# erlang

### MODULE

erlang

### MODULE SUMMARY

The Erlang BIFs

### DESCRIPTION

By convention, most built-in functions (BIFs) are seen as being in the module `erlang`. A number of the BIFs are viewed more or less as part of the Erlang programming language and are auto-imported. Thus, it is not necessary to specify the module name and both the calls `atom_to_list(Erlang)` and `erlang:atom_to_list(Erlang)` are identical.

In the text, auto-imported BIFs are listed without module prefix. BIFs listed with module prefix are not auto-imported.

BIFs may fail for a variety of reasons. All BIFs fail with reason `badarg` if they are called with arguments of an incorrect type. The other reasons that may make BIFs fail are described in connection with the description of each individual BIF.

Some BIFs may be used in guard tests, these are marked with "Allowed in guard tests".

### DATA TYPES

    ext_binary()
      a binary data object,
      structured according to the Erlang external term format

    iodata() = iolist() | binary()

    iolist() = [char() | binary() | iolist()]
      a binary is allowed as the tail of the list

### EXPORTS

`abs(Number) -> int() | float()`

Types:

`Number = number()`

Returns an integer or float which is the arithmetical absolute value of `Number`.

    > abs(-3.33).
    3.33000
    > abs(-3).
    3

Allowed in guard tests.

`erlang:append_element(Tuple1, Term) -> Tuple2`

Types:

`Tuple1 = Tuple2 = tuple()`
`Term = term()`

Returns a new tuple which has one element more than `Tuple1`, and contains the elements in `Tuple1` followed by `Term` as the last element. Semantically equivalent to `list_to_tuple(tuple_to_list(Tuple ++ [Term])`, but much faster.

    > erlang:append_element({one, two}, three).
    {one,two,three}

`apply(Fun, Args) -> term() | empty()`

Types:

`Fun = fun()`
`Args = [term()]`

Call a fun, passing the elements in `Args` as arguments.

Note: If the number of elements in the arguments are known at compile-time, the call is better written as `Fun(Arg1, Arg2, ... ArgN)`.

| Earlier, `Fun` could also be given as `{Module, Function}`, equivalent to `apply(Module, Function, Args)`. This usage is deprecated and will stop working in a future release of Erlang/OTP.

`apply(Module, Function, Args) -> term() | empty()`

Types:

`Module = Function = atom()`
`Args = [term()]`

Returns the result of applying `Function` in `Module` to `Args`. The applied function must be exported from `Module`. The arity of the function is the length of `Args`.

    > apply(lists, reverse, [[a, b, c]]).
    [c,b,a]

`apply` can be used to evaluate BIFs by using the module name `erlang`.

    > apply(erlang, atom_to_list, ['Erlang']).
    "Erlang"

Note: If the number of arguments are known at compile-time, the call is better written as `Module:Function(Arg1, Arg2, ..., ArgN)`.

Failure: `error_handler:undefined_function/3` is called if the applied function is not exported. The error handler can be redefined (see process_flag/2). If the `error_handler` is undefined, or if the user has redefined the default `error_handler` so the replacement module is undefined, an error with the reason `undef` is generated.

`atom_to_list(Atom) -> string()`

Types:

`Atom = atom()`

Returns a string which corresponds to the text representation of `Atom`.

    > atom_to_list('Erlang').
    "Erlang"

`binary_to_list(Binary) -> [char()]`

Types:

`Binary = binary()`

Returns a list of integers which correspond to the bytes of `Binary`.

`binary_to_list(Binary, Start, Stop) -> [char()]`

Types:

`Binary = binary()`
`Start = Stop = 1..size(Binary)`

As `binary_to_list/1`, but returns a list of integers corresponding to the bytes from position `Start` to position `Stop` in `Binary`. Positions in the binary are numbered starting from 1.

`binary_to_term(Binary) -> term()`

Types:

`Binary = ext_binary()`

Returns an Erlang term which is the result of decoding the binary object `Binary`, which must be encoded according to the Erlang external term format. See also term_to_binary/1.

`erlang:bump_reductions(Reductions) -> void()`

Types:

`Reductions = int()`

This implementation-dependent function increments the reduction counter for the calling process. In the Beam emulator, the reduction counter is normally incremented by one for each function and BIF call, and a context switch is forced when the counter reaches 1000.

| This BIF might be removed in a future version of the Beam machine without prior warning. It is unlikely to be implemented in other Erlang implementations.

`erlang:cancel_timer(TimerRef) -> Time | false`

Types:

`TimerRef = ref()`
`Time = int()`

Cancels a timer, where `TimerRef` was returned by either erlang:send_after/3 or erlang:start_timer/3. If the timer is there to be removed, the function returns the time in milliseconds left until the timer would have expired, otherwise `false` (which means that `TimerRef` was never a timer, that it has already been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:read_timer/1.

Note: Cancelling a timer does not guarantee that the message has not already been delivered to the message queue.

`check_process_code(Pid, Module) -> bool()`

Types:

`Pid = pid()`
`Module = atom()`

Returns `true` if the process `Pid` is executing old code for `Module`. That is, if the current call of the process executes old code for this module, or if the process has references to old code for this module, or if the process contains funs that references old code for this module. Otherwise, it returns `false`.

    > check_process_code(Pid, lists).
    false

See also code(3).

`concat_binary(ListOfBinaries)`

Do not use; use list_to_binary/1 instead.

`date() -> {Year, Month, Day}`

Types:

`Year = Month = Day = int()`

Returns the current date as `{Year, Month, Day}`.

The time zone and daylight saving time correction depend on the underlying OS.

    > date().
    {1995, 2, 19}

`delete_module(Module) -> true | undefined`

Types:

`Module = atom()`

Makes the current code for `Module` become old code, and deletes all references for this module from the export table. Returns `undefined` if the module does not exist, otherwise `true`.

| This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: `badarg` if there is already an old version of `Module`.

`erlang:demonitor(MonitorRef) -> true`

Types:

`MonitorRef = ref()`

If `MonitorRef` is a reference which the calling process obtained by calling erlang:monitor/2, this monitoring is turned off. If the monitoring is already turned off, nothing happens.

Once `erlang:demonitor(MonitorRef)` has returned it is guaranteed that no `{'DOWN', MonitorRef, _, _, _}` message due to the monitor will be placed in the callers message queue in the future. A `{'DOWN', MonitorRef, _, _, _}` message might have been placed in the callers message queue prior to the call, though. Therefore, in most cases, it is advisable to remove such a `'DOWN'` message from the message queue after monitoring has been stopped. erlang:demonitor(MonitorRef, [flush]) can be used instead of `erlang:demonitor(MonitorRef)` if this cleanup is wanted.

Prior to OTP release R11B (erts version 5.5) `erlang:demonitor/1` behaved completely asynchronous, i.e., the monitor was active until the "demonitor signal" reached the monitored entity. This had one undesirable effect, though. You could never know when you were guaranteed not to receive a `DOWN` message due to the monitor.

Current behavior can be viewed as two combined operations: asynchronously send a "demonitor signal" to the monitored entity and ignore any future results of the monitor.

Failure: It is an error if `MonitorRef` refers to a monitoring started by another process. Not all such cases are cheap to check; if checking is cheap, the call fails with `badarg` (for example if `MonitorRef` is a remote reference).

`erlang:demonitor(MonitorRef, OptionList) -> true`

Types:

`MonitorRef = ref()`
`OptionList = [Option]`
`Option = flush`

`erlang:demonitor(MonitorRef, [])` is equivalent to erlang:demonitor(MonitorRef).

Currently the following `Option`s are valid:

`flush`
  Remove (one) `{'DOWN', MonitorRef, _, _, _}` message, if there is one, from the callers message queue after monitoring has been stopped.
  Calling `erlang:demonitor(MonitorRef, [flush])` is equivalent to:

          erlang:demonitor(MonitorRef),
          receive
              {'DOWN', MonitorRef, _, _, _} ->
                  true
          after 0 ->
                  true
          end

| More options may be added in the future.

Failure: `badarg` if `OptionList` is not a list, or if `Option` is not a valid option, or the same failure as for erlang:demonitor/1

`disconnect_node(Node) -> bool() | ignored`

Types:

`Node = atom()`

Forces the disconnection of a node. This will appear to the node `Node` as if the local node has crashed. This BIF is mainly used in the Erlang network authentication protocols. Returns `true` if disconnection succeeds, otherwise `false`. If the local node is not alive, the function returns `ignored`.

`erlang:display(Term) -> true`

Types:

`Term = term()`

Prints a text representation of `Term` on the standard output.

| This BIF is intended for debugging only.

`element(N, Tuple) -> term()`

Types:

`N = 1..size(Tuple)`
`Tuple = tuple()`

Returns the `N`th element (numbering from 1) of `Tuple`.

    > element(2, {a, b, c}).
    b

Allowed in guard tests.

`erase() -> [{Key, Val}]`

Types:

`Key = Val = term()`

Returns the process dictionary and deletes it.

    > put(key1, {1, 2, 3}),
      put(key2, [a, b, c]),
      erase().
    [{key1,{1,2,3}},{key2,[a,b,c]}]

`erase(Key) -> Val | undefined`

Types:

`Key = Val = term()`

Returns the value `Val` associated with `Key` and deletes it from the process dictionary. Returns `undefined` if no value is associated with `Key`.

    > put(key1, {merry, lambs, are, playing}),
      X = erase(key1),
      {X, erase(key1)}.
    {{merry,lambs,are,playing},undefined}

`erlang:error(Reason)`

Types:

`Reason = term()`

Stops the execution of the calling process with the reason `Reason`, where `Reason` is any term. The actual exit reason will be `{Reason, Where}`, where `Where` is a list of the functions most recently called (the current function first). Since evaluating this function causes the process to terminate, it has no return value.

    > catch erlang:error(foobar).
    {'EXIT',{foobar,[{erl_eval,do_apply,5},
                     {erl_eval,expr,5},
                     {shell,exprs,6},
                     {shell,eval_loop,3}]}}

`erlang:error(Reason, Args)`

Types:

`Reason = term()`
`Args = [term()]`

Stops the execution of the calling process with the reason `Reason`, where `Reason` is any term. The actual exit reason will be `{Reason, Where}`, where `Where` is a list of the functions most recently called (the current function first). `Args` is expected to be the list of arguments for the current function; in Beam it will be used to provide the actual arguments for the current function in the `Where` term. Since evaluating this function causes the process to terminate, it has no return value.

`exit(Reason)`

Types:

`Reason = term()`

Stops the execution of the calling process with the exit reason `Reason`, where `Reason` is any term. Since evaluating this function causes the process to terminate, it has no return value.

    > exit(foobar).
    ** exited: foobar **
    > catch exit(foobar).
    {'EXIT', foobar}

`exit(Pid, Reason) -> true`

Types:

`Pid = pid()`
`Reason = term()`

Sends an exit signal with exit reason `Reason` to the process `Pid`.

The following behavior apply if `Reason` is any term except `normal` or `kill`:

If `Pid` is not trapping exits, `Pid` itself will exit with exit reason `Reason`. If `Pid` is trapping exits, the exit signal is transformed into a message `{'EXIT', From, Reason}` and delivered to the message queue of `Pid`. `From` is the pid of the process which sent the exit signal. See also process_flag/2.

If `Reason` is the atom `normal`, `Pid` will not exit. If it is trapping exits, the exit signal is transformed into a message `{'EXIT', From, normal}` and delivered to its message queue.

If `Reason` is the atom `kill`, that is if `exit(Pid, kill)` is called, an untrappable exit signal is sent to `Pid` which will unconditionally exit with exit reason `killed`.

`erlang:fault(Reason)`

Types:

`Reason = term()`

Stops the execution of the calling process with the reason `Reason`. This is an old equivalent to erlang:error(Reason).

`erlang:fault(Reason, Args)`

Types:

`Reason = term()`
`Args = [term()]`

Stops the execution of the calling process with the reason `Reason`. This is an old equivalent to erlang:error(Reason, Args) .

`float(Number) -> float()`

Types:

`Number = number()`

Returns a float by converting `Number` to a float.

    > float(55).
    55.0000

Allowed in guard tests.

| Note that if used on the top-level in a guard, it will test whether the argument is a floating point number; for clarity, use is_float/1 instead. When `float/1` is used in an expression in a guard, such as '`float(A) == 4.0`', it converts a number as described above.

`float_to_list(Float) -> string()`

Types:

`Float = float()`

Returns a string which corresponds to the text representation of `Float`.

    > float_to_list(7.0).
    "7.00000000000000000000e+00"

`erlang:fun_info(Fun) -> [{Item, Info}]`

Types:

`Fun = fun()`
`Item, Info -- see below`

Returns a list containing information about the fun `Fun`. Each element of the list is a tuple. The order of the tuples is not defined, and more tuples may be added in a future release.

| This BIF is mainly intended for debugging, but it can occasionally be useful in library functions that might need to verify, for instance, the arity of a fun.

There are two types of funs with slightly different semantics:

A fun created by `fun M:F/A` is called an external fun. Calling it will always call the function `F` with arity `A` in the latest code for module `M`. Note that module `M` does not even need to be loaded when the fun `fun M:F/A` is created.

All other funs are called local. When a local fun is called, the same version of the code that created the fun will be called (even if newer version of the module has been loaded).

The following elements will always be present in the list for both local and external funs:

`{type, Type}`
  `Type` is either `local` or `external`.
`{module, Module}`
  `Module` (an atom) is the module name.
  If `Fun` is a local fun, `Module` is the module in which the fun is defined.
  If `Fun` is an external fun, `Module` is the module that the fun refers to.
`{name, Name}`
  `Name` (an atom) is a function name.
  If `Fun` is a local fun, `Name` is the name of the local function that implements the fun. (This name was generated by the compiler, and is generally only of informational use. As it is a local function, it is not possible to call it directly.) If no code is currently loaded for the fun, `[]` will be returned instead of an atom.
  If `Fun` is an external fun, `Name` is the name of the exported function that the fun refers to.
`{arity, Arity}`
  `Arity` is the number of arguments that the fun should be called with.
`{env, Env}`
  `Env` (a list) is the environment or free variables for the fun. (For external funs, the returned list is always empty.)

