ets(3erl) Erlang Module Definition ets(3erl)
NAME
ets - Built-in term storage.
DESCRIPTION
This module is an interface to the Erlang built-in term storage BIFs.
These provide the ability to store very large quantities of data in an
Erlang runtime system, and to have constant access time to the data.
(In the case of ordered_set, see below, access time is proportional to
the logarithm of the number of stored objects.)
Data is organized as a set of dynamic tables, which can store tuples.
Each table is created by a process. When the process terminates, the
table is automatically destroyed. Every table has access rights set at
creation.
Tables are divided into four different types, set, ordered_set, bag,
and duplicate_bag. A set or ordered_set table can only have one object
associated with each key. A bag or duplicate_bag table can have many
objects associated with each key.
Note:
The number of tables stored at one Erlang node used to be limited. This
is no longer the case (except by memory usage). The previous default
limit was about 1400 tables and could be increased by setting the envi-
ronment variable ERL_MAX_ETS_TABLES or the command line option +e be-
fore starting the Erlang runtime system. This hard limit has been re-
moved, but it is currently useful to set the ERL_MAX_ETS_TABLES anyway.
It should be set to an approximate of the maximum amount of tables
used. This since an internal table for named tables is sized using this
value. If large amounts of named tables are used and ERL_MAX_ETS_TABLES
hasn't been increased, the performance of named table lookup will de-
grade.
Notice that there is no automatic garbage collection for tables. Even
if there are no references to a table from any process, it is not auto-
matically destroyed unless the owner process terminates. To destroy a
table explicitly, use function delete/1. The default owner is the
process that created the table. To transfer table ownership at process
termination, use option heir or call give_away/3.
Some implementation details:
* In the current implementation, every object insert and look-up op-
eration results in a copy of the object.
* '$end_of_table' is not to be used as a key, as this atom is used to
mark the end of the table when using functions first/1 and next/2.
Notice the subtle difference between matching and comparing equal,
which is demonstrated by table types set and ordered_set:
* Two Erlang terms match if they are of the same type and have the
same value, so that 1 matches 1, but not 1.0 (as 1.0 is a float()
and not an integer()).
* Two Erlang terms compare equal if they either are of the same type
and value, or if both are numeric types and extend to the same
value, so that 1 compares equal to both 1 and 1.0.
* The ordered_set works on the Erlang term order and no defined order
exists between an integer() and a float() that extends to the same
value. Hence the key 1 and the key 1.0 are regarded as equal in an
ordered_set table.
FAILURES
Functions in this module fail by raising an error exception with error
reason:
badarg:
If any argument has the wrong format.
badarg:
If the table identifier is invalid.
badarg:
If the operation is denied because of table access rights (pro-
tected or private).
system_limit:
Modification of a value causes it to not be representable inter-
nally in the VM. For example, incrementation of a counter past the
largest integer representable.
system_limit:
If a match specification passed as argument has excessive nesting
which causes scheduler stack exhaustion for the scheduler that the
calling process is executing on. Scheduler stack size can be con-
figured when starting the runtime system.
CONCURRENCY
This module provides some limited support for concurrent access. All
updates to single objects are guaranteed to be both atomic and iso-
lated. This means that an updating operation to a single object either
succeeds or fails completely without any effect (atomicity) and that no
intermediate results of the update can be seen by other processes (iso-
lation). Some functions that update many objects state that they even
guarantee atomicity and isolation for the entire operation. In database
terms the isolation level can be seen as "serializable", as if all iso-
lated operations are carried out serially, one after the other in a
strict order.
TABLE TRAVERSAL
There are different ways to traverse through the objects of a table.
* Single-step traversal one key at at time, using first/1, next/2,
last/1 and prev/2.
* Search with simple match patterns, using match/1/2/3,
match_delete/2 and match_object/1/2/3.
* Search with more powerful match specifications, using select/1/2/3,
select_count/2, select_delete/2, select_replace/2 and select_re-
verse/1/2/3.
* Table conversions, using tab2file/2/3 and tab2list/1.
None of these ways of table traversal will guarantee a consistent table
snapshot if the table is also updated during the traversal. Moreover,
traversals not done in a safe way, on tables where keys are inserted or
deleted during the traversal, may yield the following undesired ef-
fects:
* Any key may be missed.
* Any key may be found more than once.
* The traversal may fail with badarg exception if keys are deleted.
A table traversal is safe if either
* the table is of type ordered_set.
* the entire table traversal is done within one ETS function call.
* function safe_fixtable/2 is used to keep the table fixated during
the entire traversal.
Note:
Even though the access of a single object is always guaranteed to be
atomic and isolated, each traversal through a table to find the next
key is not done with such guarantees. This is often not a problem, but
may cause rare subtle "unexpected" effects if a concurrent process in-
serts objects during a traversal. For example, consider one process do-
ing
ets:new(t, [ordered_set, named_table]),
ets:insert(t, {1}),
ets:insert(t, {2}),
ets:insert(t, {3}),
A concurrent call to ets:first(t), done by another process, may then in
rare cases return 2 even though 2 has never existed in the table or-
dered as the first key. In the same way, a concurrent call to
ets:next(t, 1) may return 3 even though 3 never existed in the table
ordered directly after 1.
Effects like this are improbable but possible. The probability will
further be reduced (if not vanish) if table option write_concurrency is
not enabled. This can also only be a potential concern for ordered_set
where the traversal order is defined.
Traversals using match and select functions may not need to scan the
entire table depending on how the key is specified. A match pattern
with a fully bound key (without any match variables) will optimize the
operation to a single key lookup without any table traversal at all.
For ordered_set a partially bound key will limit the traversal to only
scan a subset of the table based on term order. A partially bound key
is either a list or a tuple with a prefix that is fully bound. Example:
1> T = ets:new(t,[ordered_set]), ets:insert(T, {"555-1234", "John Smith"}).
true
2> %% Efficient search of all with area code 555
2> ets:match(T,{[$5,$5,$5,$- |'$1'],'$2'}).
[["1234","John Smith"]]
MATCH SPECIFICATIONS
Some of the functions use a match specification, match_spec. For a
brief explanation, see select/2. For a detailed description, see sec-
tion Match Specifications in Erlang in ERTS User's Guide.
A match specifications with excessive nesting will cause a system_limit
error exception to be raised.
DATA TYPES
access() = public | protected | private
continuation()
Opaque continuation used by select/1,3, select_reverse/1,3,
match/1,3, and match_object/1,3.
match_spec() = [{match_pattern(), [term()], [term()]}]
A match specification, see above.
comp_match_spec()
A compiled match specification.
match_pattern() = atom() | tuple()
tab() = atom() | tid()
tid()
A table identifier, as returned by new/2.
type() = set | ordered_set | bag | duplicate_bag
EXPORTS
all() -> [Tab]
Types:
Tab = tab()
Returns a list of all tables at the node. Named tables are spec-
ified by their names, unnamed tables are specified by their ta-
ble identifiers.
