Erlang -- erlang

erlang

The Erlang BIFs

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".

A binary data object, structured according to the Erlang external term format.

See now/0.

Types:

Float = float()

Int = integer()

Returns an integer or float which is the arithmetical absolute value of Float or Int.

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

Allowed in guard tests.

Types:

Data = iodata()

Computes and returns the adler32 checksum for Data.

Types:

OldAdler = integer() >= 0

Data = iodata()

Continue computing the adler32 checksum by combining the previous checksum, OldAdler, with the checksum of Data.

The following code:

	X = erlang:adler32(Data1),
	Y = erlang:adler32(X,Data2).

- would assign the same value to Y as this would:

	Y = erlang:adler32([Data1,Data2]).

Types:

FirstAdler = SecondAdler = SecondSize = integer() >= 0

Combines two previously computed adler32 checksums. This computation requires the size of the data object for the second checksum to be known.

The following code:

	Y = erlang:adler32(Data1),
	Z = erlang:adler32(Y,Data2).

- would assign the same value to Z as this would:

	X = erlang:adler32(Data1),
	Y = erlang:adler32(Data2),
	Z = erlang:adler32_combine(X,Y,iolist_size(Data2)).

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(Tuple1) ++ [Term]), but much faster.

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

Types:

Fun = function()

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).

Warning

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.

Types:

Module = 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.

Types:

Atom = atom()

Encoding = latin1 | unicode | utf8

Returns a binary which corresponds to the text representation of Atom. If Encoding is latin1, there will be one byte for each character in the text representation. If Encoding is utf8 or unicode, the characters will be encoded using UTF-8 (meaning that characters from 16#80 up to 0xFF will be encoded in two bytes).

Note

Currently, atom_to_binary(Atom, latin1) can never fail because the text representation of an atom can only contain characters from 0 to 16#FF. In a future release, the text representation of atoms might be allowed to contain any Unicode character and atom_to_binary(Atom, latin1) will fail if the text representation for the Atom contains a Unicode character greater than 16#FF.

> atom_to_binary('Erlang', latin1).
<<"Erlang">>

Types:

Atom = atom()

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

> atom_to_list('Erlang').
"Erlang"

Types:

Subject = binary()

PosLen = {Start :: integer() >= 0, Length :: integer()}

Extracts the part of the binary described by PosLen.

Negative length can be used to extract bytes at the end of a binary:

1> Bin = <<1,2,3,4,5,6,7,8,9,10>>.
2> binary_part(Bin,{byte_size(Bin), -5}).
<<6,7,8,9,10>>

If PosLen in any way references outside the binary, a badarg exception is raised.

Start is zero-based, i.e.:

1> Bin = <<1,2,3>>
2> binary_part(Bin,{0,2}).
<<1,2>>

See the STDLIB module binary for details about the PosLen semantics.

Allowed in guard tests.

Types:

Subject = binary()

Start = integer() >= 0

Length = integer()

The same as binary_part(Subject, {Start, Length}).

Allowed in guard tests.

Types:

Binary = binary()

Encoding = latin1 | unicode | utf8

Returns the atom whose text representation is Binary. If Encoding is latin1, no translation of bytes in the binary is done. If Encoding is utf8 or unicode, the binary must contain valid UTF-8 sequences; furthermore, only Unicode characters up to 0xFF are allowed.

Note

binary_to_atom(Binary, utf8) will fail if the binary contains Unicode characters greater than 16#FF. In a future release, such Unicode characters might be allowed and binary_to_atom(Binary, utf8) will not fail in that case. For more information on Unicode support in atoms see note on UTF-8 encoded atoms in the chapter about the external term format in the ERTS User's Guide.

> binary_to_atom(<<"Erlang">>, latin1).
'Erlang'
> binary_to_atom(<<1024/utf8>>, utf8).
** exception error: bad argument
     in function  binary_to_atom/2
        called as binary_to_atom(<<208,128>>,utf8)

Types:

Binary = binary()

Encoding = latin1 | unicode | utf8

Works like binary_to_atom/2, but the atom must already exist.

Failure: badarg if the atom does not already exist.

Types:

Binary = binary()

Returns the float whose text representation is Binary.

> binary_to_float(<<"2.2017764e+0">>).
2.2017764

Failure: badarg if Binary contains a bad representation of a float.