The following elements will only be present in the list if `Fun` is local:

`{pid, Pid}`
  `Pid` is the pid of the process that originally created the fun.
`{index, Index}`
  `Index` (an integer) is an index into the module's fun table.
`{new_index, Index}`
  `Index` (an integer) is an index into the module's fun table.
`{new_uniq, Uniq}`
  `Uniq` (a binary) is a unique value for this fun.
`{uniq, Uniq}`
  `Uniq` (an integer) is a unique value for this fun.

`erlang:fun_info(Fun, Item) -> {Item, Info}`

Types:

`Fun = fun()`
`Item, Info -- see below`

Returns information about `Fun` as specified by `Item`, in the form `{Item,Info}`.

For any fun, `Item` can be any of the atoms `module`, `name`, `arity`, or `env`.

For a local fun, `Item` can also be any of the atoms `index`, `new_index`, `new_uniq`, `uniq`, and `pid`. For an external fun, the value of any of these items is always the atom `undefined`.

See erlang:fun_info/1.

`erlang:fun_to_list(Fun) -> string()`

Types:

`Fun = fun()`

Returns a string which corresponds to the text representation of `Fun`.

`erlang:function_exported(Module, Function, Arity) -> bool()`

Types:

`Module = Function = atom()`
`Arity = int()`

Returns `true` if the module `Module` is loaded and contains an exported function `Function/Arity`; otherwise `false`.

Returns `false` for any BIF (functions implemented in C rather than in Erlang).

This function is retained mainly for backwards compatibility.

`garbage_collect() -> true`

Forces an immediate garbage collection of the currently executing process. The function should not be used, unless it has been noticed -- or there are good reasons to suspect -- that the spontaneous garbage collection will occur too late or not at all. Improper use may seriously degrade system performance.

Compatibility note: In versions of OTP prior to R7, the garbage collection took place at the next context switch, not immediately. To force a context switch after a call to `erlang:garbage_collect()`, it was sufficient to make any function call.

`garbage_collect(Pid) -> bool()`

Types:

`Pid = pid()`

Works like `erlang:garbage_collect()` but on any process. The same caveats apply. Returns `false` if `Pid` refers to a dead process; `true` otherwise.

`get() -> [{Key, Val}]`

Types:

`Key = Val = term()`

Returns the process dictionary as a list of `{Key, Val}` tuples.

    > put(key1, merry),
      put(key2, lambs),
      put(key3, {are, playing}),
      get().
    [{key1,merry},{key2,lambs},{key3,{are,playing}}]

`get(Key) -> Val | undefined`

Types:

`Key = Val = term()`

Returns the value `Val`associated with `Key` in the process dictionary, or `undefined` if `Key` does not exist.

    > put(key1, merry),
      put(key2, lambs),
      put({any, [valid, term]}, {are, playing}),
      get({any, [valid, term]}).
    {are,playing}

`erlang:get_cookie() -> Cookie | nocookie`

Types:

`Cookie = atom()`

Returns the magic cookie of the local node, if the node is alive; otherwise the atom `nocookie`.

`get_keys(Val) -> [Key]`

Types:

`Val = Key = term()`

Returns a list of keys which are associated with the value `Val` in the process dictionary.

    > put(mary, {1, 2}),
      put(had, {1, 2}),
      put(a, {1, 2}),
      put(little, {1, 2}),
      put(dog, {1, 3}),
      put(lamb, {1, 2}),
      get_keys({1, 2}).
    [mary,had,a,little,lamb]

`erlang:get_stacktrace() -> [{Module, Function, Arity | Args}]`

Types:

`Module = Function = atom()`
`Arity = int()`
`Args = [term()]`

Get the call stack backtrace (stacktrace) of the last exception in the calling process as a list of `{Module,Function,Arity}` tuples. The `Arity` field in the first tuple may be the argument list of that function call instead of an arity integer, depending on the exception.

If there has not been any exceptions in a process, the stacktrace is []. After a code change for the process, the stacktrace may also be reset to [].

The stacktrace is the same data as the `catch` operator returns, for example:

`{'EXIT',{badarg,Stacktrace}} = catch abs(x)`

See also erlang:error/1 and erlang:error/2.

`group_leader() -> GroupLeader`

Types:

`GroupLeader = pid()`

Returns the pid of the group leader for the process which evaluates the function.

Every process is a member of some process group and all groups have a group leader. All IO from the group is channeled to the group leader. When a new process is spawned, it gets the same group leader as the spawning process. Initially, at system start-up, `init` is both its own group leader and the group leader of all processes.

`group_leader(GroupLeader, Pid) -> true`

Types:

`GroupLeader = Pid = pid()`

Sets the group leader of `Pid` to `GroupLeader`. Typically, this is used when a processes started from a certain shell should have another group leader than `init`.

See also group_leader/0.

`halt()`

Halts the Erlang runtime system and indicates normal exit to the calling environment. Has no return value.

    > halt().
    os_prompt%

`halt(Status)`

Types:

`Status = int()>=0 | string()`

`Status` must be a non-negative integer, or a string. Halts the Erlang runtime system. Has no return value. If `Status` is an integer, it is returned as an exit status of Erlang to the calling environment. If `Status` is a string, produces an Erlang crash dump with `String` as slogan, and then exits with a non-zero status code.

Note that on many platforms, only the status codes 0-255 are supported by the operating system.

`erlang:hash(Term, Range) -> Hash`

Returns a hash value for `Term` within the range `1..Range`. The allowed range is 1..2^27-1.

| This BIF is deprecated as the hash value may differ on different architectures. Also the hash values for integer terms larger than 2^27 as well as large binaries are very poor. The BIF is retained for backward compatibility reasons (it may have been used to hash records into a file), but all new code should use one of the BIFs `erlang:phash/2` or `erlang:phash2/1,2` instead.

`hd(List) -> term()`

Types:

`List = [term()]`

Returns the head of `List`, that is, the first element.

    > hd([1,2,3,4,5]).
    1

Allowed in guard tests.

Failure: `badarg` if `List` is the empty list [].

`erlang:hibernate(Module, Function, Args)`

Types:

`Module = Function = atom()`
`Args = [term()]`

Puts the calling process into a wait state where its memory allocation has been reduced as much as possible, which is useful if the process does not expect to receive any messages in the near future.

The process will be awaken when a message is sent to it, and control will resume in `Module:Function` with the arguments given by `Args` with the call stack emptied, meaning that the process will terminate when that function returns. Thus `erlang:hibernate/3` will never return to its caller.

If the process has any message in its message queue, the process will be awaken immediately in the same way as described above.

In more technical terms, what `erlang:hibernate/3` does is the following. It discards the call stack for the process. Then it garbage collects the process. After the garbage collection, all live data is in one continuous heap. The heap is then shrunken to the exact same size as the live data which it holds (even if that size is less than the minimum heap size for the process).

If the size of the live data in the process is less than the minimum heap size, the first garbage collection occurring after the process has been awaken will ensure that the heap size is changed to a size not smaller than the minimum heap size.

Note that emptying the call stack means that any surrounding `catch` is removed and has to be re-inserted after hibernation. One effect of this is that processes started using `proc_lib` (also indirectly, such as `gen_server` processes), should use proc_lib:hibernate/3 instead to ensure that the exception handler continues to work when the process wakes up.

`erlang:info(Type) -> Res`

This BIF is now equivalent to erlang:system_info/1.

`integer_to_list(Integer) -> string()`

Types:

`Integer = int()`

Returns a string which corresponds to the text representation of `Integer`.

    > integer_to_list(77).
    "77"

`erlang:integer_to_list(Integer, Base) -> string()`

Types:

`Integer = int()`
`Base = 2..36`

Returns a string which corresponds to the text representation of `Integer` in base `Base`.

    > erlang:integer_to_list(1023, 16).
    "3FF"

`iolist_to_binary(IoListOrBinary) -> binary()`

Types:

`IoListOrBinary = iolist() | binary()`

Returns a binary which is made from the integers and binaries in `IoListOrBinary`.

    > Bin1 = <<1,2,3>>.
    <<1,2,3>>
    > Bin2 = <<4,5>>.
    <<4,5>>
    > Bin3 = <<6>>.
    <<6>>
    > iolist_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
    <<1,2,3,1,2,3,4,5,4,6>>

`iolist_size(Item) -> int()`

Types:

`Item = iolist() | binary()`

Returns an integer which is the size in bytes of the binary that would be the result of `iolist_to_binary(Item)`.

    > iolist_size([1,2|<<3,4>>]).
    4

`is_alive() -> bool()`

Returns `true` if the local node is alive; that is, if the node can be part of a distributed system. Otherwise, it returns `false`.

`is_atom(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is an atom; otherwise returns `false`.

Allowed in guard tests.

`is_binary(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a binary; otherwise returns `false`.

Allowed in guard tests.

`is_boolean(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is either the atom `true` or the atom `false` (i.e. a boolean); otherwise returns `false`.

Allowed in guard tests.

`erlang:is_builtin(Module, Function, Arity) -> bool()`

Types:

`Module = Function = atom()`
`Arity = int()`

Returns `true` if `Module:Function/Arity` is a BIF implemented in C; otherwise returns `false`. This BIF is useful for builders of cross reference tools.

`is_float(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a floating point number; otherwise returns `false`.

Allowed in guard tests.

`is_function(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a fun; otherwise returns `false`.

Allowed in guard tests.

`is_function(Term, Arity) -> bool()`

Types:

`Term = term()`
`Arity = int()`

Returns `true` if `Term` is a fun that can be applied with `Arity` number of arguments; otherwise returns `false`.

Allowed in guard tests.

| Currently, `is_function/2` will also return `true` if the first argument is a tuple fun (a tuple containing two atoms). In a future release, tuple funs will no longer be supported and `is_function/2` will return `false` if given a tuple fun.

`is_integer(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is an integer; otherwise returns `false`.

Allowed in guard tests.

`is_list(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a list with zero or more elements; otherwise returns `false`.

Allowed in guard tests.

`is_number(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is either an integer or a floating point number; otherwise returns `false`.

Allowed in guard tests.

`is_pid(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a pid (process identifier); otherwise returns `false`.

Allowed in guard tests.

`is_port(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a port identifier; otherwise returns `false`.

Allowed in guard tests.

`is_process_alive(Pid) -> bool()`

Types:

`Pid = pid()`

`Pid` must refer to a process at the local node. Returns `true` if the process exists and is alive, that is, has not exited. Otherwise, returns `false`.

`is_record(Term, RecordTag) -> bool()`

Types:

`Term = term()`
`RecordTag = atom()`

Returns `true` if `Term` is a tuple and its first element is `RecordTag`. Otherwise, returns `false`.

| Normally the compiler treats calls to `is_record/2` specially. It emits code to verify that `Term` is a tuple, that its first element is `RecordTag`, and that the size is correct. However, if the `RecordTag` is not a literal atom, the `is_record/2` BIF will be called instead and the size of the tuple will not be verified.

Allowed in guard tests, if `RecordTag` is a literal atom.

`is_record(Term, RecordTag, Size) -> bool()`

Types:

`Term = term()`
`RecordTag = atom()`
`Size = int()`

`RecordTag` must be an atom. Returns `true` if `Term` is a tuple, its first element is `RecordTag`, and its size is `Size`. Otherwise, returns `false`.

Allowed in guard tests, provided that `RecordTag` is a literal atom and `Size` is a literal integer.

| This BIF is documented for completeness. In most cases `is_record/2` should be used.

`is_reference(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a reference; otherwise returns `false`.

Allowed in guard tests.

`is_tuple(Term) -> bool()`

Types:

`Term = term()`

Returns `true` if `Term` is a tuple; otherwise returns `false`.

Allowed in guard tests.

`length(List) -> int()`

Types:

`List = [term()]`

Returns the length of `List`.

    > length([1,2,3,4,5,6,7,8,9]).
    9

Allowed in guard tests.

`link(Pid) -> true`

Types:

`Pid = pid() | port()`

Creates a link between the calling process and another process (or port) `Pid`, if there is not such a link already. If a process attempts to create a link to itself, nothing is done. Returns `true`.

If `Pid` does not exist, the behavior of the BIF depends on if the calling process is trapping exits or not (see process_flag/2):

- If the calling process is not trapping exits, and checking `Pid` is cheap -- that is, if `Pid` is local -- `link/1` fails with reason `noproc`.
- Otherwise, if the calling process is trapping exits, and/or `Pid` is remote, `link/1` returns `true`, but an exit signal with reason `noproc` is sent to the calling process.

`list_to_atom(String) -> atom()`

Types:

`String = string()`

Returns the atom whose text representation is `String`.

    > list_to_atom("Erlang").
    'Erlang'

`list_to_binary(IoList) -> binary()`

Types:

`IoList = iolist()`

Returns a binary which is made from the integers and binaries in `IoList`.

    > Bin1 = <<1,2,3>>.
    <<1,2,3>>
    > Bin2 = <<4,5>>.
    <<4,5>>
    > Bin3 = <<6>>.
    <<6>>
    > list_to_binary([Bin1,1,[2,3,Bin2],4|Bin3]).
    <<1,2,3,1,2,3,4,5,4,6>>

`list_to_existing_atom(String) -> atom()`

Types:

`String = string()`

Returns the atom whose text representation is `String`, but only if there already exists such atom.

Failure: `badarg` if there does not already exist an atom whose text representation is `String`.

`list_to_float(String) -> float()`

Types:

`String = string()`

Returns the float whose text representation is `String`.

    > list_to_float("2.2017764e+0").
    2.20178

Failure: `badarg` if `String` contains a bad representation of a float.

`list_to_integer(String) -> int()`

Types:

`String = string()`

Returns an integer whose text representation is `String`.

    > list_to_integer("123").
    123

Failure: `badarg` if `String` contains a bad representation of an integer.

`erlang:list_to_integer(String, Base) -> int()`

Types:

`String = string()`
`Base = 2..36`

Returns an integer whose text representation in base `Base` is `String`.

