NAME

hcreate, hdestroy, hsearch — manage hash search table

SYNOPSIS

#include <search.h>

int
hcreate(size_t nel);

void
hdestroy(void);

ENTRY *
hsearch(ENTRY item, ACTION action);

DESCRIPTION

The hcreate(), hdestroy(), and hsearch() 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 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).

RETURN VALUES

If successful, the hcreate() function returns a non-zero value. Otherwise, a value of 0 is returned and errno is set to indicate the error.

If successful, the hsearch() function returns a pointer to a 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.

ERRORS

The hcreate() and hsearch() functions will fail if:

[ENOMEM]: Insufficient memory is available.

STANDARDS

The hcreate(), hdestroy() and hsearch() functions conform to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).

HISTORY

The hcreate(), hdestroy() and hsearch() functions first appeared in AT&T System V UNIX.

CAVEATS

At least the following limitations can be mentioned:

The 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.