NAME
hcreate, hdestroy, hsearch, hcreater, hdestroyr, hsearchr - hash ta‐ ble management SYNOPSIS
#include
int hcreate(sizet nel); ENTRY *hsearch(ENTRY item, ACTION action); void hdestroy(void); #define GNUSOURCE /* See featuretestmacros(7) */
#include
int hcreater(sizet nel, struct hsearchdata *htab); int hsearchr(ENTRY item, ACTION action, ENTRY **retval, struct hsearchdata *htab); void hdestroyr(struct hsearchdata *htab); DESCRIPTION The three functions hcreate(), hsearch(), and hdestroy() allow the caller to create and manage a hash search table containing entries con‐ sisting of a key (a string) and associated data. Using these func‐ tions, only one hash table can be used at a time. The three functions hcreater(), hsearchr(), hdestroyr() are reen‐ trant versions that allow a program to use more than one hash search table at the same time. The last argument, htab, points to a structure that describes the table on which the function is to operate. The pro‐ grammer should treat this structure as opaque (i.e., do not attempt to directly access or modify the fields in this structure). First a hash table must be created using hcreate(). The argument nel specifies the maximum number of entries in the table. (This maximum cannot be changed later, so choose it wisely.) The implementation may adjust this value upward to improve the performance of the resulting hash table. The hcreater() function performs the same task as hcreate(), but for the table described by the structure *htab. The structure pointed to by htab must be zeroed before the first call to hcreater(). The function hdestroy() frees the memory occupied by the hash table that was created by hcreate(). After calling hdestroy() a new hash ta‐ ble can be created using hcreate(). The hdestroyr() function performs the analogous task for a hash table described by *htab, which was pre‐ viously created using hcreater(). The hsearch() function searches the hash table for an item with the same key as item (where "the same" is determined using strcmp(3)), and if successful returns a pointer to it. The argument item is of type ENTRY, which is defined in as follows: typedef struct entry { char *key; void *data; } ENTRY; The field key points to a null-terminated string which is the search key. The field data points to data that is associated with that key. The argument action determines what hsearch() does after an unsuccess‐ ful search. This argument must either have the value ENTER, meaning insert a copy of item (and return a pointer to the new hash table entry as the function result), or the value FIND, meaning that NULL should be returned. (If action is FIND, then data is ignored.) The hsearchr() function is like hsearch() but operates on the hash ta‐ ble described by *htab. The hsearchr() function differs from hsearch() in that a pointer to the found item is returned in *retval, rather than as the function result. RETURN VALUE hcreate() and hcreater() return nonzero on success. They return 0 on error, with errno set to indicate the cause of the error. On success, hsearch() returns a pointer to an entry in the hash table. hsearch() returns NULL on error, that is, if action is ENTER and the hash table is full, or action is FIND and item cannot be found in the hash table. hsearchr() returns nonzero on success, and 0 on error. In the event of an error, these two functions set errno to indicate the cause of the error. ERRORS hcreater() and hdestroyr() can fail for the following reasons: EINVAL htab is NULL. hsearch() and hsearchr() can fail for the following reasons: ENOMEM action was ENTER, key was not found in the table, and there was no room in the table to add a new entry. ESRCH action was FIND, and key was not found in the table.
POSIX.1-2001 specifies only the ENOMEM error. ATTRIBUTES For an explanation of the terms used in this section, see attributes(7). ┌──────────────────────────┬───────────────┬────────────────────────┐ │Interface │ Attribute │ Value │ ├──────────────────────────┼───────────────┼────────────────────────┤
│hcreate(), hsearch(), │ Thread safety │ MT-Unsafe race:hsearch │ │hdestroy() │ │ │ ├──────────────────────────┼───────────────┼────────────────────────┤
│hcreater(), hsearchr(), │ Thread safety │ MT-Safe race:htab │ │hdestroyr() │ │ │ └──────────────────────────┴───────────────┴────────────────────────┘ CONFORMING TO The functions hcreate(), hsearch(), and hdestroy() are from SVr4, and
are described in POSIX.1-2001. The functions hcreater(), hsearchr(), and hdestroyr() are GNU extensions. NOTES Hash table implementations are usually more efficient when the table contains enough free space to minimize collisions. Typically, this
means that nel should be at least 25% larger than the maximum number of elements that the caller expects to store in the table. The hdestroy() and hdestroyr() functions do not free the buffers pointed to by the key and data elements of the hash table entries. (It can't do this because it doesn't know whether these buffers were allo‐ cated dynamically.) If these buffers need to be freed (perhaps because the program is repeatedly creating and destroying hash tables, rather than creating a single table whose lifetime matches that of the pro‐ gram), then the program must maintain bookkeeping data structures that allow it to free them. BUGS
SVr4 and POSIX.1-2001 specify that action is significant only for unsuccessful searches, so that an ENTER should not do anything for a successful search. In libc and glibc (before version 2.3), the imple‐ mentation violates the specification, updating the data for the given key in this case. Individual hash table entries can be added, but not deleted. EXAMPLE The following program inserts 24 items into a hash table, then prints some of them.
#include
#include
#include
char *data[] = { "alpha", "bravo", "charlie", "delta", "echo", "foxtrot", "golf", "hotel", "india", "juliet", "kilo", "lima", "mike", "november", "oscar", "papa", "quebec", "romeo", "sierra", "tango", "uniform", "victor", "whisky", "x-ray", "yankee", "zulu" }; int main(void) { ENTRY e, *ep; int i; hcreate(30); for (i = 0; i < 24; i++) { e.key = data[i]; /* data is just an integer, instead of a pointer to something */ e.data = (void *) i; ep = hsearch(e, ENTER); /* there should be no failures */ if (ep == NULL) { fprintf(stderr, "entry failed\n"); exit(EXITFAILURE); } } 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); } hdestroy(); exit(EXITSUCCESS); } SEE ALSO bsearch(3), lsearch(3), malloc(3), tsearch(3) COLOPHON
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GNU 2013-07-22 HSEARCH(3)