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Man Pages
OHASH_INTERVAL(3) FreeBSD Library Functions Manual OHASH_INTERVAL(3)

ohash_interval, ohash_create_entry, ohash_qlookup, ohash_qlookupi
helper functions for open hashing

OpenBSD Utilities Library (libopenbsd, -lopenbsd)

#include <stdint.h>
#include <stddef.h>
#include <ohash.h>
uint32_t
ohash_interval(const char *start, const char **pend);
void *
ohash_create_entry(struct ohash_info *info, const char *start, const char **pend);
unsigned int
ohash_qlookupi(struct ohash *h, const char *start, const char **pend);
unsigned int
ohash_qlookup(struct ohash *h, const char *start);

These functions are commonly used to simplify open hashing usage, and use similar conventions. They operate indifferently on NUL-terminated strings (by setting *pend = NULL) or memory ranges (delimited by start and *pend). For NUL-terminated strings, as a side effect, those functions set *pend to the terminating NUL byte.
ohash_interval() is a simple hashing function that yields good results on common data sets.
ohash_create_entry() can be used to create a new record with a given key. In that case, the alloc field of info should point to a malloc(3)-like function to allocate the storage:
p = (*info->alloc)(sz, info->data);
ohash_qlookupi() is a wrapper function that simply calls ohash_interval() and ohash_lookup_interval().
ohash_qlookup() is a variation on ohash_qlookupi() designed for NUL-terminated strings.

ohash_init(3)

Those functions are completely non-standard and should be avoided in portable programs.

Those functions were designed and written for OpenBSD make(1) by Marc Espie in 1999.
June 5, 2013 FreeBSD 12.0-RELEASE

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