Types:

Binary = binary()

Returns an integer whose text representation is Binary.

> binary_to_integer(<<"123">>).
123

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

Types:

Binary = binary()

Base = 2..36

Returns an integer whose text representation in base Base is Binary.

> binary_to_integer(<<"3FF">>, 16).
1023

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

Types:

Binary = binary()

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

Types:

Binary = binary()

Start = Stop = integer() >= 1

1..byte_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.

Note

This function's indexing style of using one-based indices for binaries is deprecated. New code should use the functions in the STDLIB module binary instead. They consequently use the same (zero-based) style of indexing.

Types:

Bitstring = bitstring()

Returns a list of integers which correspond to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list will be a bitstring containing the remaining bits (1 up to 7 bits).

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.

Warning

When decoding binaries from untrusted sources, consider using binary_to_term/2 to prevent denial of service attacks.

See also term_to_binary/1 and binary_to_term/2.

Types:

Binary = ext_binary()

Opts = [safe]

As binary_to_term/1, but takes options that affect decoding of the binary.

safe

Use this option when receiving binaries from an untrusted source.

When enabled, it prevents decoding data that may be used to attack the Erlang system. In the event of receiving unsafe data, decoding fails with a badarg error.

Currently, this prevents creation of new atoms directly, creation of new atoms indirectly (as they are embedded in certain structures like pids, refs, funs, etc.), and creation of new external function references. None of those resources are currently garbage collected, so unchecked creation of them can exhaust available memory.

Failure: badarg if safe is specified and unsafe data is decoded.

Types:

Bitstring = bitstring()

Returns an integer which is the size in bits of Bitstring.

> bit_size(<<433:16,3:3>>).
19
> bit_size(<<1,2,3>>).
24

Allowed in guard tests.

Types:

Reductions = integer() >= 1

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 the maximum number of reductions for a process (2000 reductions in R12B).

Warning

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.

Types:

Bitstring = bitstring()

Returns an integer which is the number of bytes needed to contain Bitstring. (That is, if the number of bits in Bitstring is not divisible by 8, the resulting number of bytes will be rounded up.)

> byte_size(<<433:16,3:3>>).
3
> byte_size(<<1,2,3>>).
3

Allowed in guard tests.

Types:

TimerRef = reference()

Time = integer() >= 0

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).

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

Types:

Module = module()

Returns true if the Module has old code, and false otherwise.

See also exit/2.

Types:

Module = OldModule = atom()

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 auto-loading is dependent on the correct operation of the error handling module.

Returns the old value of the flag.

Types:

MinHeapSize = OldMinHeapSize = integer() >= 0

This changes the minimum heap size for the calling process.

Returns the old value of the flag.

Types:

MinBinVHeapSize = OldMinBinVHeapSize = integer() >= 0

This changes the minimum binary virtual heap size for the calling process.

Returns the old value of the flag.

Types:

Level = OldLevel = priority_level()

priority_level() = low | normal | high | max

This sets the process priority. Level is an atom. There are currently four priority levels: low, normal, high, and max. The default priority level is normal. NOTE: The max priority level is reserved for internal use in the Erlang runtime system, and should not be used by others.

Internally in each priority level processes are scheduled in a round robin fashion.

Execution of processes on priority normal and priority low will be interleaved. Processes on priority low will be selected for execution less frequently than processes on priority normal.

When there are runnable processes on priority high no processes on priority low, or normal will be selected for execution. Note, however, that this does not mean that no processes on priority low, or normal will be able to run when there are processes on priority high running. On the runtime system with SMP support there might be more processes running in parallel than processes on priority high, i.e., a low, and a high priority process might execute at the same time.

When there are runnable processes on priority max no processes on priority low, normal, or high will be selected for execution. As with the high priority, processes on lower priorities might execute in parallel with processes on priority max.

Scheduling is preemptive. Regardless of priority, a process is preempted when it has consumed more than a certain amount of reductions since the last time it was selected for execution.

NOTE: You should not depend on the scheduling to remain exactly as it is today. Scheduling, at least on the runtime system with SMP support, is very likely to be modified in the future in order to better utilize available processor cores.

There is currently no automatic mechanism for avoiding priority inversion, such as priority inheritance, or priority ceilings. When using priorities you have to take this into account and handle such scenarios by yourself.

