HCREATE(3)

NAME
hcreate, hcreate_r, hdestroy, hdestroy_r, hsearch, hsearch_r - manage hash search table

LIBRARY
libc

SYNOPSIS
#include <search.h>
int
hcreate(size_t nel);
int
hcreate_r(size_t nel, struct hsearch_data *table);
void
hdestroy(void);
void
hdestroy_r(struct hsearch_data *table);
ENTRY *
hsearch(ENTRY item, ACTION action);
int
hsearch_r(ENTRY item, ACTION action, ENTRY ** itemp, struct hsearch_data *table);

DESCRIPTION
The hcreate(), hcreate_r(), hdestroy(), hdestroy_r() hsearch(), and hsearch_r() functions manage hash search tables.

The hcreate() function allocates and initializes the table. The nel argument specifies an estimate of the maximum number of entries to be held by the table. Unless further memory allocation fails, supplying an insufficient nel value will not result in functional harm, although a performance degradation may occur. Initialization using the hcreate() function is mandatory prior to any access operations using hsearch().

The hdestroy() function destroys a table previously created using hcreate(). After a call to hdestroy(), the data can no longer be accessed.

The hsearch() function is used to search to the hash table. It returns a pointer into the hash table indicating the address of an item. The item argument is of type ENTRY, defined in the <search.h> header. This is a structure type that contains two pointers:

char *key
    comparison key
void *data
    pointer to data associated with key

The key comparison function used by hsearch() is strcmp(3).

The action argument is of type ACTION, an enumeration type which defines the following values:

ENTER
    Insert item into the hash table. If an existing item with the same key is found, it is not replaced. Note that the key and data elements of item are used directly by the new table entry. The storage for the key must not be modified during the lifetime of the hash table.

FIND
    Search the hash table without inserting item.

Note that the comparison key must be allocated using malloc(3) or calloc(3) if action is ENTER and hdestroy() will be called. This is because hdestroy() will call free(3) for each comparison key (but not data). Typically the comparison key is allocated by using strdup(3).

The hcreate_r(), hdestroy_r(), and hsearch_r() functions are re-entrant versions of the above functions that can operate on a table supplied by the user. The hsearch_r() function returns 0 if the action is ENTER and the element cannot be created, 1 otherwise. If the element exists or can be created, it will be placed in itemp, otherwise itemp will be set to NULL.

RETURN VALUES
If successful, the hcreate() and hcreate_r() functions return a non-zero value. Otherwise, a value of 0 is returned and errno is set to indicate the error.

The hdestroy() and hdestroy_r() functions return no value.

If successful, the hsearch() function returns a pointer to hash table entry matching the provided key. If the action is FIND and the item was not found, or if the action is ENTER and the insertion failed, NULL is returned and errno is set to indicate the error. If the action is ENTER and an entry already existed in the table matching the given key, the existing entry is returned and is not replaced.

The hsearch_r() function returns 1 unless the table is full, when it returns 0. If hsearch() returns 0 or the element is not found, errno is set to indicate the error.

ERRORS
The hcreate(), hcreate_r(), hsearch() and hsearch_r() functions will fail if:

[ENOMEM]
    Insufficient memory is available.

The hsearch() and hsearch_r() functions will also fail if the action is SEARCH and the element is not found:

[ESRCH]
    The item given is not found.

SEE ALSO
bsearch(3), lsearch(3), malloc(3), strcmp(3)

STANDARDS
The hcreate(), hdestroy() and hsearch() functions conform to xpg4.2.

HISTORY
The hcreate(), hdestroy() and hsearch() functions first appeared in AT&T UNIX. The hcreate_r(), hdestroy_r() and hsearch_r() functions are GNU extensions.

CAVEATS
At least the following limitations can be mentioned:

•   The original, non-GNU interface permits the use of only one hash table at a time.

•   Individual hash table entries can be added, but not deleted.

•   The standard is indecipherable about the internal memory usage of the functions, mentioning only that “ hcreate() and hsearch() functions may use malloc() to allocate space ”. This limits the portability of the functions, given that other implementations may not free(3) the buffer pointed by key.