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1 | .. SPDX-License-Identifier: CC-BY-SA-4.0 |
2 | ||
3 | ============= | |
4 | ID Allocation | |
5 | ============= | |
6 | ||
7 | :Author: Matthew Wilcox | |
8 | ||
9 | Overview | |
10 | ======== | |
11 | ||
12 | A common problem to solve is allocating identifiers (IDs); generally | |
13 | small numbers which identify a thing. Examples include file descriptors, | |
14 | process IDs, packet identifiers in networking protocols, SCSI tags | |
15 | and device instance numbers. The IDR and the IDA provide a reasonable | |
16 | solution to the problem to avoid everybody inventing their own. The IDR | |
17 | provides the ability to map an ID to a pointer, while the IDA provides | |
18 | only ID allocation, and as a result is much more memory-efficient. | |
19 | ||
20 | IDR usage | |
21 | ========= | |
22 | ||
23 | Start by initialising an IDR, either with :c:func:`DEFINE_IDR` | |
24 | for statically allocated IDRs or :c:func:`idr_init` for dynamically | |
25 | allocated IDRs. | |
26 | ||
27 | You can call :c:func:`idr_alloc` to allocate an unused ID. Look up | |
28 | the pointer you associated with the ID by calling :c:func:`idr_find` | |
29 | and free the ID by calling :c:func:`idr_remove`. | |
30 | ||
31 | If you need to change the pointer associated with an ID, you can call | |
32 | :c:func:`idr_replace`. One common reason to do this is to reserve an | |
33 | ID by passing a ``NULL`` pointer to the allocation function; initialise the | |
34 | object with the reserved ID and finally insert the initialised object | |
35 | into the IDR. | |
36 | ||
37 | Some users need to allocate IDs larger than ``INT_MAX``. So far all of | |
38 | these users have been content with a ``UINT_MAX`` limit, and they use | |
39 | :c:func:`idr_alloc_u32`. If you need IDs that will not fit in a u32, | |
40 | we will work with you to address your needs. | |
41 | ||
42 | If you need to allocate IDs sequentially, you can use | |
43 | :c:func:`idr_alloc_cyclic`. The IDR becomes less efficient when dealing | |
44 | with larger IDs, so using this function comes at a slight cost. | |
45 | ||
46 | To perform an action on all pointers used by the IDR, you can | |
47 | either use the callback-based :c:func:`idr_for_each` or the | |
48 | iterator-style :c:func:`idr_for_each_entry`. You may need to use | |
49 | :c:func:`idr_for_each_entry_continue` to continue an iteration. You can | |
50 | also use :c:func:`idr_get_next` if the iterator doesn't fit your needs. | |
51 | ||
52 | When you have finished using an IDR, you can call :c:func:`idr_destroy` | |
53 | to release the memory used by the IDR. This will not free the objects | |
54 | pointed to from the IDR; if you want to do that, use one of the iterators | |
55 | to do it. | |
56 | ||
57 | You can use :c:func:`idr_is_empty` to find out whether there are any | |
58 | IDs currently allocated. | |
59 | ||
60 | If you need to take a lock while allocating a new ID from the IDR, | |
61 | you may need to pass a restrictive set of GFP flags, which can lead | |
62 | to the IDR being unable to allocate memory. To work around this, | |
63 | you can call :c:func:`idr_preload` before taking the lock, and then | |
64 | :c:func:`idr_preload_end` after the allocation. | |
65 | ||
66 | .. kernel-doc:: include/linux/idr.h | |
67 | :doc: idr sync | |
68 | ||
69 | IDA usage | |
70 | ========= | |
71 | ||
72 | .. kernel-doc:: lib/idr.c | |
73 | :doc: IDA description | |
74 | ||
75 | Functions and structures | |
76 | ======================== | |
77 | ||
78 | .. kernel-doc:: include/linux/idr.h | |
79 | .. kernel-doc:: lib/idr.c |