There is no guarantee of consistency in the returned list. Ta-
bles created or deleted by other processes "during" the
ets:all() call either are or are not included in the list. Only
tables created/deleted before ets:all() is called are guaranteed
to be included/excluded.
delete(Tab) -> true
Types:
Tab = tab()
Deletes the entire table Tab.
delete(Tab, Key) -> true
Types:
Tab = tab()
Key = term()
Deletes all objects with key Key from table Tab.
delete_all_objects(Tab) -> true
Types:
Tab = tab()
Delete all objects in the ETS table Tab. The operation is guar-
anteed to be atomic and isolated.
delete_object(Tab, Object) -> true
Types:
Tab = tab()
Object = tuple()
Delete the exact object Object from the ETS table, leaving ob-
jects with the same key but other differences (useful for type
bag). In a duplicate_bag table, all instances of the object are
deleted.
file2tab(Filename) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates
the corresponding table Tab.
Equivalent to file2tab(Filename, []).
file2tab(Filename, Options) -> {ok, Tab} | {error, Reason}
Types:
Filename = file:name()
Tab = tab()
Options = [Option]
Option = {verify, boolean()}
Reason = term()
Reads a file produced by tab2file/2 or tab2file/3 and creates
the corresponding table Tab.
The only supported option is {verify,boolean()}. If verification
is turned on (by specifying {verify,true}), the function uses
whatever information is present in the file to assert that the
information is not damaged. How this is done depends on which
extended_info was written using tab2file/3.
If no extended_info is present in the file and {verify,true} is
specified, the number of objects written is compared to the size
of the original table when the dump was started. This can make
verification fail if the table was public and objects were added
or removed while the table was dumped to file. To avoid this
problem, either do not verify files dumped while updated simul-
taneously or use option {extended_info, [object_count]} to
tab2file/3, which extends the information in the file with the
number of objects written.
If verification is turned on and the file was written with op-
tion {extended_info, [md5sum]}, reading the file is slower and
consumes radically more CPU time than otherwise.
{verify,false} is the default.
first(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the first key Key in table Tab. For an ordered_set ta-
ble, the first key in Erlang term order is returned. For other
table types, the first key according to the internal order of
the table is returned. If the table is empty, '$end_of_table' is
returned.
To find subsequent keys in the table, use next/2.
foldl(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar
to lists:foldl/3. The table elements are traversed in an unspec-
ified order, except for ordered_set tables, where they are tra-
versed first to last.
If Function inserts objects into the table, or another process
inserts objects into the table, those objects can (depending on
key ordering) be included in the traversal.
foldr(Function, Acc0, Tab) -> Acc1
Types:
Function = fun((Element :: term(), AccIn) -> AccOut)
Tab = tab()
Acc0 = Acc1 = AccIn = AccOut = term()
Acc0 is returned if the table is empty. This function is similar
to lists:foldr/3. The table elements are traversed in an unspec-
ified order, except for ordered_set tables, where they are tra-
versed last to first.
If Function inserts objects into the table, or another process
inserts objects into the table, those objects can (depending on
key ordering) be included in the traversal.
from_dets(Tab, DetsTab) -> true
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created ETS table with the objects in the al-
ready opened Dets table DetsTab. Existing objects in the ETS ta-
ble are kept unless overwritten.
If any of the tables does not exist or the Dets table is not
open, a badarg exception is raised.
fun2ms(LiteralFun) -> MatchSpec
Types:
LiteralFun = function()
MatchSpec = match_spec()
Pseudo function that by a parse_transform translates LiteralFun
typed as parameter in the function call to a match specifica-
tion. With "literal" is meant that the fun must textually be
written as the parameter of the function, it cannot be held in a
variable that in turn is passed to the function.
The parse transform is provided in the ms_transform module and
the source must include file ms_transform.hrl in STDLIB for this
pseudo function to work. Failing to include the hrl file in the
source results in a runtime error, not a compile time error. The
include file is easiest included by adding line -in-
clude_lib("stdlib/include/ms_transform.hrl"). to the source
file.
The fun is very restricted, it can take only a single parameter
(the object to match): a sole variable or a tuple. It must use
the is_ guard tests. Language constructs that have no represen-
tation in a match specification (if, case, receive, and so on)
are not allowed.
The return value is the resulting match specification.
Example:
1> ets:fun2ms(fun({M,N}) when N > 3 -> M end).
[{{'$1','$2'},[{'>','$2',3}],['$1']}]
Variables from the environment can be imported, so that the fol-
lowing works:
2> X=3.
3
3> ets:fun2ms(fun({M,N}) when N > X -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}}],['$1']}]
The imported variables are replaced by match specification const
expressions, which is consistent with the static scoping for Er-
lang funs. However, local or global function calls cannot be in
the guard or body of the fun. Calls to built-in match specifica-
tion functions is of course allowed:
4> ets:fun2ms(fun({M,N}) when N > X, my_fun(M) -> M end).
Error: fun containing local Erlang function calls
('my_fun' called in guard) cannot be translated into match_spec
{error,transform_error}
5> ets:fun2ms(fun({M,N}) when N > X, is_atom(M) -> M end).
[{{'$1','$2'},[{'>','$2',{const,3}},{is_atom,'$1'}],['$1']}]
As shown by the example, the function can be called from the
shell also. The fun must be literally in the call when used from
the shell as well.
Warning:
If the parse_transform is not applied to a module that calls
this pseudo function, the call fails in runtime (with a badarg).
The ets module exports a function with this name, but it is
never to be called except when using the function in the shell.
If the parse_transform is properly applied by including header
file ms_transform.hrl, compiled code never calls the function,
but the function call is replaced by a literal match specifica-
tion.
For more information, see ms_transform(3erl).
give_away(Tab, Pid, GiftData) -> true
Types:
Tab = tab()
Pid = pid()
GiftData = term()
Make process Pid the new owner of table Tab. If successful, mes-
sage {'ETS-TRANSFER',Tab,FromPid,GiftData} is sent to the new
owner.
The process Pid must be alive, local, and not already the owner
of the table. The calling process must be the table owner.
Notice that this function does not affect option heir of the ta-
ble. A table owner can, for example, set heir to itself, give
the table away, and then get it back if the receiver terminates.
i() -> ok
Displays information about all ETS tables on a terminal.
i(Tab) -> ok
Types:
Tab = tab()
Browses table Tab on a terminal.
info(Tab) -> InfoList | undefined
Types:
Tab = tab()
InfoList = [InfoTuple]
InfoTuple =
{compressed, boolean()} |
{decentralized_counters, boolean()} |
{heir, pid() | none} |
{id, tid()} |
{keypos, integer() >= 1} |
{memory, integer() >= 0} |
{name, atom()} |
{named_table, boolean()} |
{node, node()} |
{owner, pid()} |
{protection, access()} |
{size, integer() >= 0} |
{type, type()} |
{write_concurrency, boolean()} |
{read_concurrency, boolean()}
Returns information about table Tab as a list of tuples. If Tab
has the correct type for a table identifier, but does not refer
to an existing ETS table, undefined is returned. If Tab is not
of the correct type, a badarg exception is raised.
{compressed, boolean()}:
Indicates if the table is compressed.
{decentralized_counters, boolean()}:
Indicates whether the table uses decentralized_counters.
{heir, pid() | none}:
The pid of the heir of the table, or none if no heir is set.
{id,tid()}:
The table identifier.
{keypos, integer() >= 1}:
The key position.
{memory, integer() >= 0:
The number of words allocated to the table.
{name, atom()}:
The table name.
{named_table, boolean()}:
Indicates if the table is named.
