INTMAP(2) INTMAP(2)
NAME
Intmap, allocmap, freemap, insertkey, caninsertkey,
lookupkey, deletekey - integer to data structure maps
SYNOPSIS
#include <u.h>
#include <libc.h>
#include <fcall.h>
#include <thread.h>
#include <9p.h>
Intmap* allocmap(void (*inc)(void*))
void freemap(Intmap *map, void (*dec)(void*))
void* lookupkey(Intmap *map, ulong key)
void* insertkey(Intmap *map, ulong key, void *val)
int caninsertkey(Intmap *map, ulong key, void *val)
void* lookupkey(Intmap *map, ulong key)
void* deletekey(Intmap *map, ulong key)
DESCRIPTION
An Intmap is an arbitrary mapping from integers to pointers.
Allocmap creates a new map, and freemap destroys it. The
inc function is called each time a new pointer is added to
the map; similarly, dec is called on each pointer left in
the map when it is being freed. Typically these functions
maintain reference counts. New entries are added to the map
by calling insertkey, which will return the previous value
associated with the given key, or zero if there was no pre-
vious value. Caninsertkey is like insertkey but only
inserts val if there is no current mapping. It returns 1 if
val was inserted, 0 otherwise. Lookupkey returns the
pointer associated with key, or zero if there is no such
pointer. Deletekey removes the entry for id from the map,
returning the associated pointer, if any.
Concurrent access to Intmaps is safe, moderated via a QLock
stored in the Intmap structure.
In anticipation of the storage of reference-counted struc-
tures, an increment function inc may be specified at map
creation time. Lookupkey calls inc (if non-zero) on point-
ers before returning them. If the reference count adjust-
ments were left to the caller (and thus not protected by the
lock), it would be possible to accidentally reclaim a struc-
ture if, for example, it was deleted from the map and its
reference count decremented between the return of insertkey
and the external increment. Insertkey and caninsertkey do
not call inc when inserting val into the map, nor do
insertkey or deletekey call inc when returning old map
entries. The rationale is that calling an insertion
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INTMAP(2) INTMAP(2)
function transfers responsibility for the reference to the
map, and responsibility is given back via the return value
of deletekey or the next insertkey.
Intmaps are used by the 9P library to implement Fidpools and
Reqpools.
SOURCE
/sys/src/lib9p/intmap.c
SEE ALSO
9p(2), 9pfid(2).
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