Making calls from a high priority process into code that you don't have control over may cause the high priority process to wait for a processes with lower priority, i.e., effectively decreasing the priority of the high priority process during the call. Even if this isn't the case with one version of the code that you don't have under your control, it might be the case in a future version of it. This might, for example, happen if a high priority process triggers code loading, since the code server runs on priority normal.

Other priorities than normal are normally not needed. When other priorities are used, they need to be used with care, especially the high priority must be used with care. A process on high priority should only perform work for short periods of time. Busy looping for long periods of time in a high priority process will most likely cause problems, since there are important servers in OTP running on priority normal.

Returns the old value of the flag.

Types:

N = OldN = 0..10000

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.

Returns the old value of the flag.

Types:

Boolean = OldBoolean = boolean()

Set or clear the sensitive flag for the current process. When a process has been marked as sensitive by calling process_flag(sensitive, true), features in the run-time system that can be used for examining the data and/or inner working of the process are silently disabled.

Features that are disabled include (but are not limited to) the following:

Tracing: Trace flags can still be set for the process, but no trace messages of any kind will be generated. (If the sensitive flag is turned off, trace messages will again be generated if there are any trace flags set.)

Sequential tracing: The sequential trace token will be propagated as usual, but no sequential trace messages will be generated.

process_info/1,2 cannot be used to read out the message queue or the process dictionary (both will be returned as empty lists).

Stack back-traces cannot be displayed for the process.

In crash dumps, the stack, messages, and the process dictionary will be omitted.

If {save_calls,N} has been set for the process, no function calls will be saved to the call saving list. (The call saving list will not be cleared; furthermore, send, receive, and timeout events will still be added to the list.)

Returns the old value of the flag.

Types:

Pid = pid()

Flag = save_calls

Value = OldValue = integer() >= 0

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.

Types:

Pid = pid()

Info = [InfoTuple] | undefined

InfoTuple = process_info_result_item()

process_info_result_item() = {backtrace, Bin :: binary()}
                           | {binary,
                              BinInfo ::
                                  [{integer() >= 0,
                                    integer() >= 0,
                                    integer() >= 0}]}
                           | {catchlevel,
                              CatchLevel :: integer() >= 0}
                           | {current_function,
                              {Module :: module(),
                               Function :: atom(),
                               Arity :: arity()}}
                           | {current_location,
                              {Module :: module(),
                               Function :: atom(),
                               Arity :: arity(),
                               Location ::
                                   [{file,
                                     Filename :: string()} |
                                    {line,
                                     Line :: integer() >= 1}]}}
                           | {current_stacktrace,
                              Stack :: [stack_item()]}
                           | {dictionary,
                              Dictionary ::
                                  [{Key :: term(),
                                    Value :: term()}]}
                           | {error_handler,
                              Module :: module()}
                           | {garbage_collection,
                              GCInfo ::
                                  [{atom(), integer() >= 0}]}
                           | {group_leader,
                              GroupLeader :: pid()}
                           | {heap_size,
                              Size :: integer() >= 0}
                           | {initial_call, mfa()}
                           | {links,
                              PidsAndPorts :: [pid() | port()]}
                           | {last_calls,
                              false | (Calls :: [mfa()])}
                           | {memory,
                              Size :: integer() >= 0}
                           | {message_queue_len,
                              MessageQueueLen ::
                                  integer() >= 0}
                           | {messages,
                              MessageQueue :: [term()]}
                           | {min_heap_size,
                              MinHeapSize :: integer() >= 0}
                           | {min_bin_vheap_size,
                              MinBinVHeapSize ::
                                  integer() >= 0}
                           | {monitored_by, Pids :: [pid()]}
                           | {monitors,
                              Monitors ::
                                  [{process,
                                    Pid :: pid()
                                         | {RegName :: atom(),
                                            Node :: node()}}]}
                           | {priority,
                              Level :: priority_level()}
                           | {reductions,
                              Number :: integer() >= 0}
                           | {registered_name, Atom :: atom()}
                           | {sequential_trace_token,
                              [] |
                              (SequentialTraceToken :: term())}
                           | {stack_size,
                              Size :: integer() >= 0}
                           | {status,
                              Status :: exiting
                                      | garbage_collecting
                                      | waiting
                                      | running
                                      | runnable
                                      | suspended}
                           | {suspending,
                              SuspendeeList ::
                                  [{Suspendee :: pid(),
                                    ActiveSuspendCount ::
                                        integer() >= 0,
                                    OutstandingSuspendCount ::
                                        integer() >= 0}]}
                           | {total_heap_size,
                              Size :: integer() >= 0}
                           | {trace,
                              InternalTraceFlags ::
                                  integer() >= 0}
                           | {trap_exit, Boolean :: boolean()}

priority_level() = low | normal | high | max

stack_item() =
    {Module :: module(),
     Function :: atom(),
     Arity :: arity() | (Args :: [term()]),
     Location ::
         [{file, Filename :: string()} |
          {line, Line :: integer() >= 1}]}