    > erlang:list_to_integer("3FF", 16).
    1023

Failure: `badarg` if `String` contains a bad representation of an integer.

`list_to_pid(String) -> pid()`

Types:

`String = string()`

Returns a pid whose text representation is `String`.

| This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

    > list_to_pid("<0.4.1>").
    <0.4.1>

Failure: `badarg` if `String` contains a bad representation of a pid.

`list_to_tuple(List) -> tuple()`

Types:

`List = [term()]`

Returns a tuple which corresponds to `List`. `List` can contain any Erlang terms.

    > list_to_tuple([share, ['Ericsson_B', 163]]).
    {share, ['Ericsson_B', 163]}

`load_module(Module, Binary) -> {module, Module} | {error, Reason}`

Types:

`Module = atom()`
`Binary = binary()`
`Reason = badfile | not_purged | badfile`

If `Binary` contains the object code for the module `Module`, this BIF loads that object code. Also, if the code for the module `Module` already exists, all export references are replaced so they point to the newly loaded code. The previously loaded code is kept in the system as old code, as there may still be processes which are executing that code. It returns either `{module, Module}`, or `{error, Reason}` if loading fails. `Reason` is one of the following:

`badfile`
  The object code in `Binary` has an incorrect format.
`not_purged`
  `Binary` contains a module which cannot be loaded because old code for this module already exists.
`badfile`
  The object code contains code for another module than `Module`

| This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

`erlang:loaded() -> [Module]`

Types:

`Module = atom()`

Returns a list of all loaded Erlang modules (current and/or old code), including preloaded modules.

See also code(3).

`erlang:localtime() -> {Date, Time}`

Types:

`Date = {Year, Month, Day}`
`Time = {Hour, Minute, Second}`
`Year = Month = Day = Hour = Minute = Second = int()`

Returns the current local date and time `{{Year, Month, Day}, {Hour, Minute, Second}}`.

The time zone and daylight saving time correction depend on the underlying OS.

    > erlang:localtime().
    {{1996,11,6},{14,45,17}}

`erlang:localtime_to_universaltime({Date1, Time1}) -> {Date2, Time2}`

Types:

`Date1 = Date2 = {Year, Month, Day}`
`Time1 = Time2 = {Hour, Minute, Second}`
`Year = Month = Day = Hour = Minute = Second = int()`

Converts local date and time to Universal Time Coordinated (UTC), if this is supported by the underlying OS. Otherwise, no conversion is done and `{Date1, Time1}` is returned.

    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}).
    {{1996,11,6},{13,45,17}}

Failure: `badarg` if `Date1` or `Time1` do not denote a valid date or time.

`erlang:localtime_to_universaltime({Date1, Time1}, IsDst) -> {Date2, Time2}`

Types:

`Date1 = Date2 = {Year, Month, Day}`
`Time1 = Time2 = {Hour, Minute, Second}`
`Year = Month = Day = Hour = Minute = Second = int()`
`IsDst = true | false | undefined`

Converts local date and time to Universal Time Coordinated (UTC) just like `erlang:localtime_to_universaltime/1`, but the caller decides if daylight saving time is active or not.

If `IsDst == true` the `{Date1, Time1}` is during daylight saving time, if `IsDst == false` it is not, and if `IsDst == undefined` the underlying OS may guess, which is the same as calling `erlang:localtime_to_universaltime({Date1, Time1})`.

    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, true).
    {{1996,11,6},{12,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, false).
    {{1996,11,6},{13,45,17}}
    > erlang:localtime_to_universaltime({{1996,11,6},{14,45,17}}, undefined).
    {{1996,11,6},{13,45,17}}

Failure: `badarg` if `Date1` or `Time1` do not denote a valid date or time.

`make_ref() -> ref()`

Returns an almost unique reference.

The returned reference will reoccur after approximately 2^82 calls; therefore it is unique enough for practical purposes.

    > make_ref().
    #Ref<0.0.0.135>

`erlang:make_tuple(Arity, InitialValue) -> tuple()`

Types:

`Arity = int()`
`InitialValue = term()`

Returns a new tuple of the given `Arity`, where all elements are `InitialValue`.

    > erlang:make_tuple(4, []).
    {[],[],[],[]}

`erlang:md5(Data) -> Digest`

Types:

`Data = iodata()`
`Digest = binary()`

Computes an `MD5` message digest from `Data`, where the length of the digest is 128 bits (16 bytes). `Data` is a binary or a list of small integers and binaries.

See The MD5 Message Digest Algorithm (RFC 1321) for more information about MD5.

`erlang:md5_final(Context) -> Digest`

Types:

`Context = Digest = binary()`

Finishes the update of an MD5 `Context` and returns the computed `MD5` message digest.

`erlang:md5_init() -> Context`

Types:

`Context = binary()`

Creates an MD5 context, to be used in subsequent calls to `md5_update/2`.

`erlang:md5_update(Context, Data) -> NewContext`

Types:

`Data = iodata()`
`Context = NewContext = binary()`

Updates an MD5 `Context` with `Data`, and returns a `NewContext`.

`erlang:memory() -> [{Type, Size}]`

Types:

`Type, Size -- see below`

Returns a list containing information about memory dynamically allocated by the Erlang emulator. Each element of the list is a tuple `{Type, Size}`. The first element `Type`is an atom describing memory type. The second element `Size`is memory size in bytes. A description of each memory type follows:

`total`
  The total amount of memory currently allocated, which is the same as the sum of memory size for `processes` and `system`.
`processes`
  The total amount of memory currently allocated by the Erlang processes.
`processes_used`
  The total amount of memory currently used by the Erlang processes.
  This memory is part of the memory presented as `processes` memory.
`system`
  The total amount of memory currently allocated by the emulator that is not directly related to any Erlang process.
  Memory presented as `processes` is not included in this memory.
`atom`
  The total amount of memory currently allocated for atoms.
  This memory is part of the memory presented as `system` memory.
`atom_used`
  The total amount of memory currently used for atoms.
  This memory is part of the memory presented as `atom` memory.
`binary`
  The total amount of memory currently allocated for binaries.
  This memory is part of the memory presented as `system` memory.
`code`
  The total amount of memory currently allocated for Erlang code.
  This memory is part of the memory presented as `system` memory.
`ets`
  The total amount of memory currently allocated for ets tables.
  This memory is part of the memory presented as `system` memory.
`maximum`
  The maximum total amount of memory allocated since the emulator was started.
  This tuple is only present when the emulator is run with instrumentation.
  For information on how to run the emulator with instrumentation see instrument(3) and/or erl(1).

The `system` value is not complete. Some allocated memory that should be part of the `system` value are not. For example, memory allocated by drivers is missing.

When the emulator is run with instrumentation, the `system` value is more accurate, but memory directly allocated by `malloc` (and friends) are still not part of the `system` value. Direct calls to `malloc` are only done from OS specific runtime libraries and perhaps from user implemented Erlang drivers that do not use the memory allocation functions in the driver interface.

Since the `total` value is the sum of `processes` and `system` the error in `system` will propagate to the `total` value.

The different values has the following relation to each other. Values beginning with an uppercase letter is not part of the result.

            total = processes + system
            processes = processes_used + ProcessesNotUsed
            system = atom + binary + code + ets + OtherSystem
            atom = atom_used + AtomNotUsed

            RealTotal = processes + RealSystem
            RealSystem = system + MissedSystem

The `total` value is supposed to be the total amount of memory dynamically allocated by the emulator. Shared libraries, the code of the emulator itself, and the emulator stack(s) are not supposed to be included. That is, the `total` value is not supposed to be equal to the total size of all pages mapped to the emulator. Furthermore, due to fragmentation and pre-reservation of memory areas, the size of the memory segments which contain the dynamically allocated memory blocks can be substantially larger than the total size of the dynamically allocated memory blocks.

More tuples in the returned list may be added in the future.

`erlang:memory(Type | [Type]) -> Size | [{Type, Size}]`

Types:

`Type, Size -- see below`

Returns the memory size in bytes allocated for memory of type `Type`. The argument can also be given as a list of `Type` atoms, in which case a corresponding list of `{Type, Size}` tuples is returned.

See erlang:memory/0.

Failure: `badarg` if the emulator is not run with instrumentation when `Type == maximum`.

`module_loaded(Module) -> bool()`

Types:

`Module = atom()`

Returns `true` if the module `Module` is loaded, otherwise returns `false`. It does not attempt to load the module.

| This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

`erlang:monitor(Type, Item) -> MonitorRef`

Types:

`Type = process`
`Item = pid() | {RegName, Node} | RegName`
`RegName = atom()`
`Node = node()`
`MonitorRef = reference()`

The calling process starts monitoring `Item` which is an object of type `Type`.

Currently only processes can be monitored, i.e. the only allowed `Type` is `process`, but other types may be allowed in the future.

`Item` can be:

`pid()`
  The pid of the process to monitor.
`{RegName, Node}`
  A tuple consisting of a registered name of a process and a node name. The process residing on the node `Node` with the registered name `RegName` will be monitored.
`RegName`
  The process locally registered as `RegName` will be monitored.

| When a process is monitored by registered name, the process that has the registered name at the time when `erlang:monitor/2` is called will be monitored. The monitor will not be effected, if the registered name is unregistered.

A `'DOWN'` message will be sent to the monitoring process if `Item` dies, if `Item` does not exist, or if the connection is lost to the node which `Item` resides on. A `'DOWN'` message has the following pattern:

    {'DOWN', MonitorRef, Type, Object, Info}

where `MonitorRef` and `Type` are the same as described above, and:

`Object`
  A reference to the monitored object:
  - the pid of the monitored process, if `Item` was specified as a pid.
  - `{RegName, Node}`, if `Item` was specifed as `{RegName, Node}`.
  - `{RegName, Node}`, if `Item` was specified as `RegName`. `Node` will in this case be the name of the local node (`node()`).
`Info`
  Either the exit reason of the process, `noproc` (non-existing process), or `noconnection` (no connection to `Node`).

| If/when `erlang:monitor/2` is extended (e.g. to handle other item types than `process`), other possible values for `Object`, and `Info` in the `'DOWN'` message will be introduced.

The monitoring is turned off either when the `'DOWN'` message is sent, or when erlang:demonitor/1 is called.

If an attempt is made to monitor a process on an older node (where remote process monitoring is not implemented or one where remote process monitoring by registered name is not implemented), the call fails with `badarg`.

Making several calls to `erlang:monitor/2` for the same `Item` is not an error; it results in as many, completely independent, monitorings.

The format of the `'DOWN'` message changed in the 5.2 version of the emulator (OTP release R9B) for monitor by registered name. The `Object` element of the `'DOWN'` message could in earlier versions sometimes be the pid of the monitored process and sometimes be the registered name. Now the `Object` element is always a tuple consisting of the registered name and the node name. Processes on new nodes (emulator version 5.2 or greater) will always get `'DOWN'` messages on the new format even if they are monitoring processes on old nodes. Processes on old nodes will always get `'DOWN'` messages on the old format.

`monitor_node(Node, Flag) -> true`

Types:

`Node = node()`
`Flag = bool()`

Monitors the status of the node `Node`. If `Flag` is `true`, monitoring is turned on; if `Flag` is `false`, monitoring is turned off.

Making several calls to `monitor_node(Node, true)` for the same `Node` is not an error; it results in as many, completely independent, monitorings.

If `Node` fails or does not exist, the message `{nodedown, Node}` is delivered to the process. If a process has made two calls to `monitor_node(Node, true)` and `Node` terminates, two `nodedown` messages are delivered to the process. If there is no connection to `Node`, there will be an attempt to create one. If this fails, a `nodedown` message is delivered.

Nodes connected through hidden connections can be monitored as any other node.

Failure: `badarg`if the local node is not alive.

`node() -> Node`

Types:

`Node = node()`

Returns the name of the local node. If the node is not alive, `nonode@nohost` is returned instead.

Allowed in guard tests.

`node(Arg) -> Node`

Types:

`Arg = pid() | port() | ref()`
`Node = node()`

Returns the node where `Arg` is located. `Arg` can be a pid, a reference, or a port. If the local node is not alive, `nonode@nohost` is returned.

Allowed in guard tests.

`nodes() -> Nodes`

Types:

`Nodes = [node()]`

Returns a list of all visible nodes in the system, excluding the local node. Same as `nodes(visible)`.

`nodes(Arg | [Arg]) -> Nodes`

Types:

`Arg = visible | hidden | connected | this | known`
`Nodes = [node()]`

Returns a list of nodes according to argument given. The result returned when the argument is a list, is the list of nodes satisfying the disjunction(s) of the list elements.

`Arg` can be any of the following:

`visible`
  Nodes connected to this node through normal connections.
`hidden`
  Nodes connected to this node through hidden connections.
`connected`
  All nodes connected to this node.
`this`
  This node.
`known`
  Nodes which are known to this node, i.e., connected, previously connected, etc.

Some equalities: `[node()] = nodes(this)`, `nodes(connected) = nodes([visible, hidden])`, and `nodes() = nodes(visible)`.

If the local node is not alive, `nodes(this) == nodes(known) == [nonode@nohost]`, for any other `Arg` the empty list [] is returned.

`now() -> {MegaSecs, Secs, MicroSecs}`

Types:

`MegaSecs = Secs = MicroSecs = int()`

Returns the tuple `{MegaSecs, Secs, MicroSecs}` which is the elapsed time since 00:00 GMT, January 1, 1970 (zero hour) on the assumption that the underlying OS supports this. Otherwise, some other point in time is chosen. It is also guaranteed that subsequent calls to this BIF returns continuously increasing values. Hence, the return value from `now()` can be used to generate unique time-stamps. It can only be used to check the local time of day if the time-zone info of the underlying operating system is properly configured.

