1.. SPDX-License-Identifier: GPL-2.0+ 2 3============= 4ID Allocation 5============= 6 7:Author: Matthew Wilcox 8 9Overview 10======== 11 12A common problem to solve is allocating identifiers (IDs); generally 13small numbers which identify a thing. Examples include file descriptors, 14process IDs, packet identifiers in networking protocols, SCSI tags 15and device instance numbers. The IDR and the IDA provide a reasonable 16solution to the problem to avoid everybody inventing their own. The IDR 17provides the ability to map an ID to a pointer, while the IDA provides 18only ID allocation, and as a result is much more memory-efficient. 19 20The IDR interface is deprecated; please use the :doc:`XArray <xarray>` 21instead. 22 23IDR usage 24========= 25 26Start by initialising an IDR, either with DEFINE_IDR() 27for statically allocated IDRs or idr_init() for dynamically 28allocated IDRs. 29 30You can call idr_alloc() to allocate an unused ID. Look up 31the pointer you associated with the ID by calling idr_find() 32and free the ID by calling idr_remove(). 33 34If you need to change the pointer associated with an ID, you can call 35idr_replace(). One common reason to do this is to reserve an 36ID by passing a ``NULL`` pointer to the allocation function; initialise the 37object with the reserved ID and finally insert the initialised object 38into the IDR. 39 40Some users need to allocate IDs larger than ``INT_MAX``. So far all of 41these users have been content with a ``UINT_MAX`` limit, and they use 42idr_alloc_u32(). If you need IDs that will not fit in a u32, 43we will work with you to address your needs. 44 45If you need to allocate IDs sequentially, you can use 46idr_alloc_cyclic(). The IDR becomes less efficient when dealing 47with larger IDs, so using this function comes at a slight cost. 48 49To perform an action on all pointers used by the IDR, you can 50either use the callback-based idr_for_each() or the 51iterator-style idr_for_each_entry(). You may need to use 52idr_for_each_entry_continue() to continue an iteration. You can 53also use idr_get_next() if the iterator doesn't fit your needs. 54 55When you have finished using an IDR, you can call idr_destroy() 56to release the memory used by the IDR. This will not free the objects 57pointed to from the IDR; if you want to do that, use one of the iterators 58to do it. 59 60You can use idr_is_empty() to find out whether there are any 61IDs currently allocated. 62 63If you need to take a lock while allocating a new ID from the IDR, 64you may need to pass a restrictive set of GFP flags, which can lead 65to the IDR being unable to allocate memory. To work around this, 66you can call idr_preload() before taking the lock, and then 67idr_preload_end() after the allocation. 68 69.. kernel-doc:: include/linux/idr.h 70 :doc: idr sync 71 72IDA usage 73========= 74 75.. kernel-doc:: lib/idr.c 76 :doc: IDA description 77 78Functions and structures 79======================== 80 81.. kernel-doc:: include/linux/idr.h 82 :functions: 83.. kernel-doc:: lib/idr.c 84 :functions: 85