{node, node()}:
The node where the table is stored. This field is no longer
meaningful, as tables cannot be accessed from other nodes.
{owner, pid()}:
The pid of the owner of the table.
{protection,access()}:
The table access rights.
{size, integer() >= 0:
The number of objects inserted in the table.
{type,type()}:
The table type.
{read_concurrency, boolean()}:
Indicates whether the table uses read_concurrency or not.
{write_concurrency, boolean()}:
Indicates whether the table uses write_concurrency.
Note:
The execution time of this function is affected by the decen-
tralized_counters table option. The execution time is much
longer when the decentralized_counters option is set to true
than when the decentralized_counters option is set to false.
info(Tab, Item) -> Value | undefined
Types:
Tab = tab()
Item =
binary | compressed | decentralized_counters | fixed |
heir |
id | keypos | memory | name | named_table | node | owner
|
protection | safe_fixed | safe_fixed_monotonic_time |
size |
stats | type | write_concurrency | read_concurrency
Value = term()
Returns the information associated with Item for table Tab, or
returns undefined if Tab does not refer an existing ETS table.
If Tab is not of the correct type, or if Item is not one of the
allowed values, a badarg exception is raised.
In addition to the {Item,Value} pairs defined for info/1, the
following items are allowed:
* Item=binary, Value=BinInfo
BinInfo is a list containing miscellaneous information about
binaries kept by the table. This Item can be changed or re-
moved without prior notice. In the current implementation
BinInfo is a list of tuples {BinaryId,BinarySize,BinaryRefc-
Count}.
* Item=fixed, Value=boolean()
Indicates if the table is fixed by any process.
*
Item=safe_fixed|safe_fixed_monotonic_time, Value={Fixation-
Time,Info}|false
If the table is fixed using safe_fixtable/2, the call re-
turns a tuple where FixationTime is the last time when the
table changed from unfixed to fixed.
The format and value of FixationTime depends on Item:
safe_fixed:
FixationTime corresponds to the result returned by er-
lang:timestamp/0 at the time of fixation. Notice that when
the system uses single or multi time warp modes this can
produce strange results, as the use of safe_fixed is not
time warp safe. Time warp safe code must use
safe_fixed_monotonic_time instead.
safe_fixed_monotonic_time:
FixationTime corresponds to the result returned by er-
lang:monotonic_time/0 at the time of fixation. The use of
safe_fixed_monotonic_time is time warp safe.
Info is a possibly empty lists of tuples {Pid,RefCount}, one
tuple for every process the table is fixed by now. RefCount
is the value of the reference counter and it keeps track of
how many times the table has been fixed by the process.
Table fixations are not limited to safe_fixtable/2. Tempo-
rary fixations may also be done by for example traversing
functions like select and match. Such table fixations are
automatically released before the corresponding functions
returns, but they may be seen by a concurrent call to
ets:info(T,safe_fixed|safe_fixed_monotonic_time).
If the table is not fixed at all, the call returns false.
* Item=stats, Value=tuple()
Returns internal statistics about tables on an internal for-
mat used by OTP test suites. Not for production use.
Note:
The execution time of this function is affected by the decen-
tralized_counters table option when the second argument of the
function is size or memory. The execution time is much longer
when the decentralized_counters option is set to true than when
the decentralized_counters option is set to false.
init_table(Tab, InitFun) -> true
Types:
Tab = tab()
InitFun = fun((Arg) -> Res)
Arg = read | close
Res = end_of_input | {Objects :: [term()], InitFun} | term()
Replaces the existing objects of table Tab with objects created
by calling the input function InitFun, see below. This function
is provided for compatibility with the dets module, it is not
more efficient than filling a table by using insert/2.
When called with argument read, the function InitFun is assumed
to return end_of_input when there is no more input, or {Objects,
Fun}, where Objects is a list of objects and Fun is a new input
function. Any other value Value is returned as an error {error,
{init_fun, Value}}. Each input function is called exactly once,
and if an error occur, the last function is called with argument
close, the reply of which is ignored.
If the table type is set and more than one object exists with a
given key, one of the objects is chosen. This is not necessarily
the last object with the given key in the sequence of objects
returned by the input functions. This holds also for duplicated
objects stored in tables of type bag.
insert(Tab, ObjectOrObjects) -> true
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
Inserts the object or all of the objects in list ObjectOrObjects
into table Tab.
* If the table type is set and the key of the inserted objects
matches the key of any object in the table, the old object
is replaced.
* If the table type is ordered_set and the key of the inserted
object compares equal to the key of any object in the table,
the old object is replaced.
* If the list contains more than one object with matching keys
and the table type is set, one is inserted, which one is not
defined. The same holds for table type ordered_set if the
keys compare equal.
The entire operation is guaranteed to be atomic and isolated,
even when a list of objects is inserted.
insert_new(Tab, ObjectOrObjects) -> boolean()
Types:
Tab = tab()
ObjectOrObjects = tuple() | [tuple()]
Same as insert/2 except that instead of overwriting objects with
the same key (for set or ordered_set) or adding more objects
with keys already existing in the table (for bag and dupli-
cate_bag), false is returned.
If ObjectOrObjects is a list, the function checks every key be-
fore inserting anything. Nothing is inserted unless all keys
present in the list are absent from the table. Like insert/2,
the entire operation is guaranteed to be atomic and isolated.
is_compiled_ms(Term) -> boolean()
Types:
Term = term()
Checks if a term represent a valid compiled match specification.
A compiled match specifications is only valid on the Erlang node
where it was compiled by calling match_spec_compile/1.
Note:
Before STDLIB 3.4 (OTP 20.0) compiled match specifications did
not have an external representation. If passed through bi-
nary_to_term(term_to_binary(CMS)) or sent to another node and
back, the result was always an empty binary <<>>.
After STDLIB 3.4 (OTP 20.0) compiled match specifications have
an external representation as a node specific reference to the
original compiled match specification. If passed through bi-
nary_to_term(term_to_binary(CMS)) or sent to another node and
back, the result may or may not be a valid compiled match speci-
fication depending on if the original compiled match specifica-
tion was still alive.
last(Tab) -> Key | '$end_of_table'
Types:
Tab = tab()
Key = term()
Returns the last key Key according to Erlang term order in table
Tab of type ordered_set. For other table types, the function is
synonymous to first/1. If the table is empty, '$end_of_table' is
returned.
To find preceding keys in the table, use prev/2.
lookup(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns a list of all objects with key Key in table Tab.
* For tables of type set, bag, or duplicate_bag, an object is
returned only if the specified key matches the key of the
object in the table.
* For tables of type ordered_set, an object is returned if the
specified key compares equal to the key of an object in the
table.
The difference is the same as between =:= and ==.
As an example, one can insert an object with integer() 1 as a
key in an ordered_set and get the object returned as a result of
doing a lookup/2 with float() 1.0 as the key to search for.
For tables of type set or ordered_set, the function returns ei-
ther the empty list or a list with one element, as there cannot
be more than one object with the same key. For tables of type
bag or duplicate_bag, the function returns a list of arbitrary
length.
Notice that the time order of object insertions is preserved;
the first object inserted with the specified key is the first in
the resulting list, and so on.
