stdlib/FreeBSD/hcreate.3

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  29 .\" $FreeBSD: src/lib/libc/stdlib/hcreate.3,v 1.7 2008/07/06 17:03:37 danger Exp $
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  31 .Dd July 6, 2008
  32 .Os
  33 .Dt HCREATE 3
  34 .Sh NAME
  35 .Nm hcreate , hdestroy , hsearch
  36 .Nd manage hash search table
  37 .Sh LIBRARY
  38 .Lb libc
  39 .Sh SYNOPSIS
  40 .In search.h
  41 .Ft int
  42 .Fn hcreate "size_t nel"
  43 .Ft void
  44 .Fn hdestroy void
  45 .Ft ENTRY *
  46 .Fn hsearch "ENTRY item" "ACTION action"
  47 .Sh DESCRIPTION
  48 The
  49 .Fn hcreate ,
  50 .Fn hdestroy ,
  51 and
  52 .Fn hsearch
  53 functions manage hash search tables.
  54 .Pp
  55 The
  56 .Fn hcreate
  57 function allocates sufficient space for the table, and the application should
  58 ensure it is called before
  59 .Fn hsearch
  60 is used.
  61 The
  62 .Fa nel
  63 argument is an estimate of the maximum
  64 number of entries that the table should contain.
  65 This number may be adjusted upward by the
  66 algorithm in order to obtain certain mathematically favorable circumstances.
  67 .Pp
  68 The
  69 .Fn hdestroy
  70 function disposes of the search table, and may be followed by another call to
  71 .Fn hcreate .
  72 After the call to
  73 .Fn hdestroy ,
  74 the data can no longer be considered accessible.
  75 The
  76 .Fn hdestroy
  77 function calls
  78 .Xr free 3
  79 for each comparison key in the search table
  80 but not the data item associated with the key.
  81 .Pp
  82 The
  83 .Fn hsearch
  84 function is a hash-table search routine.
  85 It returns a pointer into a hash table
  86 indicating the location at which an entry can be found.
  87 The
  88 .Fa item
  89 argument is a structure of type
  90 .Vt ENTRY
  91 (defined in the
  92 .In search.h
  93 header) containing two pointers:
  94 .Fa item.key
  95 points to the comparison key (a
  96 .Vt "char *" ) ,
  97 and
  98 .Fa item.data
  99 (a
 100 .Vt "void *" )
 101 points to any other data to be associated with
 102 that key.
 103 The comparison function used by
 104 .Fn hsearch
 105 is
 106 .Xr strcmp 3 .
 107 The
 108 .Fa action
 109 argument is a
 110 member of an enumeration type
 111 .Vt ACTION
 112 indicating the disposition of the entry if it cannot be
 113 found in the table.
 114 .Dv ENTER
 115 indicates that the
 116 .Fa item
 117 should be inserted in the table at an
 118 appropriate point.
 119 .Dv FIND
 120 indicates that no entry should be made.
 121 Unsuccessful resolution is
 122 indicated by the return of a
 123 .Dv NULL
 124 pointer.
 125 .Pp
 126 The comparison key (passed to
 127 .Fn hsearch
 128 as
 129 .Fa item.key )
 130 must be allocated using
 131 .Xr malloc 3
 132 if
 133 .Fa action
 134 is
 135 .Dv ENTER
 136 and
 137 .Fn hdestroy
 138 is called.
 139 .Sh RETURN VALUES
 140 The
 141 .Fn hcreate
 142 function returns 0 if the table creation failed and the global variable
 143 .Va errno
 144 is set to indicate the error;
 145 otherwise, a non-zero value is returned.
 146 .Pp
 147 The
 148 .Fn hdestroy
 149 function does not return a value.
 150 .Pp
 151 The
 152 .Fn hsearch
 153 function returns a
 154 .Dv NULL
 155 pointer if either the
 156 .Fa action
 157 is
 158 .Dv FIND
 159 and the
 160 .Fa item
 161 could not be found or the
 162 .Fa action
 163 is
 164 .Dv ENTER
 165 and the table is full.
 166 .Sh EXAMPLES
 167 The following example reads in strings followed by two numbers
 168 and stores them in a hash table, discarding duplicates.
 169 It then reads in strings and finds the matching entry in the hash
 170 table and prints it out.
 171 .Bd -literal
 172 #include <stdio.h>
 173 #include <search.h>
 174 #include <string.h>
 175 #include <stdlib.h>
 176
 177 struct info {                   /* This is the info stored in the table */
 178         int age, room;          /* other than the key. */
 179 };
 180
 181 #define NUM_EMPL        5000    /* # of elements in search table. */
 182
 183 int
 184 main(void)
 185 {
 186         char str[BUFSIZ]; /* Space to read string */
 187         struct info info_space[NUM_EMPL]; /* Space to store employee info. */
 188         struct info *info_ptr = info_space; /* Next space in info_space. */
 189         ENTRY item;
 190         ENTRY *found_item; /* Name to look for in table. */
 191         char name_to_find[30];
 192         int i = 0;
 193
 194         /* Create table; no error checking is performed. */
 195         (void) hcreate(NUM_EMPL);
 196
 197         while (scanf("%s%d%d", str, &info_ptr->age,
 198             &info_ptr->room) != EOF && i++ < NUM_EMPL) {
 199                 /* Put information in structure, and structure in item. */
 200                 item.key = strdup(str);
 201                 item.data = info_ptr;
 202                 info_ptr++;
 203                 /* Put item into table. */
 204                 (void) hsearch(item, ENTER);
 205         }
 206
 207         /* Access table. */
 208         item.key = name_to_find;
 209         while (scanf("%s", item.key) != EOF) {
 210                 if ((found_item = hsearch(item, FIND)) != NULL) {
 211                         /* If item is in the table. */
 212                         (void)printf("found %s, age = %d, room = %d\en",
 213                             found_item->key,
 214                             ((struct info *)found_item->data)->age,
 215                             ((struct info *)found_item->data)->room);
 216                 } else
 217                         (void)printf("no such employee %s\en", name_to_find);
 218         }
 219         hdestroy();
 220         return 0;
 221 }
 222 .Ed
 223 .Sh ERRORS
 224 The
 225 .Fn hcreate
 226 and
 227 .Fn hsearch
 228 functions may fail if:
 229 .Bl -tag -width Er
 230 .It Bq Er ENOMEM
 231 Insufficient storage space is available.
 232 .It Bq Er EINVAL
 233 A table already exists.
 234 .El
 235 .Sh SEE ALSO
 236 .Xr bsearch 3 ,
 237 .Xr lsearch 3 ,
 238 .Xr malloc 3 ,
 239 .Xr strcmp 3 ,
 240 .Xr tsearch 3
 241 .Sh STANDARDS
 242 The
 243 .Fn hcreate ,
 244 .Fn hdestroy ,
 245 and
 246 .Fn hsearch
 247 functions conform to
 248 .St -xpg4.2 .
 249 .Sh HISTORY
 250 The
 251 .Fn hcreate ,
 252 .Fn hdestroy ,
 253 and
 254 .Fn hsearch
 255 functions first appeared in
 256 .At V .
 257 .Sh BUGS
 258 The interface permits the use of only one hash table at a time.