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
.
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 size argument, may be used).
bfd_hash_table_init
returns false
if some sort of
error occurs.
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
. See section Deriving a new hash table type for why you would want to use a
different value for this argument.
bfd_hash_table_init
will create an objalloc which will be
used to allocate new entries. You may allocate memory on this
objalloc using bfd_hash_allocate
.
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.
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 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 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 create argument is true
, and a new entry is
created, the copy argument is used to decide whether to
copy the string onto the hash table objalloc or not. If
copy is passed as false
, you must be careful not to
deallocate or modify the string as long as the hash table
exists.
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.
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.
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
.
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. function_name is the name of the routine. entry_type is the type of an entry in the hash table you are creating. base_newfunc is the name of the creation routine of the hash table type your hash table is derived from.
struct bfd_hash_entry * function_name (entry, table, string) struct bfd_hash_entry *entry; struct bfd_hash_table *table; const char *string; { struct entry_type *ret = (entry_type *) entry; /* Allocate the structure if it has not already been allocated by a derived class. */ if (ret == (entry_type *) NULL) { ret = ((entry_type *) bfd_hash_allocate (table, sizeof (entry_type))); if (ret == (entry_type *) NULL) return NULL; } /* Call the allocation method of the base class. */ ret = ((entry_type *) 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.
function_name is _bfd_link_hash_newfunc
.
entry_type is struct bfd_link_hash_entry
.
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
.
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.
This document was generated on 7 April 1999 using the texi2html translator version 1.52.