#include "libbfd.h"
#include "obstack.h"
+/*
+SECTION
+ Hash Tables
+
+@cindex Hash tables
+ BFD provides a simple set of hash table functions. Routines
+ are provided to initialize a hash table, to free a hash table,
+ to look up a string in a hash table and optionally create an
+ entry for it, and to traverse a hash table. There is
+ currently no routine to delete an string from a hash table.
+
+ The basic hash table does not permit any data to be stored
+ with a string. However, a hash table is designed to present a
+ base class from which other types of hash tables may be
+ derived. These derived types may store additional information
+ with the string. Hash tables were implemented in this way,
+ rather than simply providing a data pointer in a hash table
+ entry, because they were designed for use by the linker back
+ ends. The linker may create thousands of hash table entries,
+ and the overhead of allocating private data and storing and
+ following pointers becomes noticeable.
+
+ The basic hash table code is in <<hash.c>>.
+
+@menu
+@* Creating and Freeing a Hash Table::
+@* Looking Up or Entering a String::
+@* Traversing a Hash Table::
+@* Deriving a New Hash Table Type::
+@end menu
+
+INODE
+Creating and Freeing a Hash Table, Looking Up or Entering a String, Hash Tables, Hash Tables
+SUBSECTION
+ Creating and freeing a hash table
+
+@findex bfd_hash_table_init
+@findex bfd_hash_table_init_n
+ To create a hash table, create an instance of a <<struct
+ bfd_hash_table>> (defined in <<bfd.h>>) and call
+ <<bfd_hash_table_init>> (if you know approximately how many
+ entries you will need, the function <<bfd_hash_table_init_n>>,
+ which takes a @var{size} argument, may be used).
+ <<bfd_hash_table_init>> returns <<false>> if some sort of
+ error occurs.
+
+@findex bfd_hash_newfunc
+ The function <<bfd_hash_table_init>> take as an argument a
+ function to use to create new entries. For a basic hash
+ table, use the function <<bfd_hash_newfunc>>. @xref{Deriving
+ a New Hash Table Type} for why you would want to use a
+ different value for this argument.
+
+@findex bfd_hash_allocate
+ <<bfd_hash_table_init>> will create an obstack which will be
+ used to allocate new entries. You may allocate memory on this
+ obstack using <<bfd_hash_allocate>>.
+
+@findex bfd_hash_table_free
+ Use <<bfd_hash_table_free>> to free up all the memory that has
+ been allocated for a hash table. This will not free up the
+ <<struct bfd_hash_table>> itself, which you must provide.
+
+INODE
+Looking Up or Entering a String, Traversing a Hash Table, Creating and Freeing a Hash Table, Hash Tables
+SUBSECTION
+ Looking up or entering a string
+
+@findex bfd_hash_lookup
+ The function <<bfd_hash_lookup>> is used both to look up a
+ string in the hash table and to create a new entry.
+
+ If the @var{create} argument is <<false>>, <<bfd_hash_lookup>>
+ will look up a string. If the string is found, it will
+ returns a pointer to a <<struct bfd_hash_entry>>. If the
+ string is not found in the table <<bfd_hash_lookup>> will
+ return <<NULL>>. You should not modify any of the fields in
+ the returns <<struct bfd_hash_entry>>.
+
+ If the @var{create} argument is <<true>>, the string will be
+ entered into the hash table if it is not already there.
+ Either way a pointer to a <<struct bfd_hash_entry>> will be
+ returned, either to the existing structure or to a newly
+ created one. In this case, a <<NULL>> return means that an
+ error occurred.
+
+ If the @var{create} argument is <<true>>, and a new entry is
+ created, the @var{copy} argument is used to decide whether to
+ copy the string onto the hash table obstack or not. If
+ @var{copy} is passed as <<false>>, you must be careful not to
+ deallocate or modify the string as long as the hash table
+ exists.
+
+INODE
+Traversing a Hash Table, Deriving a New Hash Table Type, Looking Up or Entering a String, Hash Tables
+SUBSECTION
+ Traversing a hash table
+
+@findex bfd_hash_traverse
+ The function <<bfd_hash_traverse>> may be used to traverse a
+ hash table, calling a function on each element. The traversal
+ is done in a random order.
+
+ <<bfd_hash_traverse>> takes as arguments a function and a
+ generic <<void *>> pointer. The function is called with a
+ hash table entry (a <<struct bfd_hash_entry *>>) and the
+ generic pointer passed to <<bfd_hash_traverse>>. The function
+ must return a <<boolean>> value, which indicates whether to
+ continue traversing the hash table. If the function returns
+ <<false>>, <<bfd_hash_traverse>> will stop the traversal and
+ return immediately.
+
+INODE
+Deriving a New Hash Table Type, , Traversing a Hash Table, Hash Tables
+SUBSECTION
+ Deriving a new hash table type
+
+ Many uses of hash tables want to store additional information
+ which each entry in the hash table. Some also find it
+ convenient to store additional information with the hash table
+ itself. This may be done using a derived hash table.
+
+ Since C is not an object oriented language, creating a derived
+ hash table requires sticking together some boilerplate
+ routines with a few differences specific to the type of hash
+ table you want to create.
+
+ An example of a derived hash table is the linker hash table.
+ The structures for this are defined in <<bfdlink.h>>. The
+ functions are in <<linker.c>>.
