HCREATE(3) BSD Library Functions Manual HCREATE(3)
hcreate, hcreate_r, hdestroy, hdestroy_r, hsearch, hsearch_r -- manage hash search table
Standard C Library (libc, -lc)
hcreate_r(size_t nel, struct hsearch_data *table);
hdestroy_r(struct hsearch_data *table);
hsearch(ENTRY item, ACTION action);
hsearch_r(ENTRY item, ACTION action, ENTRY ** itemp, struct hsearch_data *table);
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 val-
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
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
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 exist-
ing 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.
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 ele-
ment is not found:
[ESRCH] The item given is not found.
bsearch(3), lsearch(3), malloc(3), strcmp(3)
The hcreate(), hdestroy() and hsearch() functions conform to X/Open Portability Guide
Issue 4, Version 2 (``XPG4.2'').
The hcreate(), hdestroy() and hsearch() functions first appeared in AT&T System V UNIX. The
hcreate_r(), hdestroy_r() and hsearch_r() functions are GNU extensions.
At least the following limitations can be mentioned:
o The original, non-GNU interface permits the use of only one hash table at a time.
o Individual hash table entries can be added, but not deleted.
o The standard is indecipherable about the internal memory usage of the functions, men-
tioning 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.
BSD September 14, 2011 BSD