Insert and lookup times in tables of type set, bag, and dupli-
cate_bag are constant, regardless of the table size. For the or-
dered_set datatype, time is proportional to the (binary) loga-
rithm of the number of objects.
lookup_element(Tab, Key, Pos) -> Elem
Types:
Tab = tab()
Key = term()
Pos = integer() >= 1
Elem = term() | [term()]
For a table Tab of type set or ordered_set, the function returns
the Pos:th element of the object with key Key.
For tables of type bag or duplicate_bag, the functions returns a
list with the Pos:th element of every object with key Key.
If no object with key Key exists, the function exits with reason
badarg.
The difference between set, bag, and duplicate_bag on one hand,
and ordered_set on the other, regarding the fact that or-
dered_set view keys as equal when they compare equal whereas the
other table types regard them equal only when they match, holds
for lookup_element/3.
match(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = [term()]
Continuation = continuation()
Continues a match started with match/3. The next chunk of the
size specified in the initial match/3 call is returned together
with a new Continuation, which can be used in subsequent calls
to this function.
When there are no more objects in the table, '$end_of_table' is
returned.
match(Tab, Pattern) -> [Match]
Types:
Tab = tab()
Pattern = match_pattern()
Match = [term()]
Matches the objects in table Tab against pattern Pattern.
A pattern is a term that can contain:
* Bound parts (Erlang terms)
* '_' that matches any Erlang term
* Pattern variables '$N', where N=0,1,...
The function returns a list with one element for each matching
object, where each element is an ordered list of pattern vari-
able bindings, for example:
6> ets:match(T, '$1'). % Matches every object in table
[[{rufsen,dog,7}],[{brunte,horse,5}],[{ludde,dog,5}]]
7> ets:match(T, {'_',dog,'$1'}).
[[7],[5]]
8> ets:match(T, {'_',cow,'$1'}).
[]
If the key is specified in the pattern, the match is very effi-
cient. If the key is not specified, that is, if it is a variable
or an underscore, the entire table must be searched. The search
time can be substantial if the table is very large.
For tables of type ordered_set, the result is in the same order
as in a first/next traversal.
match(Tab, Pattern, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Match = [term()]
Continuation = continuation()
Works like match/2, but returns only a limited (Limit) number of
matching objects. Term Continuation can then be used in subse-
quent calls to match/1 to get the next chunk of matching ob-
jects. This is a space-efficient way to work on objects in a ta-
ble, which is faster than traversing the table object by object
using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent
calls to match/1.
match_delete(Tab, Pattern) -> true
Types:
Tab = tab()
Pattern = match_pattern()
Deletes all objects that match pattern Pattern from table Tab.
For a description of patterns, see match/2.
match_object(Continuation) ->
{[Object], Continuation} | '$end_of_table'
Types:
Object = tuple()
Continuation = continuation()
Continues a match started with match_object/3. The next chunk of
the size specified in the initial match_object/3 call is re-
turned together with a new Continuation, which can be used in
subsequent calls to this function.
When there are no more objects in the table, '$end_of_table' is
returned.
match_object(Tab, Pattern) -> [Object]
Types:
Tab = tab()
Pattern = match_pattern()
Object = tuple()
Matches the objects in table Tab against pattern Pattern. For a
description of patterns, see match/2. The function returns a
list of all objects that match the pattern.
If the key is specified in the pattern, the match is very effi-
cient. If the key is not specified, that is, if it is a variable
or an underscore, the entire table must be searched. The search
time can be substantial if the table is very large.
For tables of type ordered_set, the result is in the same order
as in a first/next traversal.
match_object(Tab, Pattern, Limit) ->
{[Object], Continuation} | '$end_of_table'
Types:
Tab = tab()
Pattern = match_pattern()
Limit = integer() >= 1
Object = tuple()
Continuation = continuation()
Works like match_object/2, but only returns a limited (Limit)
number of matching objects. Term Continuation can then be used
in subsequent calls to match_object/1 to get the next chunk of
matching objects. This is a space-efficient way to work on ob-
jects in a table, which is faster than traversing the table ob-
ject by object using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent
calls to match_object/1.
match_spec_compile(MatchSpec) -> CompiledMatchSpec
Types:
MatchSpec = match_spec()
CompiledMatchSpec = comp_match_spec()
Transforms a match specification into an internal representation
that can be used in subsequent calls to match_spec_run/2. The
internal representation is opaque. To check the validity of a
compiled match specification, use is_compiled_ms/1.
If term MatchSpec does not represent a valid match specifica-
tion, a badarg exception is raised.
Note:
This function has limited use in normal code. It is used by the
dets module to perform the dets:select() operations.
match_spec_run(List, CompiledMatchSpec) -> list()
Types:
List = [term()]
CompiledMatchSpec = comp_match_spec()
Executes the matching specified in a compiled match specifica-
tion on a list of terms. Term CompiledMatchSpec is to be the re-
sult of a call to match_spec_compile/1 and is hence the internal
representation of the match specification one wants to use.
The matching is executed on each element in List and the func-
tion returns a list containing all results. If an element in
List does not match, nothing is returned for that element. The
length of the result list is therefore equal or less than the
length of parameter List.
Example:
The following two calls give the same result (but certainly not
the same execution time):
Table = ets:new...
MatchSpec = ...
% The following call...
ets:match_spec_run(ets:tab2list(Table),
ets:match_spec_compile(MatchSpec)),
% ...gives the same result as the more common (and more efficient)
ets:select(Table, MatchSpec),
Note:
This function has limited use in normal code. It is used by the
dets module to perform the dets:select() operations and by Mne-
sia during transactions.
member(Tab, Key) -> boolean()
Types:
Tab = tab()
Key = term()
Works like lookup/2, but does not return the objects. Returns
true if one or more elements in the table has key Key, otherwise
false.
new(Name, Options) -> tid() | atom()
Types:
Name = atom()
Options = [Option]
Option =
Type | Access | named_table |
{keypos, Pos} |
{heir, Pid :: pid(), HeirData} |
{heir, none} |
Tweaks
Type = type()
Access = access()
Tweaks =
{write_concurrency, boolean()} |
{read_concurrency, boolean()} |
{decentralized_counters, boolean()} |
compressed
Pos = integer() >= 1
HeirData = term()
Creates a new table and returns a table identifier that can be
used in subsequent operations. The table identifier can be sent
to other processes so that a table can be shared between differ-
ent processes within a node.
Parameter Options is a list of options that specifies table
type, access rights, key position, and whether the table is
named. Default values are used for omitted options. This means
that not specifying any options ([]) is the same as specifying
[set, protected, {keypos,1}, {heir,none}, {write_concur-
rency,false}, {read_concurrency,false}, {decentralized_coun-
ters,false}].
set:
The table is a set table: one key, one object, no order
among objects. This is the default table type.
ordered_set:
The table is a ordered_set table: one key, one object, or-
dered in Erlang term order, which is the order implied by
the < and > operators. Tables of this type have a somewhat
different behavior in some situations than tables of other
types. Most notably, the ordered_set tables regard keys as
equal when they compare equal, not only when they match.
This means that to an ordered_set table, integer() 1 and
float() 1.0 are regarded as equal. This also means that the
key used to lookup an element not necessarily matches the
key in the returned elements, if float()'s and integer()'s
are mixed in keys of a table.
bag:
The table is a bag table, which can have many objects, but
only one instance of each object, per key.
duplicate_bag:
The table is a duplicate_bag table, which can have many ob-
jects, including multiple copies of the same object, per
key.
public:
Any process can read or write to the table.
protected:
The owner process can read and write to the table. Other
processes can only read the table. This is the default set-
ting for the access rights.
private:
Only the owner process can read or write to the table.
named_table:
If this option is present, the table is registered under its
Name which can then be used instead of the table identifier
in subsequent operations.
