* hash.c: Added some documentation.

2 files changed, 270 insertions, 0 deletions

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index 18fdc63..035c25f 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -1,3 +1,7 @@
+Mon Jan  3 11:41:45 1994  Ian Lance Taylor  (ian@tweedledumb.cygnus.com)
+
+	* hash.c: Added some documentation.
+
 Mon Jan  3 11:09:28 1994  Jim Kingdon  (kingdon@lioth.cygnus.com)
 
 	* aout-target.h, netbsd386.c: Replace NO_SWAP_MAGIC with SWAP_MAGIC,
diff --git a/bfd/hash.c b/bfd/hash.c
index 991b2c3..9effba8 100644
--- a/bfd/hash.c
+++ b/bfd/hash.c
@@ -23,6 +23,272 @@ the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.  */
 #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