doc/examples/rculfhash/cds_lfht_add_unique.c

   1 // SPDX-FileCopyrightText: 2013 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   2 //
   3 // SPDX-License-Identifier: MIT
   4
   5 /*
   6  * This example shows how to add unique nodes into a RCU lock-free hash
   7  * table. We use a "seqnum" field to show which node is staying in the
   8  * hash table. This hash table requires using a RCU scheme.
   9  */
  10
  11 #include <stdio.h>
  12 #include <stdlib.h>
  13 #include <time.h>
  14
  15 #include <urcu/urcu-memb.h>     /* RCU flavor */
  16 #include <urcu/rculfhash.h>     /* RCU Lock-free hash table */
  17 #include <urcu/compiler.h>      /* For CAA_ARRAY_SIZE */
  18 #include "jhash.h"              /* Example hash function */
  19
  20 /*
  21  * Nodes populated into the hash table.
  22  */
  23 struct mynode {
  24         int value;                      /* Node content */
  25         int seqnum;                     /* Our node sequence number */
  26         struct cds_lfht_node node;      /* Chaining in hash table */
  27 };
  28
  29 static
  30 int match(struct cds_lfht_node *ht_node, const void *_key)
  31 {
  32         struct mynode *node =
  33                 caa_container_of(ht_node, struct mynode, node);
  34         const int *key = _key;
  35
  36         return *key == node->value;
  37 }
  38
  39 int main(void)
  40 {
  41         int values[] = { -5, 42, 42, 36, 24, }; /* 42 is duplicated */
  42         struct cds_lfht *ht;    /* Hash table */
  43         unsigned int i;
  44         int ret = 0, seqnum = 0;
  45         uint32_t seed;
  46         struct cds_lfht_iter iter;      /* For iteration on hash table */
  47         struct cds_lfht_node *ht_node;
  48         struct mynode *node;
  49
  50         /*
  51          * Each thread need using RCU read-side need to be explicitly
  52          * registered.
  53          */
  54         urcu_memb_register_thread();
  55
  56         /* Use time as seed for hash table hashing. */
  57         seed = (uint32_t) time(NULL);
  58
  59         /*
  60          * Allocate hash table.
  61          */
  62         ht = cds_lfht_new_flavor(1, 1, 0,
  63                 CDS_LFHT_AUTO_RESIZE | CDS_LFHT_ACCOUNTING,
  64                 &urcu_memb_flavor, NULL);
  65         if (!ht) {
  66                 printf("Error allocating hash table\n");
  67                 ret = -1;
  68                 goto end;
  69         }
  70
  71         /*
  72          * Add nodes to hash table.
  73          */
  74         for (i = 0; i < CAA_ARRAY_SIZE(values); i++) {
  75                 unsigned long hash;
  76                 int value;
  77
  78                 node = malloc(sizeof(*node));
  79                 if (!node) {
  80                         ret = -1;
  81                         goto end;
  82                 }
  83
  84                 cds_lfht_node_init(&node->node);
  85                 value = values[i];
  86                 node->value = value;
  87                 node->seqnum = seqnum++;
  88                 hash = jhash(&value, sizeof(value), seed);
  89
  90                 /*
  91                  * cds_lfht_add() needs to be called from RCU read-side
  92                  * critical section.
  93                  */
  94                 urcu_memb_read_lock();
  95                 ht_node = cds_lfht_add_unique(ht, hash, match, &value,
  96                         &node->node);
  97                 /*
  98                  * cds_lfht_add_unique() returns the added node if not
  99                  * already present, else returns the node matching the
 100                  * key.
 101                  */
 102                 if (ht_node != &node->node) {
 103                         /*
 104                          * We found a match. Therefore, to ensure we
 105                          * don't add duplicates, cds_lfht_add_unique()
 106                          * acted as a RCU lookup, returning the found
 107                          * match. It did not add the new node to the
 108                          * hash table, so we can free it on the spot.
 109                          */
 110                         printf("Not adding duplicate (key: %d, seqnum: %d)\n",
 111                                 node->value, node->seqnum);
 112                         free(node);
 113                 } else {
 114                         printf("Add (key: %d, seqnum: %d)\n",
 115                                 node->value, node->seqnum);
 116                 }
 117                 urcu_memb_read_unlock();
 118         }
 119
 120         /*
 121          * Iterate over each hash table node. Those will appear in
 122          * random order, depending on the hash seed. Iteration needs to
 123          * be performed within RCU read-side critical section.
 124          */
 125         printf("hash table content (random order):");
 126         urcu_memb_read_lock();
 127         cds_lfht_for_each_entry(ht, &iter, node, node) {
 128                 printf(" (key: %d, seqnum: %d)",
 129                         node->value, node->seqnum);
 130         }
 131         urcu_memb_read_unlock();
 132         printf("\n");
 133
 134 end:
 135         urcu_memb_unregister_thread();
 136         return ret;
 137 }