The function will also return the Name instead of the table
identifier. To get the table identifier of a named table,
use whereis/1.
{keypos,Pos}:
Specifies which element in the stored tuples to use as key.
By default, it is the first element, that is, Pos=1. How-
ever, this is not always appropriate. In particular, we do
not want the first element to be the key if we want to store
Erlang records in a table.
Notice that any tuple stored in the table must have at least
Pos number of elements.
{heir,Pid,HeirData} | {heir,none}:
Set a process as heir. The heir inherits the table if the
owner terminates. Message {'ETS-TRANS-
FER',tid(),FromPid,HeirData} is sent to the heir when that
occurs. The heir must be a local process. Default heir is
none, which destroys the table when the owner terminates.
{write_concurrency,boolean()}:
Performance tuning. Defaults to false, in which case an op-
eration that mutates (writes to) the table obtains exclusive
access, blocking any concurrent access of the same table un-
til finished. If set to true, the table is optimized to con-
current write access. Different objects of the same table
can be mutated (and read) by concurrent processes. This is
achieved to some degree at the expense of memory consumption
and the performance of sequential access and concurrent
reading.
The write_concurrency option can be combined with the op-
tions read_concurrency and decentralized_counters. You typi-
cally want to combine write_concurrency with read_concur-
rency when large concurrent read bursts and large concurrent
write bursts are common; for more information, see option
read_concurrency. The decentralized_counters option is
turned on by default for tables of type ordered_set with the
write_concurrency option enabled, and the decentral-
ized_counters option is turned off by default for all other
table types. For more information, see the documentation for
the decentralized_counters option.
Notice that this option does not change any guarantees about
atomicity and isolation. Functions that makes such promises
over many objects (like insert/2) gain less (or nothing)
from this option.
The memory consumption inflicted by both write_concurrency
and read_concurrency is a constant overhead per table for
set, bag and duplicate_bag. For ordered_set the memory over-
head depends on the number of inserted objects and the
amount of actual detected concurrency in runtime. The memory
overhead can be especially large when both options are com-
bined.
Note:
Prior to stdlib-3.7 (OTP-22.0) write_concurrency had no effect
on ordered_set.
{read_concurrency,boolean()}:
Performance tuning. Defaults to false. When set to true, the
table is optimized for concurrent read operations. When this
option is enabled on a runtime system with SMP support, read
operations become much cheaper; especially on systems with
multiple physical processors. However, switching between
read and write operations becomes more expensive.
You typically want to enable this option when concurrent
read operations are much more frequent than write opera-
tions, or when concurrent reads and writes comes in large
read and write bursts (that is, many reads not interrupted
by writes, and many writes not interrupted by reads).
You typically do not want to enable this option when the
common access pattern is a few read operations interleaved
with a few write operations repeatedly. In this case, you
would get a performance degradation by enabling this option.
Option read_concurrency can be combined with option
write_concurrency. You typically want to combine these when
large concurrent read bursts and large concurrent write
bursts are common.
{decentralized_counters,boolean()}:
Performance tuning. Defaults to true for tables of type or-
dered_set with the write_concurrency option enabled, and de-
faults to false for all other table types. This option has
no effect if the write_concurrency option is set to false.
When this option is set to true, the table is optimized for
frequent concurrent calls to operations that modify the ta-
bles size and/or its memory consumption (e.g., insert/2 and
delete/2). The drawback is that calls to info/1 and info/2
with size or memory as the second argument can get much
slower when the decentralized_counters option is turned on.
When this option is enabled the counters for the table size
and memory consumption are distributed over several cache
lines and the scheduling threads are mapped to one of those
cache lines. The erl option +dcg can be used to control the
number of cache lines that the counters are distributed
over.
compressed:
If this option is present, the table data is stored in a
more compact format to consume less memory. However, it will
make table operations slower. Especially operations that
need to inspect entire objects, such as match and select,
get much slower. The key element is not compressed.
next(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the next key Key2, following key Key1 in table Tab. For
table type ordered_set, the next key in Erlang term order is re-
turned. For other table types, the next key according to the in-
ternal order of the table is returned. If no next key exists,
'$end_of_table' is returned.
To find the first key in the table, use first/1.
Unless a table of type set, bag, or duplicate_bag is fixated us-
ing safe_fixtable/2, a call to next/2 will fail if Key1 no
longer exists in the table. For table type ordered_set, the
function always returns the next key after Key1 in term order,
regardless whether Key1 ever existed in the table.
prev(Tab, Key1) -> Key2 | '$end_of_table'
Types:
Tab = tab()
Key1 = Key2 = term()
Returns the previous key Key2, preceding key Key1 according to
Erlang term order in table Tab of type ordered_set. For other
table types, the function is synonymous to next/2. If no previ-
ous key exists, '$end_of_table' is returned.
To find the last key in an ordered_set table, use last/1.
rename(Tab, Name) -> Name
Types:
Tab = tab()
Name = atom()
Renames the named table Tab to the new name Name. Afterwards,
the old name cannot be used to access the table. Renaming an un-
named table has no effect.
repair_continuation(Continuation, MatchSpec) -> Continuation
Types:
Continuation = continuation()
MatchSpec = match_spec()
Restores an opaque continuation returned by select/3 or select/1
if the continuation has passed through external term format
(been sent between nodes or stored on disk).
The reason for this function is that continuation terms contain
compiled match specifications and may therefore be invalidated
if converted to external term format. Given that the original
match specification is kept intact, the continuation can be re-
stored, meaning it can once again be used in subsequent select/1
calls even though it has been stored on disk or on another node.
Examples:
The following sequence of calls may fail:
T=ets:new(x,[]),
...
MS = ets:fun2ms(fun({N,_}=A) when (N rem 10) =:= 0 -> A end),
{_,C} = ets:select(T, MS, 10),
MaybeBroken = binary_to_term(term_to_binary(C)),
ets:select(MaybeBroken).
The following sequence works, as the call to repair_continua-
tion/2 reestablishes the MaybeBroken continuation.
T=ets:new(x,[]),
...
MS = ets:fun2ms(fun({N,_}=A) when (N rem 10) =:= 0 -> A end),
{_,C} = ets:select(T,MS,10),
MaybeBroken = binary_to_term(term_to_binary(C)),
ets:select(ets:repair_continuation(MaybeBroken,MS)).
Note:
This function is rarely needed in application code. It is used
by Mnesia to provide distributed select/3 and select/1 se-
quences. A normal application would either use Mnesia or keep
the continuation from being converted to external format.
The actual behavior of compiled match specifications when recre-
ated from external format has changed and may change in future
releases, but this interface remains for backward compatibility.
See is_compiled_ms/1.
safe_fixtable(Tab, Fix) -> true
Types:
Tab = tab()
Fix = boolean()
Fixes a table of type set, bag, or duplicate_bag for safe tra-
versal using first/1 & next/2, match/3 & match/1, match_object/3
& match_object/1, or select/3 & select/1.