`open_port(PortName, PortSettings) -> port()`

Types:

`PortName = {spawn, Command} | {fd, In, Out}`
`Command = string()`
`In = Out = int()`
`PortSettings = [Opt]`
`Opt = {packet, N} | stream | {line, L} | {cd, Dir} | {env, Env} | exit_status | use_stdio | nouse_stdio | stderr_to_stdout | in | out | binary | eof`
`N = 1 | 2 | 4`
`L = int()`
`Dir = string()`
`Env = [{Name, Val}]`
`Name = string()`
`Val = string() | false`

Returns a port identifier as the result of opening a new Erlang port. A port can be seen as an external Erlang process. `PortName` is one of the following:

`{spawn, Command}`
  Starts an external program. `Command` is the name of the external program which will be run. `Command` runs outside the Erlang work space unless an Erlang driver with the name `Command` is found. If found, that driver will be started. A driver runs in the Erlang workspace, which means that it is linked with the Erlang runtime system.
  When starting external programs on Solaris, the system call `vfork` is used in preference to `fork` for performance reasons, although it has a history of being less robust. If there are problems with using `vfork`, setting the environment variable `ERL_NO_VFORK` to any value will cause `fork` to be used instead.
`{fd, In, Out}`
  Allows an Erlang process to access any currently opened file descriptors used by Erlang. The file descriptor `In` can be used for standard input, and the file descriptor `Out` for standard output. It is only used for various servers in the Erlang operating system (`shell` and `user`). Hence, its use is very limited.

`PortSettings` is a list of settings for the port. Valid settings are:

`{packet, N}`
  Messages are preceded by their length, sent in `N` bytes, with the most significant byte first. Valid values for `N` are 1, 2, or 4.
`stream`
  Output messages are sent without packet lengths. A user-defined protocol must be used between the Erlang process and the external object.
`{line, L}`
  Messages are delivered on a per line basis. Each line (delimited by the OS-dependent newline sequence) is delivered in one single message. The message data format is `{Flag, Line}`, where `Flag` is either `eol` or `noeol` and `Line` is the actual data delivered (without the newline sequence).
  `L` specifies the maximum line length in bytes. Lines longer than this will be delivered in more than one message, with the `Flag` set to `noeol` for all but the last message. If end of file is encountered anywhere else than immediately following a newline sequence, the last line will also be delivered with the `Flag` set to `noeol`. In all other cases, lines are delivered with `Flag` set to `eol`.
  The `{packet, N}` and `{line, L}` settings are mutually exclusive.
`{cd, Dir}`
  This is only valid for `{spawn, Command}`. The external program starts using `Dir` as its working directory. `Dir` must be a string. Not available on VxWorks.
`{env, Env}`
  This is only valid for `{spawn, Command}`. The environment of the started process is extended using the environment specifications in `Env`.
  `Env` should be a list of tuples `{Name, Val}`, where `Name` is the name of an environment variable, and `Val` is the value it is to have in the spawned port process. Both `Name` and `Val` must be strings. The one exception is `Val` being the atom `false` (in analogy with `os:getenv/1`), which removes the environment variable. Not available on VxWorks.
`exit_status`
  This is only valid for `{spawn, Command}` where `Command` refers to an external program.
  When the external process connected to the port exits, a message of the form `{Port,{exit_status,Status}}` is sent to the connected process, where `Status` is the exit status of the external process. If the program aborts, on Unix the same convention is used as the shells do (i.e., 128+signal).
  If the `eof` option has been given as well, the `eof` message and the `exit_status` message appear in an unspecified order.
  If the port program closes its stdout without exiting, the `exit_status` option will not work.
`use_stdio`
  This is only valid for `{spawn, Command}`. It allows the standard input and output (file descriptors 0 and 1) of the spawned (UNIX) process for communication with Erlang.
`nouse_stdio`
  The opposite of `use_stdio`. Uses file descriptors 3 and 4 for communication with Erlang.
`stderr_to_stdout`
  Affects ports to external programs. The executed program gets its standard error file redirected to its standard output file. `stderr_to_stdout` and `nouse_stdio` are mutually exclusive.
`in`
  The port can only be used for input.
`out`
  The port can only be used for output.
`binary`
  All IO from the port are binary data objects as opposed to lists of bytes.
`eof`
  The port will not be closed at the end of the file and produce an exit signal. Instead, it will remain open and a `{Port, eof}` message will be sent to the process holding the port.

The default is `stream` for all types of port and `use_stdio` for spawned ports.

Failure: If the port cannot be opened, the exit reason is the Posix error code which most closely describes the error, or `einval` if no Posix code is appropriate. The following Posix error codes may appear:

`enomem`
  There was not enough memory to create the port.
`eagain`
  There are no more available operating system processes.
`enametoolong`
  The external command given was too long.
`emfile`
  There are no more available file descriptors.
`enfile`
  A file or port table is full.

During use of a port opened using `{spawn, Name}`, errors arising when sending messages to it are reported to the owning process using signals of the form `{'EXIT', Port, PosixCode}`. See `file(3)` for possible values of `PosixCode`.

The maximum number of ports that can be open at the same time is 1024 by default, but can be configured by the environment variable `ERL_MAX_PORTS`.

`erlang:phash(Term, Range) -> Hash`

Types:

`Term = term()`
`Range = 1..2^32`
`Hash = 1..Range`

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 4.9.1.1). Range can be between 1 and 2^32, the function returns a hash value for `Term` within the range `1..Range`.

This BIF could be used instead of the old deprecated `erlang:hash/2` BIF, as it calculates better hashes for all datatypes, but consider using `phash2/1,2` instead.

`erlang:phash2(Term [, Range]) -> Hash`

Types:

`Term = term()`
`Range = 1..2^32`
`Hash = 0..Range-1`

Portable hash function that will give the same hash for the same Erlang term regardless of machine architecture and ERTS version (the BIF was introduced in ERTS 5.2). Range can be between 1 and 2^32, the function returns a hash value for `Term` within the range `0..Range-1`. When called without the `Range` argument, a value in the range `0..2^27-1` is returned.

This BIF should always be used for hashing terms. It distributes small integers better than `phash/2`, and it is faster for bignums and binaries.

Note that the range `0..Range-1` is different from the range of `phash/2` (`1..Range`).

`pid_to_list(Pid) -> string()`

Types:

`Pid = pid()`

Returns a string which corresponds to the text representation of `Pid`.

| This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

`port_close(Port) -> true`

Types:

`Port = port() | atom()`

Closes an open port. Roughly the same as `Port ! {self(), close}` except for the error behaviour (see below), and that the port does not reply with `{Port, closed}`. Any process may close a port with `port_close/1`, not only the port owner (the connected process).

For comparison: `Port ! {self(), close}` fails with `badarg` if `Port` cannot be sent to (i.e., `Port` refers neither to a port nor to a process). If `Port` is a closed port nothing happens. If `Port` is an open port and the calling process is the port owner, the port replies with `{Port, closed}` when all buffers have been flushed and the port really closes, but if the calling process is not the port owner the port owner fails with `badsig`.

Note that any process can close a port using `Port ! {PortOwner, close}` just as if it itself was the port owner, but the reply always goes to the port owner.

In short: `port_close(Port)` has a cleaner and more logical behaviour than `Port ! {self(), close}`.

Failure: `badarg` if `Port` is not an open port or the registered name of an open port.

`port_command(Port, Data) -> true`

Types:

`Port = port() | atom()`
`Data = iodata()`

Sends data to a port. Same as `Port ! {self(), {command, Data}}` except for the error behaviour (see below). Any process may send data to a port with `port_command/2`, not only the port owner (the connected process).

For comparison: `Port ! {self(), {command, Data}}` fails with `badarg` if `Port` cannot be sent to (i.e., `Port` refers neither to a port nor to a process). If `Port` is a closed port the data message disappears without a sound. If `Port` is open and the calling process is not the port owner, the port owner fails with `badsig`. The port owner fails with `badsig` also if `Data` is not a valid IO list.

Note that any process can send to a port using `Port ! {PortOwner, {command, Data}}` just as if it itself was the port owner.

In short: `port_command(Port, Data)` has a cleaner and more logical behaviour than `Port ! {self(), {command, Data}}`.

Failure: `badarg` if `Port` is not an open port or the registered name of an open port.

`port_connect(Port, Pid) -> true`

Types:

`Port = port() | atom()`
`Pid = pid()`

Sets the port owner (the connected port) to `Pid`. Roughly the same as `Port ! {self(), {connect, Pid}}` except for the following:

- The error behavior differs, see below.
- The port does not reply with `{Port,connected}`.
- The new port owner gets linked to the port.

The old port owner stays linked to the port and have to call `unlink(Port)` if this is not desired. Any process may set the port owner to be any process with `port_connect/2`.

For comparison: `Port ! {self(), {connect, Pid}}` fails with `badarg` if `Port` cannot be sent to (i.e., `Port` refers neither to a port nor to a process). If `Port` is a closed port nothing happens. If `Port` is an open port and the calling process is the port owner, the port replies with `{Port, connected}` to the old port owner. Note that the old port owner is still linked to the port, and that the new is not. If `Port` is an open port and the calling process is not the port owner, the port owner fails with `badsig`. The port owner fails with `badsig` also if `Pid` is not an existing local pid.

Note that any process can set the port owner using `Port ! {PortOwner, {connect, Pid}}` just as if it itself was the port owner, but the reply always goes to the port owner.

In short: `port_connect(Port, Pid)` has a cleaner and more logical behaviour than `Port ! {self(),{connect,Pid}}`.

Failure: `badarg` if `Port` is not an open port or the registered name of an open port, or if `Pid` is not an existing local pid.

`port_control(Port, Operation, Data) -> Res`

Types:

`Port = port() | atom()`
`Operation = int()`
`Data = Res = iodata()`

Performs a synchronous control operation on a port. The meaning of `Operation` and `Data` depends on the port, i.e., on the port driver. Not all port drivers support this control feature.

Returns: a list of integers in the range 0 through 255, or a binary, depending on the port driver. The meaning of the returned data also depends on the port driver.

Failure: `badarg` if `Port` is not an open port or the registered name of an open port, if `Operation` cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with `Operation` or `Data`).

`erlang:port_call(Port, Operation, Data) -> term()`

Types:

`Port = port() | atom()`
`Operation = int()`
`Data = term()`

Performs a synchronous call to a port. The meaning of `Operation` and `Data` depends on the port, i.e., on the port driver. Not all port drivers support this feature.

`Port` is a port identifier, referring to a driver.

`Operation` is an integer, which is passed on to the driver.

`Data` is any Erlang term. This data is converted to binary term format and sent to the port.

Returns: a term from the driver. The meaning of the returned data also depends on the port driver.

Failure: `badarg` if `Port` is not an open port or the registered name of an open port, if `Operation` cannot fit in a 32-bit integer, if the port driver does not support synchronous control operations, or if the port driver so decides for any reason (probably something wrong with `Operation` or `Data`).

`erlang:port_info(Port) -> [{Item, Info}] | undefined`

Types:

`Port = port() | atom()`
`Item, Info -- see below`

Returns a list containing tuples with information about the `Port`, or `undefined` if the port is not open. The order of the tuples is not defined, nor are all the tuples mandatory.

`{registered_name, RegName}`
  `RegName` (an atom) is the registered name of the port. If the port has no registered name, this tuple is not present in the list.
`{id, Index}`
  `Index` (an integer) is the internal index of the port. This index may be used to separate ports.
`{connected, Pid}`
  `Pid` is the process connected to the port.
`{links, Pids}`
  `Pids` is a list of pids to which processes the port is linked.
`{name, String}`
  `String` is the command name set by `open_port`.
`{input, Bytes}`
  `Bytes` is the total number of bytes read from the port.
`{output, Bytes}`
  `Bytes` is the total number of bytes written to the port.

Failure: `badarg` if `Port` is not a local port.

`erlang:port_info(Port, Item) -> {Item, Info} | undefined | []`

Types:

`Port = port() | atom()`
`Item, Info -- see below`

Returns information about `Port` as specified by `Item`, or `undefined` if the port is not open. Also, if `Item == registered_name` and the port has no registered name, [] is returned.

For valid values of `Item`, and corresponding values of `Info`, see erlang:port_info/1.

Failure: `badarg` if `Port` is not a local port.

`erlang:port_to_list(Port) -> string()`

Types:

`Port = port()`

Returns a string which corresponds to the text representation of the port identifier `Port`.

| This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

`erlang:ports() -> [port()]`

Returns a list of all ports on the local node.

`pre_loaded() -> [Module]`

Types:

`Module = atom()`

Returns a list of Erlang modules which are pre-loaded in the system. As all loading of code is done through the file system, the file system must have been loaded previously. Hence, at least the module `init` must be pre-loaded.

`erlang:process_display(Pid, Type) -> void()`

Types:

`Pid = pid()`
`Type = backtrace`

Writes information about the local process `Pid` on standard error. The currently allowed value for the atom `Type` is `backtrace`, which shows the contents of the call stack, including information about the call chain, with the most recent data printed last. The format of the output is not further defined.

`process_flag(Flag, Value) -> OldValue`

Types:

`Flag, Value, OldValue -- see below`

Sets certain flags for the process which calls this function. Returns the old value of the flag.

`process_flag(trap_exit, Boolean)`
  When `trap_exit` is set to `true`, exit signals arriving to a process are converted to `{'EXIT', From, Reason}` messages, which can be received as ordinary messages. If `trap_exit` is set to `false`, the process exits if it receives an exit signal other than `normal` and the exit signal is propagated to its linked processes. Application processes should normally not trap exits.
  See also exit/2.
`process_flag(error_handler, Module)`
  This is used by a process to redefine the error handler for undefined function calls and undefined registered processes. Inexperienced users should not use this flag since code autoloading is dependent on the correct operation of the error handling module.
`process_flag(min_heap_size, MinHeapSize)`
  This changes the minimum heap size for the calling process.
`process_flag(priority, Level)`
  This sets the process priority. `Level` is an atom. All implementations support three priority levels, `low`, `normal`, and `high`. The default is `normal`.
`process_flag(save_calls, N)`
  `N` must be an integer in the interval 0..10000. If `N` > 0, call saving is made active for the process, which means that information about the `N` most recent global function calls, BIF calls, sends and receives made by the process are saved in a list, which can be retrieved with `process_info(Pid, last_calls)`. A global function call is one in which the module of the function is explicitly mentioned. Only a fixed amount of information is saved: a tuple `{Module, Function, Arity}` for function calls, and the mere atoms `send`, `'receive'` and `timeout` for sends and receives (`'receive'` when a message is received and `timeout` when a receive times out). If `N` = 0, call saving is disabled for the process, which is the default. Whenever the size of the call saving list is set, its contents are reset.

