HCREATE(3) | Library Functions Manual | HCREATE(3)

# 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.

# SEE ALSO

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

# 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.

January 30, 2018 | Debian