hash32_strn(9): hash - general kernel hashing functions

HASH(9) FreeBSD Kernel Developer's Manual HASH(9)


hash, hash32, hash32_buf, hash32_str, hash32_strn, hash32_stre, hash32_strnegeneral kernel hashing functions


#include <sys/hash.h>
hash32_buf(const void *buf, size_t len, uint32_t hash);
hash32_str(const void *buf, uint32_t hash);
hash32_strn(const void *buf, size_t len, uint32_t hash);
hash32_stre(const void *buf, int end, const char **ep, uint32_t hash);
hash32_strne(const void *buf, size_t len, int end, const char **ep, uint32_t hash);


The hash32() functions are used to give a consistent and general interface to a decent hashing algorithm within the kernel. These functions can be used to hash ASCII NUL terminated strings, as well as blocks of memory.
The hash32_buf() function is used as a general buffer hashing function. The argument buf is used to pass in the location, and len is the length of the buffer. The argument hash is used to extend an existing hash, or is passed the initial value HASHINIT to start a new hash.
The hash32_str() function is used to hash a NUL terminated string passed in buf with initial hash value given in hash.
The hash32_strn() function is like the hash32_str() function, except it also takes a len argument, which is the maximal length of the expected string.
The hash32_stre() and hash32_strne() functions are helper functions used by the kernel to hash pathname components. These functions have the additional termination condition of terminating when they find a character given by end in the string to be hashed. If the argument ep is not NULL, it is set to the point in the buffer at which the hash function terminated hashing.


The hash32() functions return a 32 bit hash value of the buffer or string.


LIST_HEAD(head, cache) *hashtbl = NULL; 
u_long mask = 0; 
        hashtbl = hashinit(numwanted, type, flags, &mask); 
sample_use(char *str, int len) 
        uint32_t hash; 
        hash = hash32_str(str, HASHINIT); 
        hash = hash32_buf(&len, sizeof(len), hash); 
        hashtbl[hash & mask] = len; 


The hash32() functions are only 32 bit functions. They will prove to give poor 64 bit performance, especially for the top 32 bits. At the current time, this is not seen as a great limitation, as these hash values are usually used to index into an array. Should these hash values be used for other means, this limitation should be revisited.


The hash functions were first committed to NetBSD 1.6. The OpenBSD versions were written and massaged for OpenBSD 2.3 by Tobias Weingartner, and finally committed for OpenBSD 3.2.
April 3, 2007 FreeBSD 9.0

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