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1ba7e8fe22
Stated the obvious in the doc: Altering the LUID returned by luid_get() breaks the guarantee that it is locally unique, as it than could collide with other LUIDs returned by luid_get().
122 lines
4.0 KiB
C
122 lines
4.0 KiB
C
/*
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* Copyright (C) 2017 Freie Universität Berlin
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*
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* This file is subject to the terms and conditions of the GNU Lesser
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* General Public License v2.1. See the file LICENSE in the top level
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* directory for more details.
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*/
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/**
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* @defgroup sys_luid Locally Unique ID Generator
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* @ingroup sys
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* @brief Generate system wide (locally) unique IDs
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*
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* This module generates system wide, variable length unique IDs based on on the
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* cpuid module. If the cpuid module is not present, the module falls back to a
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* pre-defined, constant seed for creating unique IDs.
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*
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* The main purpose of this module is to have a unified way for e.g. creating
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* hardware addresses and similar.
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*
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* The IDs generated by this module are base created from a constant base part
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* and a dynamic portion, which is XORed into the base ID. If the cpuid module
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* is present, the base ID is created from the CPU's unique ID. If not, the
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* base ID is created by simple 'memsetting' the base ID with the defined
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* backup seed value.
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*
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* Once the base ID is generated, a LUID is generated by (i) XORing a counter
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* value with the LSB of the base ID, or (ii) by XORing the least significant
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* byes with a value given by the user.
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*
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* Example: Calling `luid_base(&buf, 8)` will always yield an identical value,
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* independent how often the function is called. But calling
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* `luid_base(&buf, 2)` afterwards will results in a different value, if the
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* cpuid module is present, and in the same (but shorter) value if not.
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*
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* Example: Calling `luid_get(&buf, 8)` four times in a row, will yield four
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* different IDs, differing in their LSB.
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*
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* Example: Calling `luid_custom(&buf, 8, 123)` will always yield the same
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* value, but calling `luid_custom(&buf, 8, 124)` will differ.
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*
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* @note This module generates locally unique IDs without any guarantees
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* on their structure. These LUIDs are not compatible nor conform
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* to UUIDs as defined in RFC4122.
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*
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* @{
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* @file
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* @brief Header of the unique ID generation module
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*
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* @author Hauke Petersen <hauke.petersen@fu-berlin.de>
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*/
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#ifndef LUID_H
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#define LUID_H
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#include <stddef.h>
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#ifdef __cplusplus
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extern "C" {
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#endif
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/**
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* @brief Value used as based for the LUIDs in case the cpuid module is not
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* present
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*/
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#ifndef LUID_BACKUP_SEED
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#define LUID_BACKUP_SEED 0x23
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#endif
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/**
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* @brief Get a unique ID
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*
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* The resulting ID is built from the base ID generated with luid_base(), which
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* isXORed with an 8-bit incrementing counter value into the most significant
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* byte.
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*
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* @note The resulting LUID will repeat after 255 calls.
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* @post The generated LUID must be used unchanged, as it could collide with
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* other LUIDs if altered.
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*
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* @param[out] buf memory location to copy the LUID into. MUST be able to
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* hold at least @p len bytes
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* @param[in] len length of the LUID in bytes
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*/
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void luid_get(void *buf, size_t len);
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/**
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* @brief Get a custom unique ID based on a user given generator value
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*
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* The resulting ID is built from the base ID XORed with @p gen in the base
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* ID's most significant bytes.
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*
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* @note Calling this function with identical values for @p gen and @p len
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* will always result in identical LUIDs.
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*
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* @param[out] buf memory location to copy the LUID into. MUST be able to
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* hold at least @p len bytes
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* @param[in] len length of the LUID in bytes
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* @param[in] gen custom LUID generator value
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*/
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void luid_custom(void *buf, size_t len, int gen);
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/**
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* @brief Get a LUID base value
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*
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* The luid module creates the value dependent on the given @p len value using
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* the cpuid module if present or a static seed value (@ref LUID_BACKUP_SEED) if
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* not.
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*
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* @param[out] buf memory location to copy the LUID into. MUST be able to
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* hold at least @p len bytes
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* @param[in] len length of the LUID in bytes
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*/
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void luid_base(void *buf, size_t len);
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#ifdef __cplusplus
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}
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#endif
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#endif /* LUID_H */
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/** @} */
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