+
+ You may also derive a hash table from an already derived hash
+ table. For example, the a.out linker backend code uses a hash
+ table derived from the linker hash table.
+
+@menu
+@* Define the Derived Structures::
+@* Write the Derived Creation Routine::
+@* Write Other Derived Routines::
+@end menu
+
+INODE
+Define the Derived Structures, Write the Derived Creation Routine, Deriving a New Hash Table Type, Deriving a New Hash Table Type
+SUBSUBSECTION
+ Define the derived structures
+
+ You must define a structure for an entry in the hash table,
+ and a structure for the hash table itself.
+
+ The first field in the structure for an entry in the hash
+ table must be of the type used for an entry in the hash table
+ you are deriving from. If you are deriving from a basic hash
+ table this is <<struct bfd_hash_entry>>, which is defined in
+ <<bfd.h>>. The first field in the structure for the hash
+ table itself must be of the type of the hash table you are
+ deriving from itself. If you are deriving from a basic hash
+ table, this is <<struct bfd_hash_table>>.
+
+ For example, the linker hash table defines <<struct
+ bfd_link_hash_entry>> (in <<bfdlink.h>>). The first field,
+ <<root>>, is of type <<struct bfd_hash_entry>>. Similarly,
+ the first field in <<struct bfd_link_hash_table>>, <<table>>,
+ is of type <<struct bfd_hash_table>>.
+
+INODE
+Write the Derived Creation Routine, Write Other Derived Routines, Define the Derived Structures, Deriving a New Hash Table Type
+SUBSUBSECTION
+ Write the derived creation routine
+
+ You must write a routine which will create and initialize an
+ entry in the hash table. This routine is passed as the
+ function argument to <<bfd_hash_table_init>>.
+
+ In order to permit other hash tables to be derived from the
+ hash table you are creating, this routine must be written in a
+ standard way.
+
+ The first argument to the creation routine is a pointer to a
+ hash table entry. This may be <<NULL>>, in which case the
+ routine should allocate the right amount of space. Otherwise
+ the space has already been allocated by a hash table type
+ derived from this one.
+
+ After allocating space, the creation routine must call the
+ creation routine of the hash table type it is derived from,
+ passing in a pointer to the space it just allocated. This
+ will initialize any fields used by the base hash table.
+
+ Finally the creation routine must initialize any local fields
+ for the new hash table type.
+
+ Here is a boilerplate example of a creation routine.
+ @var{function_name} is the name of the routine.
+ @var{entry_type} is the type of an entry in the hash table you
+ are creating. @var{base_newfunc} is the name of the creation
+ routine of the hash table type your hash table is derived
+ from.
+
+EXAMPLE
+
+.struct bfd_hash_entry *
+.@var{function_name} (entry, table, string)
+. struct bfd_hash_entry *entry;
+. struct bfd_hash_table *table;
+. const char *string;
+.{
+. struct @var{entry_type} *ret = (@var{entry_type} *) entry;
+.
+. {* Allocate the structure if it has not already been allocated by a
+. derived class. *}
+. if (ret == (@var{entry_type} *) NULL)
+. ret = ((@var{entry_type} *)
+. bfd_hash_allocate (table, sizeof (@var{entry_type})));
+.
+. {* Call the allocation method of the base class. *}
+. ret = ((@var{entry_type} *)
+. @var{base_newfunc} ((struct bfd_hash_entry *) ret, table, string));
+.
+. {* Initialize the local fields here. *}
+.
+. return (struct bfd_hash_entry *) ret;
+.}
+
+DESCRIPTION
+ The creation routine for the linker hash table, which is in
+ <<linker.c>>, looks just like this example.
+ @var{function_name} is <<_bfd_link_hash_newfunc>>.
+ @var{entry_type} is <<struct bfd_link_hash_entry>>.
+ @var{base_newfunc} is <<bfd_hash_newfunc>>, the creation
+ routine for a basic hash table.
+
+ <<_bfd_link_hash_newfunc>> also initializes the local fields
+ in a linker hash table entry: <<type>>, <<written>> and
+ <<next>>.
+
+INODE
+Write Other Derived Routines, , Write the Derived Creation Routine, Deriving a New Hash Table Type
+SUBSUBSECTION
+ Write other derived routines
+
+ You will want to write other routines for your new hash table,
+ as well.
+
+ You will want an initialization routine which calls the
+ initialization routine of the hash table you are deriving from
+ and initializes any other local fields. For the linker hash
+ table, this is <<_bfd_link_hash_table_init>> in <<linker.c>>.
+
+ You will want a lookup routine which calls the lookup routine
+ of the hash table you are deriving from and casts the result.
+ The linker hash table uses <<bfd_link_hash_lookup>> in
+ <<linker.c>> (this actually takes an additional argument which
+ it uses to decide how to return the looked up value).
+
+ You may want a traversal routine. This should just call the
+ traversal routine of the hash table you are deriving from with
+ appropriate casts. The linker hash table uses
+ <<bfd_link_hash_traverse>> in <<linker.c>>.
+
+ These routines may simply be defined as macros. For example,
+ the a.out backend linker hash table, which is derived from the
+ linker hash table, uses macros for the lookup and traversal
+ routines. These are <<aout_link_hash_lookup>> and
+ <<aout_link_hash_traverse>> in aoutx.h.
+*/
+
/* Obstack allocation and deallocation routines. */
#define obstack_chunk_alloc bfd_xmalloc_by_size_t
#define obstack_chunk_free free
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