207 lines
4.3 KiB
Groff
207 lines
4.3 KiB
Groff
.\" $FreeBSD$
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.\"
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.Dd May 8, 2001
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.Os
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.Dt HCREATE 3
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.Sh NAME
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.Nm hcreate , hdestroy , hsearch
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.Nd manage hash search table
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In search.h
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.Ft int
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.Fn hcreate "size_t nel"
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.Ft void
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.Fn hdestroy void
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.Ft ENTRY *
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.Fn hsearch "ENTRY item" "ACTION action"
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.Sh DESCRIPTION
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The
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.Fn hcreate ,
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.Fn hdestroy ,
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and
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.Fn hsearch
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functions manage hash search tables.
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.Pp
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The
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.Fn hcreate
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function allocates sufficient space for the table, and the application should
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ensure it is called before
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.Fn hsearch
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is used.
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The
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.Fa nel
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argument is an estimate of the maximum
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number of entries that the table should contain.
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This number may be adjusted upward by the
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algorithm in order to obtain certain mathematically favorable circumstances.
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.Pp
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The
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.Fn hdestroy
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function disposes of the search table, and may be followed by another call to
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.Fn hcreate .
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After the call to
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.Fn hdestroy ,
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the data can no longer be considered accessible.
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.Pp
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The
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.Fn hsearch
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function is a hash-table search routine.
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It returns a pointer into a hash table
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indicating the location at which an entry can be found.
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The
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.Fa item
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argument is a structure of type
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.Vt ENTRY
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(defined in the
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.Aq Pa search.h
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header) containing two pointers:
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.Fa item.key
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points to the comparison key (a
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.Vt "char *" ) ,
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and
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.Fa item.data
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(a
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.Vt "void *" )
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points to any other data to be associated with
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that key.
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The comparison function used by
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.Fn hsearch
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is
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.Xr strcmp 3 .
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The
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.Fa action
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argument is a
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member of an enumeration type
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.Vt ACTION
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indicating the disposition of the entry if it cannot be
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found in the table.
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.Dv ENTER
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indicates that the
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.Fa item
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should be inserted in the table at an
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appropriate point.
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.Dv FIND
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indicates that no entry should be made.
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Unsuccessful resolution is
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indicated by the return of a
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.Dv NULL
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pointer.
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.Sh RETURN VALUES
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The
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.Fn hcreate
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function returns 0 if it cannot allocate sufficient space for the table;
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otherwise, it returns non-zero.
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.Pp
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The
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.Fn hdestroy
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function does not return a value.
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.Pp
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The
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.Fn hsearch
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function returns a
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.Dv NULL
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pointer if either the
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.Fa action
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is
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.Dv FIND
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and the
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.Fa item
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could not be found or the
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.Fa action
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is
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.Dv ENTER
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and the table is full.
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.Sh ERRORS
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The
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.Fn hcreate
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and
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.Fn hsearch
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functions may fail if:
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.Bl -tag -width Er
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.It Bq Er ENOMEM
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Insufficient storage space is available.
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.El
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.Sh EXAMPLES
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The following example reads in strings followed by two numbers
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and stores them in a hash table, discarding duplicates.
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It then reads in strings and finds the matching entry in the hash
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table and prints it out.
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.Bd -literal
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#include <stdio.h>
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#include <search.h>
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#include <string.h>
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struct info { /* This is the info stored in the table */
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int age, room; /* other than the key. */
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};
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#define NUM_EMPL 5000 /* # of elements in search table. */
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int
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main(void)
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{
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char string_space[NUM_EMPL*20]; /* Space to store strings. */
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struct info info_space[NUM_EMPL]; /* Space to store employee info. */
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char *str_ptr = string_space; /* Next space in string_space. */
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struct info *info_ptr = info_space; /* Next space in info_space. */
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ENTRY item;
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ENTRY *found_item; /* Name to look for in table. */
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char name_to_find[30];
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int i = 0;
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/* Create table; no error checking is performed. */
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(void) hcreate(NUM_EMPL);
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while (scanf("%s%d%d", str_ptr, &info_ptr->age,
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&info_ptr->room) != EOF && i++ < NUM_EMPL) {
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/* Put information in structure, and structure in item. */
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item.key = str_ptr;
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item.data = info_ptr;
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str_ptr += strlen(str_ptr) + 1;
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info_ptr++;
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/* Put item into table. */
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(void) hsearch(item, ENTER);
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}
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/* Access table. */
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item.key = name_to_find;
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while (scanf("%s", item.key) != EOF) {
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if ((found_item = hsearch(item, FIND)) != NULL) {
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/* If item is in the table. */
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(void)printf("found %s, age = %d, room = %d\en",
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found_item->key,
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((struct info *)found_item->data)->age,
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((struct info *)found_item->data)->room);
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} else
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(void)printf("no such employee %s\en", name_to_find);
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}
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return 0;
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}
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.Ed
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.Sh SEE ALSO
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.Xr bsearch 3 ,
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.Xr lsearch 3 ,
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.Xr malloc 3 ,
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.Xr strcmp 3 ,
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.Xr tsearch 3
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.Sh STANDARDS
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The
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.Fn hcreate ,
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.Fn hdestroy ,
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and
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.Fn hsearch
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functions conform to
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.St -xpg4.2 .
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.Sh HISTORY
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The
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.Fn hcreate ,
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.Fn hdestroy ,
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and
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.Fn hsearch
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functions first appeared in
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.At V .
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.Sh BUGS
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The interface permits the use of only one hash table at a time.
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