@section Hash Tables@cindex Hash tablesBFD provides a simple set of hash table functions.  Routinesare provided to initialize a hash table, to free a hash table,to look up a string in a hash table and optionally create anentry for it, and to traverse a hash table.  There iscurrently no routine to delete an string from a hash table.The basic hash table does not permit any data to be storedwith a string.  However, a hash table is designed to present abase class from which other types of hash tables may bederived.  These derived types may store additional informationwith the string.  Hash tables were implemented in this way,rather than simply providing a data pointer in a hash tableentry, because they were designed for use by the linker backends.  The linker may create thousands of hash table entries,and the overhead of allocating private data and storing andfollowing 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_nTo create a hash table, create an instance of a @code{structbfd_hash_table} (defined in @code{bfd.h}) and call@code{bfd_hash_table_init} (if you know approximately how manyentries 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 oferror occurs.@findex bfd_hash_newfuncThe function @code{bfd_hash_table_init} take as an argument afunction to use to create new entries.  For a basic hashtable, use the function @code{bfd_hash_newfunc}.  @xref{Derivinga New Hash Table Type}, for why you would want to use adifferent value for this argument.@findex bfd_hash_allocate@code{bfd_hash_table_init} will create an objalloc which will beused to allocate new entries.  You may allocate memory on thisobjalloc using @code{bfd_hash_allocate}.@findex bfd_hash_table_freeUse @code{bfd_hash_table_free} to free up all the memory that hasbeen 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_lookupThe function @code{bfd_hash_lookup} is used both to look up astring 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 willreturns a pointer to a @code{struct bfd_hash_entry}.  If thestring is not found in the table @code{bfd_hash_lookup} willreturn @code{NULL}.  You should not modify any of the fields inthe returns @code{struct bfd_hash_entry}.If the @var{create} argument is @code{TRUE}, the string will beentered into the hash table if it is not already there.Either way a pointer to a @code{struct bfd_hash_entry} will bereturned, either to the existing structure or to a newlycreated one.  In this case, a @code{NULL} return means that anerror occurred.If the @var{create} argument is @code{TRUE}, and a new entry iscreated, the @var{copy} argument is used to decide whether tocopy the string onto the hash table objalloc or not.  If@var{copy} is passed as @code{FALSE}, you must be careful not todeallocate or modify the string as long as the hash tableexists.@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_traverseThe function @code{bfd_hash_traverse} may be used to traverse ahash table, calling a function on each element.  The traversalis done in a random order.@code{bfd_hash_traverse} takes as arguments a function and ageneric @code{void *} pointer.  The function is called with ahash table entry (a @code{struct bfd_hash_entry *}) and thegeneric pointer passed to @code{bfd_hash_traverse}.  The functionmust return a @code{boolean} value, which indicates whether tocontinue traversing the hash table.  If the function returns@code{FALSE}, @code{bfd_hash_traverse} will stop the traversal andreturn immediately.@node Deriving a New Hash Table Type, , Traversing a Hash Table, Hash Tables@subsection Deriving a new hash table typeMany uses of hash tables want to store additional informationwhich each entry in the hash table.  Some also find itconvenient to store additional information with the hash tableitself.  This may be done using a derived hash table.Since C is not an object oriented language, creating a derivedhash table requires sticking together some boilerplateroutines with a few differences specific to the type of hashtable 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}.  Thefunctions are in @code{linker.c}.You may also derive a hash table from an already derived hashtable.  For example, the a.out linker backend code uses a hashtable 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 structuresYou 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 hashtable must be of the type used for an entry in the hash tableyou are deriving from.  If you are deriving from a basic hashtable this is @code{struct bfd_hash_entry}, which is defined in@code{bfd.h}.  The first field in the structure for the hashtable itself must be of the type of the hash table you arederiving from itself.  If you are deriving from a basic hashtable, this is @code{struct bfd_hash_table}.For example, the linker hash table defines @code{structbfd_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 routineYou must write a routine which will create and initialize anentry in the hash table.  This routine is passed as thefunction argument to @code{bfd_hash_table_init}.In order to permit other hash tables to be derived from thehash table you are creating, this routine must be written in astandard way.The first argument to the creation routine is a pointer to ahash table entry.  This may be @code{NULL}, in which case theroutine should allocate the right amount of space.  Otherwisethe space has already been allocated by a hash table typederived from this one.After allocating space, the creation routine must call thecreation routine of the hash table type it is derived from,passing in a pointer to the space it just allocated.  Thiswill initialize any fields used by the base hash table.Finally the creation routine must initialize any local fieldsfor 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 youare creating.  @var{base_newfunc} is the name of the creationroutine of the hash table type your hash table is derivedfrom.@examplestruct 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 creationroutine for a basic hash table.@code{_bfd_link_hash_newfunc} also initializes the local fieldsin 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 routinesYou will want to write other routines for your new hash table,as well.You will want an initialization routine which calls theinitialization routine of the hash table you are deriving fromand initializes any other local fields.  For the linker hashtable, this is @code{_bfd_link_hash_table_init} in @code{linker.c}.You will want a lookup routine which calls the lookup routineof 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 whichit uses to decide how to return the looked up value).You may want a traversal routine.  This should just call thetraversal routine of the hash table you are deriving from withappropriate 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 thelinker hash table, uses macros for the lookup and traversalroutines.  These are @code{aout_link_hash_lookup} and@code{aout_link_hash_traverse} in aoutx.h.