`process_flag(Pid, Flag, Value) -> OldValue`

Types:

`Pid = pid()`
`Flag, Value, OldValue -- see below`

Sets certain flags for the process `Pid`, in the same manner as process_flag/2. Returns the old value of the flag. The allowed values for `Flag` are only a subset of those allowed in `process_flag/2`, namely: `save_calls`.

Failure: `badarg` if `Pid` is not a local process.

`process_info(Pid) -> [{Item, Info}] | undefined`

Types:

`Pid = pid()`
`Item, Info -- see below`

Returns a list containing tuples with information about the process `Pid`, or `undefined` if the process is not alive. The order of the tuples is not defined, nor are all the tuples mandatory.

| This BIF is intended for debugging only.

`{current_function, {Module, Function, Args}}`
  `Module`, `Function`, `Args` is the current function call of the process.
`{dictionary, Dictionary}`
  `Dictionary` is the dictionary of the process.
`{error_handler, Module}`
  `Module` is the error handler module used by the process (for undefined function calls, for example).
`{group_leader, GroupLeader}`
  `GroupLeader` is group leader for the IO of the process.
`{heap_size, Size}`
  `Size` is the heap size of the process in words.
`{initial_call, {Module, Function, Arity}}`
  `Module`, `Function`, `Arity` is the initial function call with which the process was spawned.
`{links, Pids}`
  `Pids` is a list of pids, with processes to which the process has a link.
`{message_queue_len, MessageQueueLen}`
  `MessageQueueLen` is the number of messages currently in the message queue of the process. This is the length of the list `MessageQueue` returned as the info item `messages` (see below).
`{messages, MessageQueue}`
  `MessageQueue` is a list of the messages to the process, which have not yet been processed.
`{priority, Level}`
  `Level` is the current priority level for the process. Only `low` and `normal` are always supported.
`{reductions, Number}`
  `Number` is the number of reductions executed by the process.
`{registered_name, Atom}`
  `Atom` is the registered name of the process. If the process has no registered name, this tuple is not present in the list.
`{stack_size, Size}`
  `Size` is the stack size of the process in words.
`{status, Status}`
  `Status` is the status of the process. `Status` is `waiting` (waiting for a message), `running`, `runnable` (ready to run, but another process is running), or `suspended` (suspended on a "busy" port or by the `erlang:suspend_process/1` BIF).
`{trap_exit, Boolean}`
  `Boolean` is `true` if the process is trapping exits, otherwise it is `false`.

Failure: `badarg` if `Pid` is not a local process.

`process_info(Pid, Item) -> {Item, Info} | undefined | []`

Types:

`Pid = pid()`
`Item, Info -- see below`

Returns information about the process `Pid` as specified by `Item`, or `undefined` if the process is not alive. Also, if `Item == registered_name` and the process has no registered name, [] is returned.

The value of `Item`, and corresponding value of `Info`, can be any of the values specified for process_info/1.

In addition to the above, also the following items -- with corresponding values -- are allowed:

`{backtrace, Bin}`
  The binary `Bin` contains the same information as the output from `erlang:process_display(Pid, backtrace)`. Use `binary_to_list/1` to obtain the string of characters from the binary.
`{last_calls, false|Calls}`
  The value is `false` if call saving is not active for the process (see process_flag/3). If call saving is active, a list is returned, in which the last element is the most recent called.
`{memory, Size}`
  `Size` is the size of the process in bytes. This includes call stack, heap and internal structures.
`{monitored_by, Pids}`
  A list of pids that are monitoring the process (with `erlang:monitor/2`).
`{monitors, Monitors}`
  A list of monitors (started by `erlang:monitor/2`) that are active for the process. For a local process monitor or a remote process monitor by pid, the list item is `{process, Pid}`, and for a remote process monitor by name, the list item is `{process, {RegName, Node}}`.

Note however, that not all implementations support every one of the above `Items`.

Failure: `badarg` if `Pid` is not a local process.

`processes() -> [pid()]`

Returns a list of all processes on the local node.

    > processes().
    [<0.0.0>,
     <0.2.0>,
     <0.4.0>,
     <0.5.0>,
     <0.7.0>,
     <0.8.0>]

`purge_module(Module) -> void()`

Types:

`Module = atom()`

Removes old code for `Module`. Before this BIF is used, `erlang:check_process_code/2` should be called to check that no processes are executing old code in the module.

| This BIF is intended for the code server (see code(3)) and should not be used elsewhere.

Failure: `badarg` if there is no old code for `Module`.

`put(Key, Val) -> OldVal | undefined`

Types:

`Key = Val = OldVal = term()`

Adds a new `Key` to the process dictionary, associated with the value `Val`, and returns `undefined`. If `Key` already exists, the old value is deleted and replaced by `Val` and the function returns the old value.

| The values stored when `put` is evaluated within the scope of a `catch` will not be retracted if a `throw` is evaluated, or if an error occurs.

    > X = put(name, walrus), Y = put(name, carpenter),
      Z = get(name),
      {X, Y, Z}.
    {undefined,walrus,carpenter}

`erlang:raise(Class, Reason, Stacktrace)`

Types:

`Class = error | exit | throw`
`Reason = term()`
`Stacktrace = [{Module, Function, Arity | Args} | {Fun, Args}]`
`Module = Function = atom()`
`Arity = int()`
`Args = [term()]`
`Fun = [fun()]`

Stops the execution of the calling process with an exception of given class, reason and call stack backtrace (stacktrace).

| This BIF is intended for debugging and for use in the Erlang operating system. In general, it should be avoided in applications, unless you know very well what you are doing.

`Class` is one of `error`, `exit` or `throw`, so if it were not for the stacktrace `erlang:raise(Class, Reason, Stacktrace)` is equivalent to `erlang:Class(Reason)`. `Reason` is any term and `Stacktrace` is a list as returned from `get_stacktrace()`, that is a list of 3-tuples `{Module, Function, Arity | Args}` where `Module` and `Function` are atoms and the third element is an integer arity or an argument list. The stacktrace may also contain `{Fun, Args}` tuples where `Fun` is a local fun and `Args` is an argument list.

The stacktrace is used as the exception stacktrace for the calling process; it will be truncated to the current maximum stacktrace depth.

Because evaluating this function causes the process to terminate, it has no return value - unless the arguments are invalid, in which case the function returns the error reason, that is `badarg`. If you want to be really sure not to return you can call `erlang:error(erlang:raise(Class, Reason, Stacktrace))` and hope to distinguish exceptions later.

`erlang:read_timer(TimerRef) -> int() | false`

Types:

`TimerRef = ref()`

`TimerRef` is a timer reference returned by erlang:send_after/3 or erlang:start_timer/3. If the timer is active, the function returns the time in milliseconds left until the timer will expire, otherwise `false` (which means that `TimerRef` was never a timer, that it has been cancelled, or that it has already delivered its message).

See also erlang:send_after/3, erlang:start_timer/3, and erlang:cancel_timer/1.

`erlang:ref_to_list(Ref) -> string()`

Types:

`Ref = ref()`

Returns a string which corresponds to the text representation of `Ref`.

| This BIF is intended for debugging and for use in the Erlang operating system. It should not be used in application programs.

`register(RegName, Pid | Port) -> true`

Types:

`RegName = atom()`
`Pid = pid()`
`Port = port()`

Associates the name `RegName` with a pid or a port identifier. `RegName`, which must be an atom, can be used instead of the pid / port identifier in the send operator (`RegName ! Message`).

    > register(db, Pid).
    true

Failure: `badarg` if `Pid` is not an existing, local process or port, if `RegName` is already in use, if the process or port is already registered (already has a name), or if `RegName` is the atom `undefined`.

`registered() -> [RegName]`

Types:

`RegName = atom()`

Returns a list of names which have been registered using register/2.

    > registered().
    [code_server, file_server, init, user, my_db]

`erlang:resume_process(Pid) -> true`

Types:

`Pid = pid()`

Resume a suspended process `Pid`.

| This BIF is intended for debugging only.

Failure: `badarg` if `Pid` does not exist.

`round(Number) -> int()`

Types:

`Number = number()`

Returns an integer by rounding `Number`.

    > round(5.5).
    6

Allowed in guard tests.

`self() -> pid()`

Returns the pid (process identifier) of the calling process.

    > self().
    <0.26.0>

Allowed in guard tests.

`erlang:send(Dest, Msg) -> Msg`

Types:

`Dest = pid() | port() | RegName | {RegName, Node}`
`Msg = term()`
`RegName = atom()`
`Node = node()`

Sends a message and returns `Msg`. This is the same as `Dest ! Msg`.

`Dest` may be a remote or local pid, a (local) port, a locally registered name, or a tuple `{RegName, Node}` for a registered name at another node.

`erlang:send(Dest, Msg, [Option]) -> Res`

Types:

`Dest = pid() | port() | RegName | {RegName, Node}`
`RegName = atom()`
`Node = node()`
`Msg = term()`
`Option = nosuspend | noconnect`
`Res = ok | nosuspend | noconnect`

Sends a message and returns `ok`, or does not send the message but returns something else (see below). Otherwise the same as erlang:send/2. See also erlang:send_nosuspend/2,3. for more detailed explanation and warnings.

The possible options are:

`nosuspend`
  If the sender would have to be suspended to do the send, `nosuspend` is returned instead.
`noconnect`
  If the destination node would have to be autoconnected before doing the send, `noconnect` is returned instead.

| As with `erlang:send_nosuspend/2,3`: Use with extreme care!

`erlang:send_after(Time, Dest, Msg) -> TimerRef`

Types:

`Time = int()`
`0 <= Time <= 4294967295`
`Dest = pid() | RegName`
`LocalPid = pid() (of a process, alive or dead, on the local node)`
`Msg = term()`
`TimerRef = ref()`

Starts a timer which will send the message `Msg` to `Dest` after `Time` milliseconds.

If `Dest` is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If `Dest` is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when `Dest` is an atom.

See also erlang:start_timer/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: `badarg` if the arguments does not satisfy the requirements specified above.

`erlang:send_nosuspend(Dest, Msg) -> bool()`

Types:

`Dest = pid() | port() | RegName | {RegName, Node}`
`RegName = atom()`
`Node = node()`
`Msg = term()`

The same as erlang:send(Dest, Msg, [nosuspend]), but returns `true` if the message was sent and `false` if the message was not sent because the sender would have had to be suspended.

This function is intended for send operations towards an unreliable remote node without ever blocking the sending (Erlang) process. If the connection to the remote node (usually not a real Erlang node, but a node written in C or Java) is overloaded, this function will not send the message but return `false` instead.

The same happens, if `Dest` refers to a local port that is busy. For all other destinations (allowed for the ordinary send operator `'!'`) this function sends the message and returns `true`.

This function is only to be used in very rare circumstances where a process communicates with Erlang nodes that can disappear without any trace causing the TCP buffers and the drivers que to be overfull before the node will actually be shut down (due to tick timeouts) by `net_kernel`. The normal reaction to take when this happens is some kind of premature shutdown of the other node.

Note that ignoring the return value from this function would result in unreliable message passing, which is contradictory to the Erlang programming model. The message is not sent if this function returns `false`.

Note also that in many systems, transient states of overloaded queues are normal. The fact that this function returns `false` does not in any way mean that the other node is guaranteed to be nonrespoinsive, it could be a temporary overload. Also a return value of `true` does only mean that the message could be sent on the (TCP) channel without blocking, the message is not guaranteed to have arrived at the remote node. Also in the case of a disconnected nonresponsive node, the return value is `true` (mimics the behaviour of the `!` operator). The expected behaviour as well as the actions to take when the function returns `false` are application and hardware specific.

| Use with extreme care!

`erlang:send_nosuspend(Dest, Msg, Options) -> bool()`

Types:

`Dest = pid() | port() | RegName | {RegName, Node}`
`RegName = atom()`
`Node = node()`
`Msg = term()`
`Option = noconnect`

The same as erlang:send(Dest, Msg, [nosuspend | Options]), but with boolean return value.

This function behaves like erlang:send_nosuspend/2), but takes a third parameter, a list of options. The only currently implemented option is `noconnect`. The option `noconnect` makes the function return `false` if the remote node is not currently reachable by the local node. The normal behaviour is to try to connect to the node, which may stall the process for a shorter period. The use of the `noconnect` option makes it possible to be absolutely sure not to get even the slightest delay when sending to a remote process. This is especially useful when communicating with nodes who expect to always be the connecting part (i.e. nodes written in C or Java).

Whenever the function returns `false` (either when a suspend would occur or when `noconnect` was specified and the node was not already connected), the message is guaranteed not to have been sent.

| Use with extreme care!

`erlang:set_cookie(Node, Cookie) -> true`

Types:

`Node = node()`
`Cookie = atom()`

Sets the magic cookie of `Node` to the atom `Cookie`. If `Node` is the local node, the function also sets the cookie of all other unknown nodes to `Cookie` (see Distributed Erlang in the Erlang Reference Manual).

Failure: `function_clause` if the local node is not alive.

`setelement(Index, Tuple1, Value) -> Tuple2`

Types:

`Index = 1..size(Tuple1)`
`Tuple1 = Tuple2 = tuple()`
`Value = term()`

Returns a tuple which is a copy of the argument `Tuple1` with the element given by the integer argument `Index` (the first element is the element with index 1) replaced by the argument `Value`.

    > setelement(2, {10, green, bottles}, red).
    {10, red, bottles}

`size(Item) -> int()`

Types:

`Item = tuple() | binary()`

Returns an integer which is the size of the argument `Item`, which must be either a tuple or a binary.

    > size({morni, mulle, bwange}).
    3

Allowed in guard tests.