A process fixes a table by calling safe_fixtable(Tab, true). The
table remains fixed until the process releases it by calling
safe_fixtable(Tab, false), or until the process terminates.
If many processes fix a table, the table remains fixed until all
processes have released it (or terminated). A reference counter
is kept on a per process basis, and N consecutive fixes requires
N releases to release the table.
When a table is fixed, a sequence of first/1 and next/2 calls
are guaranteed to succeed even if keys are removed during the
traversal. The keys for objects inserted or deleted during a
traversal may or may not be returned by next/2 depending on the
ordering of keys within the table and if the key exists at the
time next/2 is called.
Example:
clean_all_with_value(Tab,X) ->
safe_fixtable(Tab,true),
clean_all_with_value(Tab,X,ets:first(Tab)),
safe_fixtable(Tab,false).
clean_all_with_value(Tab,X,'$end_of_table') ->
true;
clean_all_with_value(Tab,X,Key) ->
case ets:lookup(Tab,Key) of
[{Key,X}] ->
ets:delete(Tab,Key);
_ ->
true
end,
clean_all_with_value(Tab,X,ets:next(Tab,Key)).
Notice that deleted objects are not freed from a fixed table un-
til it has been released. If a process fixes a table but never
releases it, the memory used by the deleted objects is never
freed. The performance of operations on the table also degrades
significantly.
To retrieve information about which processes have fixed which
tables, use info(Tab, safe_fixed_monotonic_time). A system with
many processes fixing tables can need a monitor that sends
alarms when tables have been fixed for too long.
Notice that safe_fixtable/2 is not necessary for table type or-
dered_set and for traversals done by a single ETS function call,
like select/2.
select(Continuation) -> {[Match], Continuation} | '$end_of_table'
Types:
Match = term()
Continuation = continuation()
Continues a match started with select/3. The next chunk of the
size specified in the initial select/3 call is returned together
with a new Continuation, which can be used in subsequent calls
to this function.
When there are no more objects in the table, '$end_of_table' is
returned.
select(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Matches the objects in table Tab using a match specification.
This is a more general call than match/2 and match_object/2
calls. In its simplest form, the match specification is as fol-
lows:
MatchSpec = [MatchFunction]
MatchFunction = {MatchHead, [Guard], [Result]}
MatchHead = "Pattern as in ets:match"
Guard = {"Guardtest name", ...}
Result = "Term construct"
This means that the match specification is always a list of one
or more tuples (of arity 3). The first element of the tuple is
to be a pattern as described in match/2. The second element of
the tuple is to be a list of 0 or more guard tests (described
below). The third element of the tuple is to be a list contain-
ing a description of the value to return. In almost all normal
cases, the list contains exactly one term that fully describes
the value to return for each object.
The return value is constructed using the "match variables"
bound in MatchHead or using the special match variables '$_'
(the whole matching object) and '$$' (all match variables in a
list), so that the following match/2 expression:
ets:match(Tab,{'$1','$2','$3'})
is exactly equivalent to:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
And that the following match_object/2 call:
ets:match_object(Tab,{'$1','$2','$1'})
is exactly equivalent to
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
Composite terms can be constructed in the Result part either by
simply writing a list, so that the following code:
ets:select(Tab,[{{'$1','$2','$3'},[],['$$']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$3'},[],[['$1','$2','$3']]}])
That is, all the bound variables in the match head as a list. If
tuples are to be constructed, one has to write a tuple of arity
1 where the single element in the tuple is the tuple one wants
to construct (as an ordinary tuple can be mistaken for a Guard).
Therefore the following call:
ets:select(Tab,[{{'$1','$2','$1'},[],['$_']}])
gives the same output as:
ets:select(Tab,[{{'$1','$2','$1'},[],[{{'$1','$2','$3'}}]}])
This syntax is equivalent to the syntax used in the trace pat-
terns (see the dbg(3erl)) module in Runtime_Tools.
The Guards are constructed as tuples, where the first element is
the test name and the remaining elements are the test parame-
ters. To check for a specific type (say a list) of the element
bound to the match variable '$1', one would write the test as
{is_list, '$1'}. If the test fails, the object in the table does
not match and the next MatchFunction (if any) is tried. Most
guard tests present in Erlang can be used, but only the new ver-
sions prefixed is_ are allowed (is_float, is_atom, and so on).
The Guard section can also contain logic and arithmetic opera-
tions, which are written with the same syntax as the guard tests
(prefix notation), so that the following guard test written in
Erlang:
is_integer(X), is_integer(Y), X + Y < 4711
is expressed as follows (X replaced with '$1' and Y with '$2'):
[{is_integer, '$1'}, {is_integer, '$2'}, {'<', {'+', '$1', '$2'}, 4711}]
For tables of type ordered_set, objects are visited in the same
order as in a first/next traversal. This means that the match
specification is executed against objects with keys in the
first/next order and the corresponding result list is in the or-
der of that execution.
select(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like select/2, but only returns a limited (Limit) number
of matching objects. Term Continuation can then be used in sub-
sequent calls to select/1 to get the next chunk of matching ob-
jects. This is a space-efficient way to work on objects in a ta-
ble, which is still faster than traversing the table object by
object using first/1 and next/2.
If the table is empty, '$end_of_table' is returned.
Use safe_fixtable/2 to guarantee safe traversal for subsequent
calls to select/1.
select_count(Tab, MatchSpec) -> NumMatched
Types:
Tab = tab()
MatchSpec = match_spec()
NumMatched = integer() >= 0
Matches the objects in table Tab using a match specification. If
the match specification returns true for an object, that object
considered a match and is counted. For any other result from the
match specification the object is not considered a match and is
therefore not counted.
This function can be described as a match_delete/2 function that
does not delete any elements, but only counts them.
The function returns the number of objects matched.
select_delete(Tab, MatchSpec) -> NumDeleted
Types:
Tab = tab()
MatchSpec = match_spec()
NumDeleted = integer() >= 0
Matches the objects in table Tab using a match specification. If
the match specification returns true for an object, that object
is removed from the table. For any other result from the match
specification the object is retained. This is a more general
call than the match_delete/2 call.
The function returns the number of objects deleted from the ta-
ble.
Note:
The match specification has to return the atom true if the ob-
ject is to be deleted. No other return value gets the object
deleted. So one cannot use the same match specification for
looking up elements as for deleting them.
select_replace(Tab, MatchSpec) -> NumReplaced
Types:
Tab = tab()
MatchSpec = match_spec()
NumReplaced = integer() >= 0
Matches the objects in the table Tab using a match specifica-
tion. For each matched object, the existing object is replaced
with the match specification result.
The match-and-replace operation for each individual object is
guaranteed to be atomic and isolated. The select_replace table
traversal as a whole, like all other select functions, does not
give such guarantees.
The match specifiction must be guaranteed to retain the key of
any matched object. If not, select_replace will fail with badarg
without updating any objects.
For the moment, due to performance and semantic constraints, ta-
bles of type bag are not yet supported.
The function returns the total number of replaced objects.
Example
For all 2-tuples with a list in second position, add atom
'marker' first in the list:
1> T = ets:new(x,[]), ets:insert(T, {key, [1, 2, 3]}).
true
2> MS = ets:fun2ms(fun({K, L}) when is_list(L) -> {K, [marker | L]} end).