Returns a list containing InfoTuples with miscellaneous information about the process identified by Pid, or undefined if the process is not alive.

The order of the InfoTuples is not defined, nor are all the InfoTuples mandatory. The InfoTuples part of the result may be changed without prior notice. Currently InfoTuples with the following items are part of the result: current_function, initial_call, status, message_queue_len, messages, links, dictionary, trap_exit, error_handler, priority, group_leader, total_heap_size, heap_size, stack_size, reductions, and garbage_collection. If the process identified by Pid has a registered name also an InfoTuple with the item registered_name will appear.

See process_info/2 for information about specific InfoTuples.

Warning

This BIF is intended for debugging only, use process_info/2 for all other purposes.

Failure: badarg if Pid is not a local process.

Types:

Pid = pid()

ItemList = [Item]

Item = process_info_item()

InfoTupleList = [InfoTuple]

InfoTuple = process_info_result_item()

process_info_result_item() = {backtrace, Bin :: binary()}
                           | {binary,
                              BinInfo ::
                                  [{integer() >= 0,
                                    integer() >= 0,
                                    integer() >= 0}]}
                           | {catchlevel,
                              CatchLevel :: integer() >= 0}
                           | {current_function,
                              {Module :: module(),
                               Function :: atom(),
                               Arity :: arity()}}
                           | {current_location,
                              {Module :: module(),
                               Function :: atom(),
                               Arity :: arity(),
                               Location ::
                                   [{file,
                                     Filename :: string()} |
                                    {line,
                                     Line :: integer() >= 1}]}}
                           | {current_stacktrace,
                              Stack :: [stack_item()]}
                           | {dictionary,
                              Dictionary ::
                                  [{Key :: term(),
                                    Value :: term()}]}
                           | {error_handler,
                              Module :: module()}
                           | {garbage_collection,
                              GCInfo ::
                                  [{atom(), integer() >= 0}]}
                           | {group_leader,
                              GroupLeader :: pid()}
                           | {heap_size,
                              Size :: integer() >= 0}
                           | {initial_call, mfa()}
                           | {links,
                              PidsAndPorts :: [pid() | port()]}
                           | {last_calls,
                              false | (Calls :: [mfa()])}
                           | {memory,
                              Size :: integer() >= 0}
                           | {message_queue_len,
                              MessageQueueLen ::
                                  integer() >= 0}
                           | {messages,
                              MessageQueue :: [term()]}
                           | {min_heap_size,
                              MinHeapSize :: integer() >= 0}
                           | {min_bin_vheap_size,
                              MinBinVHeapSize ::
                                  integer() >= 0}
                           | {monitored_by, Pids :: [pid()]}
                           | {monitors,
                              Monitors ::
                                  [{process,
                                    Pid :: pid()
                                         | {RegName :: atom(),
                                            Node :: node()}}]}
                           | {priority,
                              Level :: priority_level()}
                           | {reductions,
                              Number :: integer() >= 0}
                           | {registered_name, Atom :: atom()}
                           | {sequential_trace_token,
                              [] |
                              (SequentialTraceToken :: term())}
                           | {stack_size,
                              Size :: integer() >= 0}
                           | {status,
                              Status :: exiting
                                      | garbage_collecting
                                      | waiting
                                      | running
                                      | runnable
                                      | suspended}
                           | {suspending,
                              SuspendeeList ::
                                  [{Suspendee :: pid(),
                                    ActiveSuspendCount ::
                                        integer() >= 0,
                                    OutstandingSuspendCount ::
                                        integer() >= 0}]}
                           | {total_heap_size,
                              Size :: integer() >= 0}
                           | {trace,
                              InternalTraceFlags ::
                                  integer() >= 0}
                           | {trap_exit, Boolean :: boolean()}

stack_item() =
    {Module :: module(),
     Function :: atom(),
     Arity :: arity() | (Args :: [term()]),
     Location ::
         [{file, Filename :: string()} |
          {line, Line :: integer() >= 1}]}

priority_level() = low | normal | high | max

Returns information about the process identified by Pid as specified by the Item or the ItemList, or undefined if the process is not alive.

