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@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 @code{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

@node 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 @code{struct

bfd_hash_table} (defined in @code{bfd.h}) and call

@code{bfd_hash_table_init} (if you know approximately how many

entries you will need, the function @code{bfd_hash_table_init_n},

which takes a @var{size} argument, may be used).

@code{bfd_hash_table_init} returns @code{false} if some sort of

error occurs.

@findex bfd_hash_newfunc

The function @code{bfd_hash_table_init} take as an argument a

function to use to create new entries.  For a basic hash

table, use the function @code{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

@code{bfd_hash_table_init} will create an objalloc which will be

used to allocate new entries.  You may allocate memory on this

objalloc using @code{bfd_hash_allocate}.

@findex bfd_hash_table_free

Use @code{bfd_hash_table_free} to free up all the memory that has

been allocated for a hash table.  This will not free up the

@code{struct bfd_hash_table} itself, which you must provide.

@node 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 @code{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 @code{false}, @code{bfd_hash_lookup}

will look up a string.  If the string is found, it will

returns a pointer to a @code{struct bfd_hash_entry}.  If the

string is not found in the table @code{bfd_hash_lookup} will

return @code{NULL}.  You should not modify any of the fields in

the returns @code{struct bfd_hash_entry}.

If the @var{create} argument is @code{true}, the string will be

entered into the hash table if it is not already there.

Either way a pointer to a @code{struct bfd_hash_entry} will be

returned, either to the existing structure or to a newly

created one.  In this case, a @code{NULL} return means that an

error occurred.

If the @var{create} argument is @code{true}, and a new entry is

created, the @var{copy} argument is used to decide whether to

copy the string onto the hash table objalloc or not.  If

@var{copy} is passed as @code{false}, you must be careful not to

deallocate or modify the string as long as the hash table

exists.

@node 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 @code{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.

@code{bfd_hash_traverse} takes as arguments a function and a

generic @code{void *} pointer.  The function is called with a

hash table entry (a @code{struct bfd_hash_entry *}) and the

generic pointer passed to @code{bfd_hash_traverse}.  The function

must return a @code{boolean} value, which indicates whether to

continue traversing the hash table.  If the function returns

@code{false}, @code{bfd_hash_traverse} will stop the traversal and

return immediately.

@node 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 @code{bfdlink.h}.  The

functions are in @code{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

@node 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 @code{struct bfd_hash_entry}, which is defined in

@code{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 @code{struct bfd_hash_table}.

For example, the linker hash table defines @code{struct

bfd_link_hash_entry} (in @code{bfdlink.h}).  The first field,

@code{root}, is of type @code{struct bfd_hash_entry}.  Similarly,

the first field in @code{struct bfd_link_hash_table}, @code{table},

is of type @code{struct bfd_hash_table}.

@node 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 @code{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 @code{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})));

      if (ret == (@var{entry_type} *) NULL)

        return NULL;

    @}

 /* 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;

@}

@end example

@strong{Description}@*

The creation routine for the linker hash table, which is in

@code{linker.c}, looks just like this example.

@var{function_name} is @code{_bfd_link_hash_newfunc}.

@var{entry_type} is @code{struct bfd_link_hash_entry}.

@var{base_newfunc} is @code{bfd_hash_newfunc}, the creation

routine for a basic hash table.

@code{_bfd_link_hash_newfunc} also initializes the local fields

in a linker hash table entry: @code{type}, @code{written} and

@code{next}.

@node 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 @code{_bfd_link_hash_table_init} in @code{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 @code{bfd_link_hash_lookup} in

@code{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

@code{bfd_link_hash_traverse} in @code{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 @code{aout_link_hash_lookup} and

@code{aout_link_hash_traverse} in aoutx.h.