[{{'$1','$2'},[{is_list,'$2'}],[{{'$1',[marker|'$2']}}]}]
3> ets:select_replace(T, MS).
1
4> ets:tab2list(T).
[{key,[marker,1,2,3]}]
A generic single object compare-and-swap operation:
[Old] = ets:lookup(T, Key),
New = update_object(Old),
Success = (1 =:= ets:select_replace(T, [{Old, [], [{const, New}]}])),
select_reverse(Continuation) ->
{[Match], Continuation} | '$end_of_table'
Types:
Continuation = continuation()
Match = term()
Continues a match started with select_reverse/3. For tables of
type ordered_set, the traversal of the table continues to ob-
jects with keys earlier in the Erlang term order. The returned
list also contains objects with keys in reverse order. For all
other table types, the behavior is exactly that of select/1.
Example:
1> T = ets:new(x,[ordered_set]).
2> [ ets:insert(T,{N}) || N <- lists:seq(1,10) ].
...
3> {R0,C0} = ets:select_reverse(T,[{'_',[],['$_']}],4).
...
4> R0.
[{10},{9},{8},{7}]
5> {R1,C1} = ets:select_reverse(C0).
...
6> R1.
[{6},{5},{4},{3}]
7> {R2,C2} = ets:select_reverse(C1).
...
8> R2.
[{2},{1}]
9> '$end_of_table' = ets:select_reverse(C2).
...
select_reverse(Tab, MatchSpec) -> [Match]
Types:
Tab = tab()
MatchSpec = match_spec()
Match = term()
Works like select/2, but returns the list in reverse order for
table type ordered_set. For all other table types, the return
value is identical to that of select/2.
select_reverse(Tab, MatchSpec, Limit) ->
{[Match], Continuation} | '$end_of_table'
Types:
Tab = tab()
MatchSpec = match_spec()
Limit = integer() >= 1
Match = term()
Continuation = continuation()
Works like select/3, but for table type ordered_set traversing
is done starting at the last object in Erlang term order and
moves to the first. For all other table types, the return value
is identical to that of select/3.
Notice that this is not equivalent to reversing the result list
of a select/3 call, as the result list is not only reversed, but
also contains the last Limit matching objects in the table, not
the first.
setopts(Tab, Opts) -> true
Types:
Tab = tab()
Opts = Opt | [Opt]
Opt = {heir, pid(), HeirData} | {heir, none}
HeirData = term()
Sets table options. The only allowed option to be set after the
table has been created is heir. The calling process must be the
table owner.
slot(Tab, I) -> [Object] | '$end_of_table'
Types:
Tab = tab()
I = integer() >= 0
Object = tuple()
This function is mostly for debugging purposes, Normally
first/next or last/prev are to be used instead.
Returns all objects in slot I of table Tab. A table can be tra-
versed by repeatedly calling the function, starting with the
first slot I=0 and ending when '$end_of_table' is returned. If
argument I is out of range, the function fails with reason
badarg.
Unless a table of type set, bag, or duplicate_bag is protected
using safe_fixtable/2, a traversal can fail if concurrent up-
dates are made to the table. For table type ordered_set, the
function returns a list containing object I in Erlang term or-
der.
tab2file(Tab, Filename) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Reason = term()
Dumps table Tab to file Filename.
Equivalent to tab2file(Tab, Filename,[])
tab2file(Tab, Filename, Options) -> ok | {error, Reason}
Types:
Tab = tab()
Filename = file:name()
Options = [Option]
Option = {extended_info, [ExtInfo]} | {sync, boolean()}
ExtInfo = md5sum | object_count
Reason = term()
Dumps table Tab to file Filename.
When dumping the table, some information about the table is
dumped to a header at the beginning of the dump. This informa-
tion contains data about the table type, name, protection, size,
version, and if it is a named table. It also contains notes
about what extended information is added to the file, which can
be a count of the objects in the file or a MD5 sum of the header
and records in the file.
The size field in the header might not correspond to the number
of records in the file if the table is public and records are
added or removed from the table during dumping. Public tables
updated during dump, and that one wants to verify when reading,
needs at least one field of extended information for the read
verification process to be reliable later.
Option extended_info specifies what extra information is written
to the table dump:
object_count:
The number of objects written to the file is noted in the
file footer, so file truncation can be verified even if the
file was updated during dump.
md5sum:
The header and objects in the file are checksummed using the
built-in MD5 functions. The MD5 sum of all objects is writ-
ten in the file footer, so that verification while reading
detects the slightest bitflip in the file data. Using this
costs a fair amount of CPU time.
Whenever option extended_info is used, it results in a file not
readable by versions of ETS before that in STDLIB 1.15.1
If option sync is set to true, it ensures that the content of
the file is written to the disk before tab2file returns. De-
faults to {sync, false}.
tab2list(Tab) -> [Object]
Types:
Tab = tab()
Object = tuple()
Returns a list of all objects in table Tab.
tabfile_info(Filename) -> {ok, TableInfo} | {error, Reason}
Types:
Filename = file:name()
TableInfo = [InfoItem]
InfoItem =
{name, atom()} |
{type, Type} |
{protection, Protection} |
{named_table, boolean()} |
{keypos, integer() >= 0} |
{size, integer() >= 0} |
{extended_info, [ExtInfo]} |
{version,
{Major :: integer() >= 0, Minor :: integer() >= 0}}
ExtInfo = md5sum | object_count
Type = bag | duplicate_bag | ordered_set | set
Protection = private | protected | public
Reason = term()
Returns information about the table dumped to file by tab2file/2
or tab2file/3.
The following items are returned:
name:
The name of the dumped table. If the table was a named ta-
ble, a table with the same name cannot exist when the table
is loaded from file with file2tab/2. If the table is not
saved as a named table, this field has no significance when
loading the table from file.
type:
The ETS type of the dumped table (that is, set, bag, dupli-
cate_bag, or ordered_set). This type is used when loading
the table again.
protection:
The protection of the dumped table (that is, private, pro-
tected, or public). A table loaded from the file gets the
same protection.
named_table:
true if the table was a named table when dumped to file,
otherwise false. Notice that when a named table is loaded
from a file, there cannot exist a table in the system with
the same name.
keypos:
The keypos of the table dumped to file, which is used when
loading the table again.
size:
The number of objects in the table when the table dump to
file started. For a public table, this number does not need
to correspond to the number of objects saved to the file, as
objects can have been added or deleted by another process
during table dump.
extended_info:
The extended information written in the file footer to allow
stronger verification during table loading from file, as
specified to tab2file/3. Notice that this function only
tells which information is present, not the values in the
file footer. The value is a list containing one or more of
the atoms object_count and md5sum.
version:
A tuple {Major,Minor} containing the major and minor version
of the file format for ETS table dumps. This version field
was added beginning with STDLIB 1.5.1. Files dumped with
older versions return {0,0} in this field.
An error is returned if the file is inaccessible, badly damaged,
or not produced with tab2file/2 or tab2file/3.
table(Tab) -> QueryHandle
table(Tab, Options) -> QueryHandle
Types:
Tab = tab()
QueryHandle = qlc:query_handle()
Options = [Option] | Option
Option = {n_objects, NObjects} | {traverse, TraverseMethod}
NObjects = default | integer() >= 1
TraverseMethod =
first_next | last_prev | select |
{select, MatchSpec :: match_spec()}
Returns a Query List Comprehension (QLC) query handle. The qlc
module provides a query language aimed mainly at Mnesia, but ETS
tables, Dets tables, and lists are also recognized by QLC as
sources of data. Calling table/1,2 is the means to make the ETS
table Tab usable to QLC.
