hsearch(3)
NAME
hsearch, hcreate, hdestroy − manage hash search tables
SYNTAX
#include <search.h>
ENTRY ∗hsearch (item, action)
ENTRY item;
ACTION action;
int hcreate (nel)
unsigned nel;
void hdestroy ( )
DESCRIPTION
The hsearch subroutine is a hash-table search routine generalized from Knuth (6.4) Algorithm D. It returns a pointer into a hash table indicating the location at which an entry can be found. The item is a structure of type ENTRY (defined in the <search.h> header file) containing two pointers: item.key points to the comparison key, and item.data points to any other data to be associated with that key. (Pointers to types other than character should be cast to pointer-to-character.) The action is a member of an enumeration type ACTION indicating the disposition of the entry if it cannot be found in the table. ENTER indicates that the item should be inserted in the table at an appropriate point. FIND indicates that no entry should be made. Unsuccessful resolution is indicated by the return of a NULL pointer.
The hcreate subroutine allocates sufficient space for the table, and must be called before hsearch is used. The nel is an estimate of the maximum number of entries that the table will contain. This number may be adjusted upward by the algorithm in order to obtain certain mathematically favorable circumstances.
The hdestroy subroutine destroys the search table, and may be followed by another call to hcreate.
NOTES
The hsearch subroutine uses open addressing with a multiplicative hash function. However, its source code has many other options available which the user may select by compiling the hsearch source with the following symbols defined to the preprocessor:
DIV Use the remainder modulo table size as the hash function instead of the multiplicative algorithm.
USCR Use a User Supplied Comparison Routine for ascertaining table membership. The routine should be named hcompar and should behave in a manner similar to strcmp. For further information, see string(3).
CHAINED
Use a linked list to resolve collisions. If this option is selected, the following other options become available.
START Place new entries at the beginning of the linked list (default is at the end).
SORTUP Keep the linked list sorted by key in ascending order.
SORTDOWN
Keep the linked list sorted by key in descending order.
Additionally, there are preprocessor flags for obtaining debugging printout (−DDEBUG) and for including a test driver in the calling routine (−DDRIVER). The source code should be consulted for further details.
RESTRICTIONS
Only one hash search table may be active at any given time.
DIAGNOSTICS
The hsearch subroutine returns a NULL pointer if either the action is FIND and the item could not be found or the action is ENTER and the table is full.
The hcreate subroutine returns zero if it cannot allocate sufficient space for the table.