`spawn(Fun) -> pid()`

Types:

`Fun = fun()`

Returns the pid of a new process started by the application of `Fun` to the empty list `[]`. Otherwise works like spawn/3.

`spawn(Node, Fun) -> pid()`

Types:

`Node = node()`
`Fun = fun()`

Returns the pid of a new process started by the application of `Fun` to the empty list `[]` on `Node`. If `Node` does not exist, a useless pid is returned. Otherwise works like spawn/3.

`spawn(Module, Function, Args) -> pid()`

Types:

`Module = Function = atom()`
`Args = [term()]`

Returns the pid of a new process started by the application of `Module:Function` to `Args`. The new process created will be placed in the system scheduler queue and be run some time later.

`error_handler:undefined_function(Module, Function, Args)` is evaluated by the new process if `Module:Function/Arity` does not exist (where `Arity` is the length of `Args`). The error handler can be redefined (see process_flag/2). If `error_handler` is undefined, or the user has redefined the default `error_handler` its replacement is undefined, a failure with the reason `undef` will occur.

    > spawn(speed, regulator, [high_speed, thin_cut]).
    <0.13.1>

`spawn(Node, Module, Function, ArgumentList) -> pid()`

Types:

`Node = node()`
`Module = Function = atom()`
`Args = [term()]`

Returns the pid of a new process started by the application of `Module:Function` to `Args` on `Node`. If `Node` does not exists, a useless pid is returned. Otherwise works like spawn/3.

`spawn_link(Fun) -> pid()`

Types:

`Fun = fun()`

Returns the pid of a new process started by the application of `Fun` to the empty list []. A link is created between the calling process and and the new process, atomically. Otherwise works like spawn/3.

`spawn_link(Node, Fun) ->`

Types:

`Node = node()`
`Fun = fun()`

Returns the pid of a new process started by the application of `Fun` to the empty list [] on `Node`. A link is created between the calling process and and the new process, atomically. If `Node` does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason `noconnection` will be received). Otherwise works like spawn/3.

`spawn_link(Module, Function, Args) -> pid()`

Types:

`Module = Function = atom()`
`Args = [term()]`

Returns the pid of a new process started by the application of `Module:Function` to `Args`. A link is created betwen the calling process and the new process, atomically. Otherwise works like spawn/3.

`spawn_link(Node, Module, Function, Args) -> pid()`

Types:

`Node = node()`
`Module = Function = atom()`
`Args = [term()]`

Returns the pid of a new process started by the application of `Module:Function` to `Args` on `Node`. A link is created betwen the calling process and the new process, atomically. If `Node` does not exist, a useless pid is returned (and due to the link, an exit signal with exit reason `noconnection` will be received). Otherwise works like spawn/3.

`spawn_opt(Fun, [Option]) -> pid()`

Types:

`Fun = fun()`
`Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}`
`Level = low | normal | high`
`Number = int()`
`Size = int()`

Returns the pid of a new process started by the application of `Fun` to the empty list `[]`. Otherwise works like spawn_opt/4.

`spawn_opt(Node, Fun, [Option]) -> pid()`

Types:

`Node = node()`
`Fun = fun()`
`Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}`
`Level = low | normal | high`
`Number = int()`
`Size = int()`

Returns the pid of a new process started by the application of `Fun` to the empty list `[]` on `Node`. If `Node` does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

`spawn_opt(Module, Function, Args, [Option]) -> pid()`

Types:

`Module = Function = atom()`
`Args = [term()]`
`Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}`
`Level = low | normal | high`
`Number = int()`
`Size = int()`

Works exactly like spawn/3, except that an extra option list is given when creating the process.

| This BIF is only useful for performance tuning. Random tweaking of the parameters without measuring execution times and memory consumption may actually make things worse. Furthermore, most of the options are inherently implementation-dependent, and they can be changed or removed in future versions of OTP.

`link`
  Sets a link to the parent process (like `spawn_link/3` does).
`{priority, Level}`
  Sets the priority of the new process. Equivalent to executing `process_flag(priority, Level)` in the start function of the new process, except that the priority will be set before the process is scheduled in the first time.
`{fullsweep_after, Number}`
  The Erlang runtime system uses a generational garbage collection scheme, using an "old heap" for data that has survived at least one garbage collection. When there is no more room on the old heap, a fullsweep garbage collection will be done.
  The `fullsweep_after` option makes it possible to specify the maximum number of generational collections before forcing a fullsweep even if there is still room on the old heap. Setting the number to zero effectively disables the general collection algorithm, meaning that all live data is copied at every garbage collection.
  Here are a few cases when it could be useful to change `fullsweep_after`. Firstly, if binaries that are no longer used should be thrown away as soon as possible. (Set `Number` to zero.) Secondly, a process that mostly have short-lived data will be fullsweeped seldom or never, meaning that the old heap will contain mostly garbage. To ensure a fullsweep once in a while, set `Number` to a suitable value such as 10 or 20. Thirdly, in embedded systems with limited amount of RAM and no virtual memory, one might want to preserve memory by setting `Number` to zero. (The value may be set globally, see erlang:system_flag/2.)
`{min_heap_size, Size}`
  Gives a minimum heap size in words. Setting this value higher than the system default might speed up some processes because less garbage collection is done. Setting too high value, however, might waste memory and slow down the system due to worse data locality. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various `Size` values.

`spawn_opt(Node, Module, Function, Args, [Option]) -> pid()`

Types:

`Node = node()`
`Module = Function = atom()`
`Args = [term()]`
`Option = link | {priority, Level} | {fullsweep_after, Number} | {min_heap_size, Size}`
`Level = low | normal | high`
`Number = int()`
`Size = int()`

Returns the pid of a new process started by the application of `Module:Function` to `Args` on `Node`. If `Node` does not exist, a useless pid is returned. Otherwise works like spawn_opt/4.

`split_binary(Bin, Pos) -> {Bin1, Bin2}`

Types:

`Bin = Bin1 = Bin2 = binary()`
`Pos = 1..size(Bin)`

Returns a tuple containing the binaries which are the result of splitting `Bin` into two parts at position `Pos`. This is not a destructive operation. After the operation, there will be three binaries altogether.

    > B = list_to_binary("0123456789").
    <<48,49,50,51,52,53,54,55,56,57>>
    > size(B).
    10
    > {B1, B2} = split_binary(B,3).
    {<<48,49,50>>,<<51,52,53,54,55,56,57>>}
    > size(B1).
    3
    > size(B2).
    7

`erlang:start_timer(Time, Dest, Msg) -> TimerRef`

Types:

`Time = int()`
`0 <= Time <= 4294967295`
`Dest = LocalPid | RegName`
`LocalPid = pid() (of a process, alive or dead, on the local node)`
`RegName = atom()`
`Msg = term()`
`TimerRef = ref()`

Starts a timer which will send the message `{timeout, TimerRef, Msg}` to `Dest` after `Time` milliseconds.

If `Dest` is an atom, it is supposed to be the name of a registered process. The process referred to by the name is looked up at the time of delivery. No error is given if the name does not refer to a process.

If `Dest` is a pid, the timer will be automatically canceled if the process referred to by the pid is not alive, or when the process exits. This feature was introduced in erts version 5.4.11. Note that timers will not be automatically canceled when `Dest` is an atom.

See also erlang:send_after/3, erlang:cancel_timer/1, and erlang:read_timer/1.

Failure: `badarg` if the arguments does not satisfy the requirements specified above.

`statistics(Type) -> Res`

Types:

`Type, Res -- see below`

Returns information about the system as specified by `Type`:

`context_switches`
  Returns `{ContextSwitches, 0}`, where `ContextSwitches` is the total number of context switches since the system started.
`exact_reductions`
  Returns `{Total_Exact_Reductions, Exact_Reductions_Since_Last_Call}`.
  NOTE: `statistics(exact_reductions)` is a more expensive operation than statistics(reductions) especially on an Erlang machine with smp support.
`garbage_collection`
  Returns `{Number_of_GCs, Words_Reclaimed, 0}`. This information may not be valid for all implementations.
`io`
  Returns `{{input, Input}, {output, Output}}`, where `Input` is the total number of bytes received through ports, and `Output` is the total number of bytes output to ports.
`reductions`
  Returns `{Total_Reductions, Reductions_Since_Last_Call}`.
  NOTE: From erts version 5.5 (OTP release R11B) this value does not include reductions performed in current time slices of currently scheduled processes. If an exact value is wanted, use statistics(exact_reductions).
`run_queue`
  Returns the length of the run queue, that is, the number of processes that are ready to run.
`runtime`
  Returns `{Total_Run_Time, Time_Since_Last_Call}`.
`wall_clock`
  Returns `{Total_Wallclock_Time, Wallclock_Time_Since_Last_Call}`. `wall_clock` can be used in the same manner as `runtime`, except that real time is measured as opposed to runtime or CPU time.

All times are in milliseconds.

    > statistics(runtime).
    {1690,1620}
    > statistics(reductions).
    {2046,11}
    > statistics(garbage_collection).
    {85,23961,0}

`erlang:suspend_process(Pid) -> true`

Types:

`Pid = pid()`

Suspends the process `Pid`.

| This BIF is intended for debugging only.

Failure: `badarg` if `Pid` does not exist.

`erlang:system_flag(Flag, Value) -> OldValue`

Types:

`Flag, Value, OldValue -- see below`

Sets various system properties of the Erlang node. Returns the old value of the flag.

`erlang:system_flag(backtrace_depth, Depth)`
  Sets the maximum depth of call stack backtraces in the exit reason element of `'EXIT'` tuples.
`erlang:system_flag(fullsweep_after, Number)`
  `Number` is a non-negative integer which indicates how many times generational garbages collections can be done without forcing a fullsweep collection. The value applies to new processes; processes already running are not affected.
  In low-memory systems (especially without virtual memory), setting the value to 0 can help to conserve memory.
  An alternative way to set this value is through the (operating system) environment variable `ERL_FULLSWEEP_AFTER`.
`erlang:system_flag(min_heap_size, MinHeapSize)`
  Sets the default minimum heap size for processes. The size is given in words. The new `min_heap_size` only effects processes spawned after the change of `min_heap_size` has been made. The `min_heap_size` can be set for individual processes by use of spawn_opt/N or process_flag/2.
`erlang:system_flag(schedulers, NO)`
  Sets the number of scheduler threads to used when the runtime system has been compiled with smp support. Valid range of `NO` is 1-1024. Returns `{OpRes, NoOfSchedulers, OldNoOfSchedulers}` where `OpRes = atom()` (`ok` or a posix error), `NoOfSchedulers = integer()` (current number of scheduler threads), and `OldNoOfSchedulers = integer()` (previous number of scheduler threads).
  NOTE! This feature might be removed without prior notice.
  NOTE! This is a quite expensive operation. Do not use it for mutual exclusion.
`erlang:system_flag(trace_control_word, TCW)`
  Sets the value of the node's trace control word to `TCW`. `TCW` should be an unsigned integer. For more information see documentation of the set_tcw function in the match specification documentation in the ERTS User's Guide.

`erlang:system_info(Type) -> Res`

Types:

`Type, Res -- see below`

Returns various information about the current system (emulator) as specified by `Type`:

