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elf: Implement a string table for ldconfig, with tail merging

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This will be used in ldconfig to reduce the ld.so.cache size slightly.

Tail merging is an optimization where a pointer points into another
string if the first string is a suffix of the second string.

The hash function FNV-1a was chosen because it is simple and achieves
good dispersion even for short strings (so that the hash table bucket
count can be a power of two).  It is clearly superior to the hsearch
hash and the ELF hash in this regard.

The hash table uses chaining for collision resolution.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>
This commit is contained in:
Florian Weimer
2020-12-04 09:13:43 +01:00
parent dfb3f101c5
commit 785969a047
5 changed files with 488 additions and 1 deletions
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elf/stringtable.h Normal file
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/* String tables for ld.so.cache construction.
Copyright (C) 2020 Free Software Foundation, Inc.
This file is part of the GNU C Library.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published
by the Free Software Foundation; version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, see <https://www.gnu.org/licenses/>. */
#ifndef _STRINGTABLE_H
#define _STRINGTABLE_H
#include <stddef.h>
#include <stdint.h>
/* An entry in the string table. Only the length and string fields are
expected to be used outside the string table code. */
struct stringtable_entry
{
struct stringtable_entry *next; /* For collision resolution. */
uint32_t length; /* Length of then string. */
uint32_t offset; /* From start of finalized table. */
char string[]; /* Null-terminated string. */
};
/* A string table. Zero-initialization produces a valid atable. */
struct stringtable
{
struct stringtable_entry **entries; /* Array of hash table buckets. */
uint32_t count; /* Number of elements in the table. */
uint32_t allocated; /* Length of the entries array. */
};
/* Adds STRING to TABLE. May return the address of an existing entry. */
struct stringtable_entry *stringtable_add (struct stringtable *table,
const char *string);
/* Result of stringtable_finalize. SIZE bytes at STRINGS should be
written to the file. */
struct stringtable_finalized
{
char *strings;
size_t size;
};
/* Assigns offsets to string table entries and computes the serialized
form of the string table. */
void stringtable_finalize (struct stringtable *table,
struct stringtable_finalized *result);
/* Deallocate the string table (but not the TABLE pointer itself).
(The table can be re-used for adding more strings without
initialization.) */
void stringtable_free (struct stringtable *table);
#endif /* _STRINGTABLE_H */