pointer you associated with the ID by calling :c:func:`idr_find`
and free the ID by calling :c:func:`idr_remove`.
If you need to change the pointer associated with an ID, you can call
:c:func:`idr_replace`. One common reason to do this is to reserve an
ID by passing a ``NULL`` pointer to the allocation function; initialise the
object with the reserved ID and finally insert the initialised object
into the IDR.
Some users need to allocate IDs larger than ``INT_MAX``. So far all of
these users have been content with a ``UINT_MAX`` limit, and they use
:c:func:`idr_alloc_u32`. If you need IDs that will not fit in a u32,
we will work with you to address your needs.
If you need to allocate IDs sequentially, you can use
:c:func:`idr_alloc_cyclic`. The IDR becomes less efficient when dealing
with larger IDs, so using this function comes at a slight cost.
To perform an action on all pointers used by the IDR, you can
either use the callback-based :c:func:`idr_for_each` or the
iterator-style :c:func:`idr_for_each_entry`. You may need to use
:c:func:`idr_for_each_entry_continue` to continue an iteration. You can
also use :c:func:`idr_get_next` if the iterator doesn't fit your needs.
When you have finished using an IDR, you can call :c:func:`idr_destroy`
to release the memory used by the IDR. This will not free the objects
pointed to from the IDR; if you want to do that, use one of the iterators
to do it.
You can use :c:func:`idr_is_empty` to find out whether there are any
IDs currently allocated.
If you need to take a lock while allocating a new ID from the IDR,
you may need to pass a restrictive set of GFP flags, which can lead
to the IDR being unable to allocate memory. To work around this,
you can call :c:func:`idr_preload` before taking the lock, and then
:c:func:`idr_preload_end` after the allocation.
.. kernel-doc:: include/linux/idr.h
:doc: idr sync
IDA usage
=========
.. kernel-doc:: lib/idr.c
:doc: IDA description
Functions and structures
========================
.. kernel-doc:: include/linux/idr.h
.. kernel-doc:: lib/idr.c
|