`allocated_areas`
  Returns a list of tuples with information about miscellaneous allocated memory areas.
  Each tuple contains an atom describing type of memory as first element and amount of allocated memory in bytes as second element. In those cases when there is information present about allocated and used memory, a third element is present. This third element contains the amount of used memory in bytes.
  `erlang:system_info(allocated_areas)` is intended for debugging, and the content is highly implementation dependent. The content of the results will therefore change when needed without prior notice.
  Note: The sum of these values is not the total amount of memory allocated by the emulator. Some values are part of other values, and some memory areas are not part of the result. If you are interested in the total amount of memory allocated by the emulator see erlang:memory/0,1.
`allocator`
  Returns `{Allocator, Version, Features, Settings}.`
  Types:
  - `Allocator = undefined | elib_malloc | glibc`
  - `Version = [int()]`
  - `Features = [atom()]`
  - `Settings = [{Subsystem, [{Parameter, Value}]}]`
  - `Subsystem = atom()`
  - `Parameter = atom()`
  - `Value = term()`
  Explanation:
  - `Allocator` corresponds to the `malloc()` implementation used. If `Allocator` equals `undefined`, the `malloc()` implementation used could not be identified. Currently `elib_malloc` and `glibc` can be identified.
  - `Version` is a list of integers (but not a string) representing the version of the `malloc()` implementation used.
  - `Features` is a list of atoms representing allocation features used.
  - `Settings` is a list of subsystems, their configurable parameters, and used values. Settings may differ between different combinations of platforms, allocators, and allocation features. Memory sizes are given in bytes.
  See also "System Flags Effecting erts_alloc" in erts_alloc(3).
`{allocator, Alloc}`
  Returns information about the specified allocator. If `Alloc` is not a recognized allocator, `undefined` is returned. If `Alloc` is disabled, `false` is returned.
  Note: The information returned is highly implementation dependent and may be changed, or removed at any time without prior notice. It was initially intended as a tool when developing new allocators, but since it might be of interest for others it has been briefly documented.
  The recognized allocators are listed in erts_alloc(3), and after reading this also the returned information should more or less speak for itself. But it can be worth explaining some things. Call counts are presented by two values. The first value is giga calls, and the second value is calls. `mbcs`, and `sbcs` are abbreviations for, respectively, multiblock carriers, and singleblock carriers. Sizes are presented in bytes. When it is not a size that is presented, it is the amount of something. Sizes and amounts are often presented by three values, the first is current value, the second is maximum value since the last call to `erlang:system_info({allocator, Alloc})`, and the third is maximum value since the emulator was started. If only one value is present, it is the current value. `fix_alloc` memory block types are presented by two values. The first value is memory pool size and the second value used memory size.
`compat_rel`
  Returns the compatibility mode of the local node as an integer. The integer returned represents the Erlang/OTP release which the current emulator has been set to be backward compatible with. The compatibility mode can be configured at startup by using the command line flag `+R`, see erl(1).
`creation`
  Returns the creation of the local node as an integer. The creation is changed when a node is restarted. The creation of a node is stored in process identifiers, port identifiers, and references. This makes it (to some extent) possible to distinguish between identifiers from different incarnations of a node. Currently valid creations are integers in the range 1..3, but this may (probably will) change in the future. If the node is not alive, 0 is returned.
`dist`
  Returns a binary containing a string of distribution information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.
`dist_ctrl`
  Returns a list of tuples `{Node, ControllingEntity}`, one entry for each connected remote node. The `Node` is the name of the node and the `ControllingEntity` is the port or pid responsible for the communication to that node. More specifically, the `ControllingEntity` for nodes connected via TCP/IP (the normal case) is the socket actually used in communication with the specific node.
`elib_malloc`
  If the emulator uses the `elib_malloc` memory allocator, a list of two-element tuples containing status information is returned; otherwise, `false` is returned. The list currently contains the following two-element tuples (all sizes are presented in bytes):
  `{heap_size, Size}`
    Where `Size` is the current heap size.
  `{max_alloced_size, Size}`
    Where `Size` is the maximum amount of memory allocated on the heap since the emulator started.
  `{alloced_size, Size}`
    Where `Size` is the current amount of memory allocated on the heap.
  `{free_size, Size}`
    Where `Size` is the current amount of free memory on the heap.
  `{no_alloced_blocks, No}`
    Where `No` is the current number of allocated blocks on the heap.
  `{no_free_blocks, No}`
    Where `No` is the current number of free blocks on the heap.
  `{smallest_alloced_block, Size}`
    Where `Size` is the size of the smallest allocated block on the heap.
  `{largest_free_block, Size}`
    Where `Size` is the size of the largest free block on the heap.
`fullsweep_after`
  Returns `{fullsweep_after, int()}` which is the `fullsweep_after` garbage collection setting used by default. For more information see `garbage_collection` described below.
`garbage_collection`
  Returns a list describing the default garbage collection settings. A process spawned on the local node by a `spawn` or `spawn_link` will use these garbage collection settings. The default settings can be changed by use of system_flag/2. spawn_opt/4 can spawn a process that does not use the default settings.
`global_heaps_size`
  Returns the current size of the shared (global) heap.
`heap_sizes`
  Returns a list of integers representing valid heap sizes in words. All Erlang heaps are sized from sizes in this list.
`heap_type`
  Returns the heap type used by the current emulator. Currently the following heap types exist:
  `private`
    Each process has a heap reserved for its use and no references between heaps of different processes are allowed. Messages passed between processes are copied between heaps.
  `shared`
    One heap for use by all processes. Messages passed between processes are passed by reference.
  `hybrid`
    A hybrid of the `private` and `shared` heap types. A shared heap as well as private heaps are used.
`info`
  Returns a binary containing a string of miscellaneous system information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.
`loaded`
  Returns a binary containing a string of loaded module information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.
`machine`
  Returns a string containing the Erlang machine name.
`process_count`
  Returns the number of processes currently existing at the local node as an integer. The same value as `length(processes())` returns.
`process_limit`
  Returns the maximum number of concurrently existing processes at the local node as an integer. This limit can be configured at startup by using the command line flag `+P`, see erl(1).
`procs`
  Returns a binary containing a string of process and port information formatted as in Erlang crash dumps. For more information see the "How to interpret the Erlang crash dumps" chapter in the ERTS User's Guide.
`scheduler_id`
  Returns the scheduler id (a positive integer) of current scheduler thread.
`schedulers`
  Returns the number of scheduler threads.
`smp_support`
  Returns `true` if the emulator has been compiled with smp support; otherwise, `false` is returned.
`system_version`
  Returns a string containing the emulator type and version as well as some important properties such as the size of the thread pool, etc.
`system_architecture`
  Returns a string containing the processor and OS architecture the emulator is built for.
`threads`
  Returns `true` if the emulator has been compiled with thread support; otherwise, `false` is returned.
`thread_pool_size`
  Returns the number of threads used for driver calls as an integer.
`trace_control_word`
  Returns the value of the node's trace control word. For more information see documentation of the function `get_tcw` in "Match Specifications in Erlang", ERTS User's Guide.
`version`
  Returns a string containing the version number of the emulator.
`wordsize`
  Returns the word size in bytes as an integer, i.e. on a 32-bit architecture 4 is returned, and on a 64-bit architecture 8 is returned.

| The `scheduler` argument has changed name to `scheduler_id`. This in order to avoid mixup with the `schedulers` argument. The `scheduler` argument was introduced in ERTS version 5.5 and renamed in ERTS version 5.5.1.

`erlang:system_monitor() -> MonSettings`

Types:

`MonSettings -> {MonitorPid, Options} | undefined`
`MonitorPid = pid()`
`Options = [Option]`
`Option = {long_gc, Time} | {large_heap, Size} | busy_port | busy_dist_port`
`Time = Size = int()`

Returns the current system monitoring settings set by erlang:system_monitor/2 as `{MonitorPid, Options}`, or `undefined` if there are no settings. The order of the options may be different from the one that was set.

`erlang:system_monitor(undefined | {MonitorPid, Options}) -> MonSettings`

Types:

`MonitorPid, Options, MonSettings -- see below`

When called with the argument `undefined`, all system performance monitoring settings are cleared.

Calling the function with `{MonitorPid, Options}` as argument, is the same as calling erlang:system_monitor(MonitorPid, Options).

Returns the previous system monitor settings just like erlang:system_monitor/0.

`erlang:system_monitor(MonitorPid, [Option]) -> MonSettings`

Types:

`MonitorPid = pid()`
`Option = {long_gc, Time} | {large_heap, Size} | busy_port | busy_dist_port`
`Time = Size = int()`
`MonSettings = {OldMonitorPid, [Option]}`
`OldMonitorPid = pid()`

Sets system performance monitoring options. `MonitorPid` is a local pid that will receive system monitor messages, and the second argument is a list of monitoring options:

`{long_gc, Time}`
  If a garbage collection in the system takes at least `Time` wallclock milliseconds, a message `{monitor, GcPid, long_gc, Info}` is sent to `MonitorPid`. `GcPid` is the pid that was garbage collected and `Info` is a list of two-element tuples describing the result of the garbage collection. One of the tuples is `{timeout, GcTime}` where `GcTime` is the actual time for the garbage collection in milliseconds. The other are the tuples tagged with `heap_size`, `stack_size`, `mbuf_size` and `heap_block_size` from the `gc_start` trace message (see erlang:trace/3).
`{large_heap, Size}`
  If a garbage collection in the system results in the allocated size of a heap being at least `Size` words, a message `{monitor, GcPid, large_heap, Info}` is sent to `MonitorPid`. `GcPid` and `Info` are the same as for `long_gc` above, except that the tuple tagged with `timeout` is not present.
`busy_port`
  If a process in the system gets suspended because it sends to a busy port, a message `{monitor, SusPid, busy_port, Port}` is sent to `MonitorPid`. `SusPid` is the pid that got suspended when sending to `Port`.
`busy_dist_port`
  If a process in the system gets suspended because it sends to a process on a remote node whose inter-node communication was handled by a busy port, a message `{monitor, SusPid, busy_dist_port, Port}` is sent to `MonitorPid`. `SusPid` is the pid that got suspended when sending through the inter-node communication port `Port`.

Returns the previous system monitor settings just like erlang:system_monitor/0.

| If a monitoring process gets so large that it itself starts to cause system monitor messages when garbage collecting, the messages will enlargen the process's message queue and probably make the problem worse. Keep the monitoring process neat and do not set the system monitor limits too tight.

Failure: `badarg` if `MonitorPid` does not exist.

`term_to_binary(Term) -> ext_binary()`

Types:

`Term = term()`

Returns a binary data object which is the result of encoding `Term` according to the Erlang external term format.

This can be used for a variety of purposes, for example writing a term to a file in an efficient way, or sending an Erlang term to some type of communications channel not supported by distributed Erlang.

See also binary_to_term/1.

`term_to_binary(Term, [Option]) -> ext_binary()`

Types:

`Term = term()`
`Option = compressed`

Returns a binary data object which is the result of encoding `Term` according to the Erlang external term format.

If the option `compressed` is provided, the external term format will be compressed. The compressed format is automatically recognized by `binary_to_term/1` in R7.

See also binary_to_term/1.

`throw(Any)`

Types:

`Any = term()`

A non-local return from a function. If evaluated within a `catch`, `catch` will return the value `Any`.

    > catch throw({hello, there}).
    {hello, there}

Failure: `nocatch` if not evaluated within a catch.

`time() -> {Hour, Minute, Second}`

Types:

`Hour = Minute = Second = int()`

Returns the current time as `{Hour, Minute, Second}`.

The time zone and daylight saving time correction depend on the underlying OS.

    > time().
    {9,42,44}

`tl(List1) -> List2`

Types:

`List1 = List2 = [term()]`

Returns the tail of `List1`, that is, the list minus the first element.

    > tl([geesties, guilies, beasties]).
    [guilies, beasties]

Allowed in guard tests.

Failure: `badarg` if `List` is the empty list [].

`erlang:trace(PidSpec, How, FlagList) -> int()`

Types:

`PidSpec = pid() | existing | new | all`
`How = bool()`
`FlagList = [Flag]`
`Flag -- see below`

Turns on (if `How == true`) or off (if `How == false`) the trace flags in `FlagList` for the process or processes represented by `PidSpec`.

`PidSpec` is either a pid for a local process, or one of the following atoms:

`existing`
  All processes currently existing.
`new`
  All processes that will be created in the future.
`all`
  All currently existing processes and all processes that will be created in the future.

`FlagList` can contain any number of the following flags (the "message tags" refers to the list of messages following below):

`all`
  Set all trace flags except `{tracer, Tracer}` and `cpu_timestamp` that are in their nature different than the others.
`send`
  Trace sending of messages.
  Message tags: `send`, `send_to_non_existing_process`.
`'receive'`
  Trace receiving of messages.
  Message tags: `'receive'`.
`procs`
  Trace process related events.
  Message tags: `spawn`, `exit`, `register`, `unregister`, `link`, `unlink`, `getting_linked`, `getting_unlinked`.
`call`
  Trace certain function calls. Specify which function calls to trace by calling erlang:trace_pattern/3.
  Message tags: `call`, `return_from`.
`silent`
  Used in conjunction with the `call` trace flag. The `call`, `return_from` and `return_to` trace messages are inhibited if this flag is set, but if there are match specs they are executed as normal.
  Silent mode is inhibited by executing `erlang:trace(_, false, [silent|_])`, or by a match spec executing the `{silent, false}` function.
  The `silent` trace flag facilitates setting up a trace on many or even all processes in the system. Then the interesting trace can be activated and deactivated using the `{silent,Bool}` match spec function, giving a high degree of control of which functions with which arguments that triggers the trace.
  Message tags: `call`, `return_from`, `return_to`. Or rather, the absence of.
`return_to`
  Used in conjunction with the `call` trace flag. Trace the actual return from a traced function back to its caller. Only works for functions traced with the `local` option to erlang:trace_pattern/3.
  The semantics is that a trace message is sent when a call traced function actually returns, that is, when a chain of tail recursive calls is ended. There will be only one trace message sent per chain of tail recursive calls, why the properties of tail recursiveness for function calls are kept while tracing with this flag. Using `call` and `return_to` trace together makes it possible to know exactly in which function a process executes at any time.
  To get trace messages containing return values from functions, use the `{return_trace}` match_spec action instead.
  Message tags: `return_to`.
`running`
  Trace scheduling of processes.
  Message tags: `in`, `out`.
`garbage_collection`
  Trace garbage collections of processes.
  Message tags: `gc_start`, `gc_end`.
`timestamp`
  Include a time stamp in all trace messages. The time stamp (Ts) is of the same form as returned by `erlang:now()`.
`cpu_timestamp`
  A global trace flag for the Erlang node that makes all trace timestamps be in CPU time, not wallclock. It is only allowed with `PidSpec==all`. If the host machine operating system does not support high resolution CPU time measurements, `trace/3` exits with `badarg`.
`arity`
  Used in conjunction with the `call` trace flag. `{M, F, Arity}` will be specified instead of `{M, F, Args}` in call trace messages.
`set_on_spawn`
  Makes any process created by a traced process inherit its trace flags, including the `set_on_spawn` flag.
`set_on_first_spawn`
  Makes the first process created by a traced process inherit its trace flags, excluding the `set_on_first_spawn` flag.
`set_on_link`
  Makes any process linked by a traced process inherit its trace flags, including the `set_on_link` flag.
`set_on_first_link`
  Makes the first process linked to by a traced process inherit its trace flags, excluding the `set_on_first_link` flag.
`{tracer, Tracer}`
  Specify where to send the trace messages. `Tracer` must be the pid of a local process or the port identifier of a local port. If this flag is not given, trace messages will be sent to the process that called `erlang:trace/3`.

The effect of combining `set_on_first_link` with `set_on_link` is the same as having `set_on_first_link` alone. Likewise for `set_on_spawn` and `set_on_first_spawn`.

If the `timestamp` flag is not given, the tracing process will receive the trace messages described below. `Pid` is the pid of the traced process in which the traced event has occurred. The third element of the tuple is the message tag.

If the `timestamp` flag is given, the first element of the tuple will be `trace_ts` instead and the timestamp is added last in the tuple.

