mirror of https://github.com/mkerrisk/man-pages
176 lines
5.5 KiB
Plaintext
176 lines
5.5 KiB
Plaintext
.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved
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.TH "HCREATE" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
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.\" hcreate
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.SH NAME
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hcreate, hdestroy, hsearch \- manage hash search table
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.SH SYNOPSIS
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.LP
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\fB#include <search.h>
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.br
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.sp
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int hcreate(size_t\fP \fInel\fP\fB);
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.br
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void hdestroy(void);
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.br
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ENTRY *hsearch(ENTRY\fP \fIitem\fP\fB, ACTION\fP \fIaction\fP\fB);
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\fP
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\fB
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.br
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\fP
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.SH DESCRIPTION
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.LP
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The \fIhcreate\fP(), \fIhdestroy\fP(), and \fIhsearch\fP() functions
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shall manage hash search tables.
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.LP
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The \fIhcreate\fP() function shall allocate sufficient space for the
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table, and the application shall ensure it is called
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before \fIhsearch\fP() is used. The \fInel\fP argument is an estimate
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of the maximum number of entries that the table shall
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contain. This number may be adjusted upward by the algorithm in order
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to obtain certain mathematically favorable circumstances.
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.LP
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The \fIhdestroy\fP() function shall dispose of the search table, and
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may be followed by another call to \fIhcreate\fP(). After
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the call to \fIhdestroy\fP(), the data can no longer be considered
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accessible.
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.LP
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The \fIhsearch\fP() function is a hash-table search routine. It shall
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return a pointer into a hash table indicating the
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location at which an entry can be found. The \fIitem\fP argument is
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a structure of type \fBENTRY\fP (defined in the \fI<search.h>\fP header)
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containing two pointers: \fIitem.key\fP points to the comparison
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key (a \fBchar *\fP), and \fIitem.data\fP (a \fBvoid *\fP) points
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to any other data to be associated with that key. The
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comparison function used by \fIhsearch\fP() is \fIstrcmp\fP(). The
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\fIaction\fP argument
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is a member of an enumeration type \fBACTION\fP indicating the disposition
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of the entry if it cannot be found in the table. ENTER
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indicates that the item should be inserted in the table at an appropriate
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point. FIND indicates that no entry should be made.
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Unsuccessful resolution is indicated by the return of a null pointer.
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.LP
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These functions need not be reentrant. A function that is not required
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to be reentrant is not required to be thread-safe.
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.SH RETURN VALUE
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.LP
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The \fIhcreate\fP() function shall return 0 if it cannot allocate
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sufficient space for the table; otherwise, it shall return
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non-zero.
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.LP
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The \fIhdestroy\fP() function shall not return a value.
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.LP
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The \fIhsearch\fP() function shall return a null pointer if either
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the action is FIND and the item could not be found or the
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action is ENTER and the table is full.
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.SH ERRORS
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.LP
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The \fIhcreate\fP() and \fIhsearch\fP() functions may fail if:
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.TP 7
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.B ENOMEM
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Insufficient storage space is available.
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.sp
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.LP
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\fIThe following sections are informative.\fP
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.SH EXAMPLES
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.LP
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The following example reads in strings followed by two numbers and
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stores them in a hash table, discarding duplicates. It then
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reads in strings and finds the matching entry in the hash table and
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prints it out.
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.sp
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.RS
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.nf
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\fB#include <stdio.h>
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#include <search.h>
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#include <string.h>
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.sp
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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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.sp
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#define NUM_EMPL 5000 /* # of elements in search table. */
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.sp
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int 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;
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/* 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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.sp
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int i = 0;
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.sp
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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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.sp
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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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.sp
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/* Put item into table. */
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(void) hsearch(item, ENTER);
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}
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.sp
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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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.sp
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/* If item is in the table. */
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(void)printf("found %s, age = %d, room = %d\\n",
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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\\n", name_to_find);
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}
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return 0;
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}
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\fP
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.fi
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.RE
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.SH APPLICATION USAGE
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.LP
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The \fIhcreate\fP() and \fIhsearch\fP() functions may use \fImalloc\fP()
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to allocate
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space.
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.SH RATIONALE
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.LP
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None.
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.SH FUTURE DIRECTIONS
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.LP
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None.
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.SH SEE ALSO
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.LP
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\fIbsearch\fP() , \fIlsearch\fP() , \fImalloc\fP() , \fIstrcmp\fP()
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, \fItsearch\fP() , the
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Base Definitions volume of IEEE\ Std\ 1003.1-2001, \fI<search.h>\fP
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.SH COPYRIGHT
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Portions of this text are reprinted and reproduced in electronic form
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from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
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-- Portable Operating System Interface (POSIX), The Open Group Base
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Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
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Electrical and Electronics Engineers, Inc and The Open Group. In the
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event of any discrepancy between this version and the original IEEE and
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The Open Group Standard, the original IEEE and The Open Group Standard
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is the referee document. The original Standard can be obtained online at
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http://www.opengroup.org/unix/online.html .
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