HSEARCH Section: Linux Programmer's Manual (3) Updated: September 30, 2025 Index   NAME hcreate, hdestroy, hsearch - hash table management   SYNOPSIS #include <search.h> ENTRY *hsearch(ENTRY item, ACTION action); int hcreate(unsigned nel); void hdestroy(void);   DESCRIPTION These three functions allow the user to create a hash table which associates a key with any data. First the table must be created with the function hcreate(). nel is an estimate of the number of entries in the table. hcreate() may adjust this value upward to improve the performance of the resulting hash table. The GNU implementation of hsearch() will also enlarge the table if it gets nearly full. The corresponding function hdestroy() frees the memory occupied by the hash table so that a new table can be constructed. item is of type ENTRY, which is a typedef defined in <search.h> and includes these elements: typedef struct entry { char *key; char *data; } ENTRY; key points to the zero-terminated ASCII string which is the search key. data points to the data associated with that key. (A pointer to a type other than character should be cast to pointer-to-character.) hsearch() searches the hash table for an item with the same key as item, and if successful returns a pointer to it. action determines what hsearch() does after an unsuccessful search. A value of ENTER instructs it to insert the new item, while a value of FIND means to return NULL.   RETURN VALUE hcreate() returns NULL if the hash table cannot be successfully installed. hsearch() returns NULL if action is ENTER and there is insufficient memory to expand the hash table, or action is FIND and item cannot be found in the hash table.   CONFORMS TO SVID, except that in SysV, the hash table cannot grow.   BUGS The implementation can manage only one hash table at a time. Individual hash table entries can be added, but not deleted.   EXAMPLE The following program inserts 24 items in to a hash table, then prints some of them. #include <stdio.h> #include <search.h> char *data[]={ "alpha", "bravo", "charley", "delta", "echo", "foxtrot", "golf", "hotel", "india", "juliette", "kilo", "lima", "mike", "november", "oscar", "papa", "quebec", "romeo", "sierra", "tango", "uniform", "victor", "whiskey", "x-ray", "yankee", "zulu" }; int main() { ENTRY e, *ep; int i; /* start with small table, and let it grow */ for (i = 0; i < 24; i++) { e.key = data[i]; /* data is just an integer, instead of a pointer to something */ e.data = (char *)i; ep = hsearch(e, ENTER); /* there should be no failures */ } for (i = 22; i < 26; i++) /* print two entries from the table, and show that two are not in the table */ { e.key = data[i]; ep = hsearch(e, FIND); printf("%9.9s -> %9.9s:%d\n", e.key, ep?ep->key:"NULL", ep?(int)(ep->data):0); } return 0; }   SEE ALSO   Index NAME SYNOPSIS DESCRIPTION RETURN VALUE CONFORMS TO BUGS EXAMPLE SEE ALSO This document was created by using the manual pages. Time: 11:58:04 GMT, March 24, 2025