`{trace, Pid, 'receive', Msg}`
  When `Pid` receives the message `Msg`.
`{trace, Pid, send, Msg, To}`
  When `Pid` sends the message `Msg` to the process `To`.
`{trace, Pid, send_to_non_existing_process, Msg, To}`
  When `Pid` sends the message `Msg` to the non-existing process `To`.
`{trace, Pid, call, {M, F, Args}}`
  When `Pid` calls a traced function. The return values of calls are never supplied, only the call and its arguments.
  Note that the trace flag `arity` can be used to change the contents of this message, so that `Arity` is specified instead of `Args`.
`{trace, Pid, return_to, {M, F, Args}}`
  When `Pid` returns to the specified function. This trace message is sent if both the `call` and the `return_to` flags are set, and the function is set to be traced on local function calls. The message is only sent when returning from a chain of tail recursive function calls where at least one call generated a `call` trace message (that is, the functions match specification matched and `{message, false}` was not an action).
`{trace, Pid, return_from, {M, F, Args}, ReturnValue}`
  When `Pid` returns from the specified function. This trace message is sent if the `call` flag is set, and the function has a match specification with a `return_trace` or `exception_trace` action.
`{trace, Pid, exception_from, {M, F, Args}, {Class, Value}}`
  When `Pid` exits from the specified function due to an exception. This trace message is sent if the `call` flag is set, and the function has a match specification with an `exception_trace` action.
`{trace, Pid, spawn, Pid2, {M, F, Args}}`
  When `Pid` spawns a new process `Pid2` with the specified function call as entry point.
  Note that `Args` is supposed to be the argument list, but may be any term in the case of an erroneous spawn.
`{trace, Pid, exit, Reason}`
  When `Pid` exits with reason `Reason`.
`{trace, Pid, link, Pid2}`
  When `Pid` links to a process `Pid2`.
`{trace, Pid, unlink, Pid2}`
  When `Pid` removes the link from a process `Pid2`.
`{trace, Pid, getting_linked, Pid2}`
  When `Pid` gets linked to a process `Pid2`.
`{trace, Pid, getting_unlinked, Pid2}`
  When `Pid` gets unlinked from a process `Pid2`.
`{trace, Pid, register, RegName}`
  When `Pid` gets the name `RegName` registered.
`{trace, Pid, unregister, RegName}`
  When `Pid` gets the name `RegName` unregistered. Note that this is done automatically when a registered process exits.
`{trace, Pid, in, {M, F, Arity} | 0}`
  When `Pid` is scheduled to run. The process will run in function `{M, F, Arity}`. On some rare occasions the current function cannot be determined, then the last element `Arity` is 0.
`{trace, Pid, out, {M, F, Arity} | 0}`
  When `Pid` is scheduled out. The process was running in function {M, F, Arity}. On some rare occasions the current function cannot be determined, then the last element `Arity` is 0.
`{trace, Pid, gc_start, Info}`
  Sent when garbage collection is about to be started. `Info` is a list of two-element tuples, where the first element is a key, and the second is the value. You should not depend on the tuples have any defined order. Currently, the following keys are defined.
  `heap_size`
    The size of the used part of the heap.
  `old_heap_size`
    The size of the used part of the old heap.
  `stack_size`
    The actual size of the stack.
  `recent_size`
    The size of the data that survived the previous garbage collection.
  `mbuf_size`
    The combined size of message buffers associated with the process.
  All sizes are in words.
`{trace, Pid, gc_end, Info}`
  Sent when garbage collection is finished. `Info` contains the same kind of list as in the `gc_start` message, but the sizes reflect the new sizes after garbage collection.

If the tracing process dies, the flags will be silently removed.

Only one process can trace a particular process. For this reason, attempts to trace an already traced process will fail.

Returns: A number indicating the number of processes that matched `PidSpec`. If `PidSpec` is a pid, the return value will be `1`. If `PidSpec` is `all` or `existing` the return value will be the number of processes running, excluding tracer processes. If `PidSpec` is `new`, the return value will be `0`.

Failure: If specified arguments are not supported. For example `cpu_timestamp` is not supported on all platforms.

`erlang:trace_info(PidOrFunc, Item) -> Res`

Types:

`PidOrFunc = pid() | new | {Module, Function, Arity} | on_load`
`Module = Function = atom()`
`Arity = int()`
`Item, Res -- see below`

Returns trace information about a process or function.

To get information about a process, `PidOrFunc` should be a pid or the atom `new`. The atom `new` means that the default trace state for processes to be created will be returned. `Item` must have one of the following values:

`flags`
  Return a list of atoms indicating what kind of traces is enabled for the process. The list will be empty if no traces are enabled, and one or more of the followings atoms if traces are enabled: `send`, `'receive'`, `set_on_spawn`, `call`, `return_to`, `procs`, `set_on_first_spawn`, `set_on_link`, `running`, `garbage_collection`, `timestamp`, and `arity`. The order is arbitrary.
`tracer`
  Return the identifier for process or port tracing this process. If this process is not being traced, the return value will be `[]`.

To get information about a function, `PidOrFunc` should be a three-element tuple: `{Module, Function, Arity}` or the atom `on_load`. No wildcards are allowed. Returns `undefined` if the function does not exist or `false` if the function is not traced at all. `Item` must have one of the following values:

`traced`
  Return `global` if this function is traced on global function calls, `local` if this function is traced on local function calls (i.e local and global function calls), and `false` if neither local nor global function calls are traced.
`match_spec`
  Return the match specification for this function, if it has one. If the function is locally or globally traced but has no match specification defined, the returned value is `[]`.
`meta`
  Return the meta trace tracer process or port for this function, if it has one. If the function is not meta traced the returned value is `false`, and if the function is meta traced but has once detected that the tracer proc is invalid, the returned value is [].
`meta_match_spec`
  Return the meta trace match specification for this function, if it has one. If the function is meta traced but has no match specification defined, the returned value is `[]`.
`call_count`
  Return the call count value for this function or `true` for the pseudo function `on_load` if call count tracing is active. Return `false` otherwise. See also erlang:trace_pattern/3 .
`all`
  Return a list contaning the `{Item, Value}` tuples for all other items, or return `false` if no tracing is active for this function.

The actual return value will be `{Item, Value}`, where `Value` is the requested information as described above. If a pid for a dead process was given, or the name of a non-existing function, `Value` will be `undefined`.

If `PidOrFunc` is the `on_load`, the information returned refers to the default value for code that will be loaded.

`erlang:trace_pattern(MFA, MatchSpec) -> int()`

The same as erlang:trace_pattern(MFA, MatchSpec, []), retained for backward compatibility.

`erlang:trace_pattern(MFA, MatchSpec, FlagList) -> int()`

Types:

`MFA, MatchSpec, FlagList -- see below`

This BIF is used to enable or disable call tracing for exported functions. It must be combined with erlang:trace/3 to set the `call` trace flag for one or more processes.

Conceptually, call tracing works like this: Inside the Erlang virtual machine there is a set of processes to be traced and a set of functions to be traced. Tracing will be enabled on the intersection of the set. That is, if a process included in the traced process set calls a function included in the traced function set, the trace action will be taken. Otherwise, nothing will happen.

Use erlang:trace/3 to add or remove one or more processes to the set of traced processes. Use `erlang:trace_pattern/2` to add or remove exported functions to the set of traced functions.

The `erlang:trace_pattern/3` BIF can also add match specifications to an exported function. A match specification comprises a pattern that the arguments to the function must match, a guard expression which must evaluate to `true` and an action to be performed. The default action is to send a trace message. If the pattern does not match or the guard fails, the action will not be executed.

The `MFA` argument should be a tuple like `{Module, Function, Arity}` or the atom `on_load` (described below). It can be the module, function, and arity for an exported function (or a BIF in any module). The `'_'` atom can be used to mean any of that kind. Wildcards can be used in any of the following ways:

`{Module,Function,'_'}`
  All exported functions of any arity named `Function` in module `Module`.
`{Module,'_','_'}`
  All exported functions in module `Module`.
`{'_','_','_'}`
  All exported functions in all loaded modules.

Other combinations, such as `{Module,'_',Arity}`, are not allowed. Local functions will match wildcards only if the `local` option is in the `FlagList`.

If the `MFA` argument is the atom `on_load`, the match specification and flag list will be used on all modules that are newly loaded.

The `MatchSpec` argument can take any of the following forms:

`false`
  Disable tracing for the matching function(s). Any match specification will be removed.
`true`
  Enable tracing for the matching function(s).
`MatchSpecList`
  A list of match specifications. An empty list is equivalent to `true`. See the ERTS User's Guide for a description of match specifications.
`restart`
  For the `FlagList` option `call_count`: restart the existing counters. The behaviour is undefined for other `FlagList` options.
`pause`
  For the `FlagList` option `call_count`: pause the existing counters. The behaviour is undefined for other `FlagList` options.

The `FlagList` parameter is a list of options. The following options are allowed:

`global`
  Turn on or off call tracing for global function calls (that is, calls specifying the module explicitly). Only exported functions will match and only global calls will generate trace messages. This is the default.
`local`
  Turn on or off call tracing for all types of function calls. Trace messages will be sent whenever any of the specified functions are called, regardless of how they are called. If the `return_to` flag is set for the process, a `return_to` message will also be sent when this function returns to its caller.
`meta | {meta, Pid}`
  Turn on or off meta tracing for all types of function calls. Trace messages will be sent to the tracer process or port `Pid` whenever any of the specified functions are called, regardless of how they are called. If no `Pid` is specified, `self()` is used as a default tracer process.
  Meta tracing traces all processes and does not care about the process trace flags set by `trace/3`, the trace flags are instead fixed to `[call, timestamp]`.
  The match spec function `{return_trace}` works with meta trace and send its trace message to the same tracer process.
`call_count`
  Starts (`MatchSpec == true`) or stops (`MatchSpec == false`) call count tracing for all types of function calls. For every function a counter is incremented when the function is called, in any process. No process trace flags need to be activated.
  If call count tracing is started while already running, the count is restarted from zero. Running counters can be paused with `MatchSpec == pause`. Paused and running counters can be restarted from zero with `MatchSpec == restart`.
  The counter value can be read with erlang:trace_info/2 .

The `global` and `local` options are mutually exclusive and `global` is the default (if no options are specified). The `call_count` and `meta` options perform a kind of local tracing, and can also not be combined with `global`. A function can be either globally or locally traced. If global tracing is specified for a specified set of functions; local, meta and call count tracing for the matching set of local functions will be disabled, and vice versa.

When disabling trace, the option must match the type of trace that is set on the function, so that local tracing must be disabled with the `local` option and global tracing with the `global` option (or no option at all), and so forth.

There is no way to directly change part of a match specification list. If a function has a match specification, you can replace it with a completely new one. If you need to change an existing match specification, use the erlang:trace_info/2 BIF to retrieve the existing match specification.

Returns the number of exported functions that matched the `MFA` argument. This will be zero if none matched at all.

`trunc(Number) -> int()`

Types:

`Number = number()`

Returns an integer by the truncating `Number`.

    > trunc(5.5).
    5

Allowed in guard tests.

`tuple_to_list(Tuple) -> [term()]`

Types:

`Tuple = tuple()`

Returns a list which corresponds to `Tuple`. `Tuple` may contain any Erlang terms.

    > tuple_to_list({share, {'Ericsson_B', 163}}).
    [share, {'Ericsson_B', 163}]

`erlang:universaltime() -> {Date, Time}`

Types:

`Date = {Year, Month, Day}`
`Time = {Hour, Minute, Second}`
`Year = Month = Day = Hour = Minute = Second = int()`

Returns the current date and time according to Universal Time Coordinated (UTC), also called GMT, in the form `{{Year, Month, Day}, {Hour, Minute, Second}}` if supported by the underlying operating system. If not, `erlang:universaltime()` is equivalent to `erlang:localtime()`.

    > erlang:universaltime().
    {{1996,11,6},{14,18,43}}

`erlang:universaltime_to_localtime({Date1, Time1}) -> {Date2, Time2}`

Types:

`Date1 = Date2 = {Year, Month, Day}`
`Time1 = Time2 = {Hour, Minute, Second}`
`Year = Month = Day = Hour = Minute = Second = int()`

Converts Universal Time Coordinated (UTC) date and time to local date and time, if this is supported by the underlying OS. Otherwise, no conversion is done, and `{Date1, Time1}` is returned.

    > erlang:universaltime_to_localtime({{1996,11,6},{14,18,43}}).
    {{1996,11,7},{15,18,43}}

Failure: `badarg` if `Date1` or `Time1` do not denote a valid date or time.

`unlink(Id) -> true`

Types:

`Id = pid() | port()`

Removes the link, if there is one, between the calling process and the process or port referred to by `Id`.

Returns `true` and does not fail, even if there is no link to `Id`, or if `Id` does not exist.

Once `unlink(Id)` has returned it is guaranteed that the link between the caller and the entity referred to by `Id` has no effect on the caller in the future (unless the link is setup again). If caller is trapping exits, an `{'EXIT', Id, _}` message due to the link might have been placed in the callers message queue prior to the call, though. Note, the `{'EXIT', Id, _}` message can be the result of the link, but can also be the result of `Id` calling `exit/2`. Therefore, it may be appropriate to cleanup the message queue when trapping exits after the call to `unlink(Id)`, as follow:

        unlink(Id),
        receive
            {'EXIT', Id, _} ->
                true
        after 0 ->
                true
        end

Prior to OTP release R11B (erts version 5.5) `unlink/1` behaved completely asynchronous, i.e., the link was active until the "unlink signal" reached the linked entity. This had one undesirable effect, though. You could never know when you were guaranteed not to be effected by the link.

Current behavior can be viewed as two combined operations: asynchronously send an "unlink signal" to the linked entity and ignore any future results of the link.

`unregister(RegName) -> true`

Types:

`RegName = atom()`

Removes the registered name `RegName`, associated with a pid or a port identifier.

    > unregister(db).
    true

Users are advised not to unregister system processes.

Failure: `badarg` if `RegName` is not a registered name.

`whereis(RegName) -> pid() | port() | undefined`

Returns the pid or port identifier with the registered name `RegName`. Returns `undefined` if the name is not registered.

    > whereis(db).
    <0.43.0>

`erlang:yield() -> true`

Voluntarily let other processes (if any) get a chance to execute. Using `erlang:yield()` is similar to `receive after 1 -> ok end`, except that `yield()` is faster.

### AUTHORS

Joe Armstrong - support@erlang.ericsson.se
Mike Williams - support@erlang.ericsson.se
Robert Virding - support@erlang.ericsson.se
Claes Wikström - support@erlang.ericsson.se
Rickard Green - support@erlang.ericsson.se

kernel 2.11.1
Copyright © 1991-2006 Ericsson AB