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.TH assoc 3 "23 January 2001" "Straylight/Edgeware" "mLib utilities library"
.SH NAME
assoc \- tables indexed by atoms
.\" @assoc_create
.\" @assoc_destroy
.\" @assoc_find
.\" @assoc_remove
.\" @assoc_mkiter
.\" @assoc_next
.\"
.\" @ASSOC_ATOM
.\"
.SH SYNOPSIS
.nf
.B "#include <mLib/assoc.h>"

.BI "void assoc_create(assoc_table *" t );
.BI "void assoc_destroy(assoc_table *" t );

.BI "void *assoc_find(assoc_table *" t ", atom *" a ", size_t " sz ", unsigned *" f );
.BI "void assoc_remove(assoc_table *" t ", void *" b );

.BI "atom *ASSOC_ATOM(const void *" p );

.BI "void assoc_mkiter(assoc_iter *" i ", assoc_table *" t );
.BI "void *assoc_next(assoc_iter *" i );
.fi
.SH DESCRIPTION
An
.I "association table"
is a data structure which maps atoms (see
.BR atom (3))
to arbitrary values.  It works in a similar way to the symbol tables
implemented by
.BR sym (3),
except that it uses atoms rather than blocks of data as keys.
.PP
Like
.BR sym (3),
it implements an
.I intrusive
table: the value structures must include an
.B assoc_base
structure.
.PP
There are three main data structures:
.TP
.B assoc_table
Keeps track of the information associated with a particular table.
.TP
.B assoc_base
The header which must be attached to the front of all the value objects.
.TP
.B assoc_iter
An iterator object, used for enumerating all of the associations stored
in the table
.PP
All of the above structures should be considered
.IR opaque :
don't try looking inside.
.SS "Creation and destruction"
The
.B assoc_table
object itself needs to be allocated by the caller.  It is initialized by
passing it to the function
.BR assoc_create .
After initialization, the table contains no entries.
.PP
Initializing an association table involves allocating some memory.  If
this allocation fails, an
.B EXC_NOMEM
exception is raised.
.PP
When an association table is no longer needed, the memory occupied by
the values and other maintenance structures can be reclaimed by calling
.BR assoc_destroy .
Any bits of user data attached to values should previously have been
destroyed.
.SS "Adding, searching and removing"
Most of the actual work is done by the function
.BR assoc_find .
It does both lookup and creation, depending on its arguments.  To do its
job, it needs to know the following bits of information:
.TP
.BI "assoc_table *" t
A pointer to an association table to manipulate.
.TP
.BI "atom *" a
The address of the atom to use as a key.
.TP
.BI "size_t " sz
The size of the value block to allocate if the key could not be found.
If this is zero, no value is allocated, and a null pointer is returned
to indicate an unsuccessful lookup.
.TP
.BI "unsigned *" f
The address of a `found' flag to set.  This is an output parameter.  On
exit,
.B assoc_find
will set the value of
.BI * f
to zero if the key could not be found, or nonzero if it was found.  This
can be used to tell whether the value returned has been newly allocated,
or whether it was already in the table.
.PP
A symbol can be removed from the table by calling
.BR assoc_remove ,
passing the association table itself, and the value block that needs
removing.
.SS "Enquiries about associations"
Given a pointer
.I a
to an association, the expression
.BI ASSOC_ATOM( a )
has as its value a poinetr to the atom which is that association's key.
.SS "Enumerating associations"
Enumerating the values in an association table is fairly simple.
Allocate an
.B assoc_iter
object from somewhere.  Attach it to an association table by calling
.BR assoc_mkiter ,
and passing in the addresses of the iterator and the symbol table.
Then, each call to
.B assoc_next
will return a different value from the association table, until all of
them have been enumerated, at which point,
.B assoc_next
returns a null pointer.
.PP
It's safe to remove the symbol you've just been returned by
.BR assoc_next .
However, it's not safe to remove any other symbol.  So don't do that.
.PP
When you've finished with an iterator, it's safe to just throw it away.
You don't need to call any functions beforehand.
.SH SEE ALSO
.BR atom (3),
.BR hash (3),
.BR sym (3),
.BR mLib (3).
.SH AUTHOR
Mark Wooding, <mdw@nsict.org>