When there are only simple restrictions on the key position, QLC
uses lookup/2 to look up the keys. When that is not possible,
the whole table is traversed. Option traverse determines how
this is done:
first_next:
The table is traversed one key at a time by calling first/1
and next/2.
last_prev:
The table is traversed one key at a time by calling last/1
and prev/2.
select:
The table is traversed by calling select/3 and select/1. Op-
tion n_objects determines the number of objects returned
(the third argument of select/3); the default is to return
100 objects at a time. The match specification (the second
argument of select/3) is assembled by QLC: simple filters
are translated into equivalent match specifications while
more complicated filters must be applied to all objects re-
turned by select/3 given a match specification that matches
all objects.
{select, MatchSpec}:
As for select, the table is traversed by calling select/3
and select/1. The difference is that the match specification
is explicitly specified. This is how to state match specifi-
cations that cannot easily be expressed within the syntax
provided by QLC.
Examples:
An explicit match specification is here used to traverse the ta-
ble:
9> true = ets:insert(Tab = ets:new(t, []), [{1,a},{2,b},{3,c},{4,d}]),
MS = ets:fun2ms(fun({X,Y}) when (X > 1) or (X < 5) -> {Y} end),
QH1 = ets:table(Tab, [{traverse, {select, MS}}]).
An example with an implicit match specification:
10> QH2 = qlc:q([{Y} || {X,Y} <- ets:table(Tab), (X > 1) or (X < 5)]).
The latter example is equivalent to the former, which can be
verified using function qlc:info/1:
11> qlc:info(QH1) =:= qlc:info(QH2).
true
qlc:info/1 returns information about a query handle, and in this
case identical information is returned for the two query han-
dles.
take(Tab, Key) -> [Object]
Types:
Tab = tab()
Key = term()
Object = tuple()
Returns and removes a list of all objects with key Key in table
Tab.
The specified Key is used to identify the object by either com-
paring equal the key of an object in an ordered_set table, or
matching in other types of tables (for details on the differ-
ence, see lookup/2 and new/2).
test_ms(Tuple, MatchSpec) -> {ok, Result} | {error, Errors}
Types:
Tuple = tuple()
MatchSpec = match_spec()
Result = term()
Errors = [{warning | error, string()}]
This function is a utility to test a match specification used in
calls to select/2. The function both tests MatchSpec for "syn-
tactic" correctness and runs the match specification against ob-
ject Tuple.
If the match specification is syntactically correct, the func-
tion either returns {ok,Result}, where Result is what would have
been the result in a real select/2 call, or false if the match
specification does not match object Tuple.
If the match specification contains errors, tuple {error, Er-
rors} is returned, where Errors is a list of natural language
descriptions of what was wrong with the match specification.
This is a useful debugging and test tool, especially when writ-
ing complicated select/2 calls.
See also: erlang:match_spec_test/3.
to_dets(Tab, DetsTab) -> DetsTab
Types:
Tab = tab()
DetsTab = dets:tab_name()
Fills an already created/opened Dets table with the objects in
the already opened ETS table named Tab. The Dets table is emp-
tied before the objects are inserted.
update_counter(Tab, Key, UpdateOp) -> Result
update_counter(Tab, Key, UpdateOp, Default) -> Result
update_counter(Tab, Key, X3 :: [UpdateOp]) -> [Result]
update_counter(Tab, Key, X3 :: [UpdateOp], Default) -> [Result]
update_counter(Tab, Key, Incr) -> Result
update_counter(Tab, Key, Incr, Default) -> Result
Types:
Tab = tab()
Key = term()
UpdateOp = {Pos, Incr} | {Pos, Incr, Threshold, SetValue}
Pos = Incr = Threshold = SetValue = Result = integer()
Default = tuple()
This function provides an efficient way to update one or more
counters, without the trouble of having to look up an object,
update the object by incrementing an element, and insert the re-
sulting object into the table again. The operation is guaranteed
to be atomic and isolated.
This function destructively update the object with key Key in
table Tab by adding Incr to the element at position Pos. The new
counter value is returned. If no position is specified, the ele-
ment directly following key (<keypos>+1) is updated.
If a Threshold is specified, the counter is reset to value Set-
Value if the following conditions occur:
* Incr is not negative (>= 0) and the result would be greater
than (>) Threshold.
* Incr is negative (< 0) and the result would be less than (<)
Threshold.
A list of UpdateOp can be supplied to do many update operations
within the object. The operations are carried out in the order
specified in the list. If the same counter position occurs more
than once in the list, the corresponding counter is thus updated
many times, each time based on the previous result. The return
value is a list of the new counter values from each update oper-
ation in the same order as in the operation list. If an empty
list is specified, nothing is updated and an empty list is re-
turned. If the function fails, no updates are done.
The specified Key is used to identify the object by either
matching the key of an object in a set table, or compare equal
to the key of an object in an ordered_set table (for details on
the difference, see lookup/2 and new/2).
If a default object Default is specified, it is used as the ob-
ject to be updated if the key is missing from the table. The
value in place of the key is ignored and replaced by the proper
key value. The return value is as if the default object had not
been used, that is, a single updated element or a list of them.
The function fails with reason badarg in the following situa-
tions:
* The table type is not set or ordered_set.
* No object with the correct key exists and no default object
was supplied.
* The object has the wrong arity.
* The default object arity is smaller than <keypos>.
* Any field from the default object that is updated is not an
integer.
* The element to update is not an integer.
* The element to update is also the key.
* Any of Pos, Incr, Threshold, or SetValue is not an integer.
update_element(Tab, Key, ElementSpec :: {Pos, Value}) -> boolean()
update_element(Tab, Key, ElementSpec :: [{Pos, Value}]) ->
boolean()
Types:
Tab = tab()
Key = term()
Value = term()
Pos = integer() >= 1
This function provides an efficient way to update one or more
elements within an object, without the trouble of having to look
up, update, and write back the entire object.
This function destructively updates the object with key Key in
table Tab. The element at position Pos is given the value Value.
A list of {Pos,Value} can be supplied to update many elements
within the same object. If the same position occurs more than
once in the list, the last value in the list is written. If the
list is empty or the function fails, no updates are done. The
function is also atomic in the sense that other processes can
never see any intermediate results.
Returns true if an object with key Key is found, otherwise
false.
The specified Key is used to identify the object by either
matching the key of an object in a set table, or compare equal
to the key of an object in an ordered_set table (for details on
the difference, see lookup/2 and new/2).
The function fails with reason badarg in the following situa-
tions:
* The table type is not set or ordered_set.
* Pos < 1.
* Pos > object arity.
* The element to update is also the key.
whereis(TableName) -> tid() | undefined
Types:
TableName = atom()
This function returns the tid() of the named table identified by
TableName, or undefined if no such table exists. The tid() can
be used in place of the table name in all operations, which is
slightly faster since the name does not have to be resolved on
each call.
If the table is deleted, the tid() will be invalid even if an-
other named table is created with the same name.
Ericsson AB stdlib 3.13 ets(3erl)