If the process is alive and a single Item is given, the returned value is the corresponding InfoTuple unless Item =:= registered_name and the process has no registered name. In this case [] is returned. This strange behavior is due to historical reasons, and is kept for backward compatibility.

If an ItemList is given, the result is an InfoTupleList. The InfoTuples in the InfoTupleList will appear with the corresponding Items in the same order as the Items appeared in the ItemList. Valid Items may appear multiple times in the ItemList.

Note

If registered_name is part of an ItemList and the process has no name registered a {registered_name, []} InfoTuple will appear in the resulting InfoTupleList. This behavior is different than when a single Item =:= registered_name is given, and than when process_info/1 is used.

Currently the following InfoTuples with corresponding Items are valid:

{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.
{binary, BinInfo}: BinInfo is a list containing miscellaneous information about binaries currently being referred to by this process. This InfoTuple may be changed or removed without prior notice.
{catchlevel, CatchLevel}: CatchLevel is the number of currently active catches in this process. This InfoTuple may be changed or removed without prior notice.
{current_function, {Module, Function, Arity}}: Module, Function, Arity is the current function call of the process.
{current_location, {Module, Function, Arity, Location}}: Module, Function, Arity is the current function call of the process. Location is a list of two-tuples that describes the location in the source code.
{current_stacktrace, Stack}: Return the current call stack back-trace (stacktrace) of the process. The stack has the same format as returned by erlang:get_stacktrace/0.
{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).
{garbage_collection, GCInfo}: GCInfo is a list which contains miscellaneous information about garbage collection for this process. The content of GCInfo may be changed without prior notice.
{group_leader, GroupLeader}: GroupLeader is group leader for the IO of the process.
{heap_size, Size}: Size is the size in words of youngest heap generation of the process. This generation currently include the stack of the process. This information is highly implementation dependent, and may change if the implementation change.
{initial_call, {Module, Function, Arity}}: Module, Function, Arity is the initial function call with which the process was spawned.
{links, PidsAndPorts}: PidsAndPorts is a list of pids and port identifiers, with processes or ports to which the process has a link.
{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 in bytes of the process. This includes call stack, heap and internal structures.
{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.
{min_heap_size, MinHeapSize}: MinHeapSize is the minimum heap size for the process.
{min_bin_vheap_size, MinBinVHeapSize}: MinBinVHeapSize is the minimum binary virtual heap size for the process.
{monitored_by, Pids}: A list of pids that are monitoring the process (with monitor/2).
{monitors, Monitors}: A list of monitors (started by 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}}.
{priority, Level}: Level is the current priority level for the process. For more information on priorities see process_flag(priority, Level).
{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.
{sequential_trace_token, [] | SequentialTraceToken}: SequentialTraceToken the sequential trace token for the process. This InfoTuple may be changed or removed without prior notice.
{stack_size, Size}: Size is the stack size of the process in words.
{status, Status}: Status is the status of the process. Status is exiting, garbage_collecting, 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/[1,2] BIF).
{suspending, SuspendeeList}: SuspendeeList is a list of {Suspendee, ActiveSuspendCount, OutstandingSuspendCount} tuples. Suspendee is the pid of a process that have been or is to be suspended by the process identified by Pid via the erlang:suspend_process/2 BIF, or the erlang:suspend_process/1 BIF. ActiveSuspendCount is the number of times the Suspendee has been suspended by Pid. OutstandingSuspendCount is the number of not yet completed suspend requests sent by Pid. That is, if ActiveSuspendCount =/= 0, Suspendee is currently in the suspended state, and if OutstandingSuspendCount =/= 0 the asynchronous option of erlang:suspend_process/2 has been used and the suspendee has not yet been suspended by Pid. Note that the ActiveSuspendCount and OutstandingSuspendCount are not the total suspend count on Suspendee, only the parts contributed by Pid.
{total_heap_size, Size}: Size is the total size in words of all heap fragments of the process. This currently include the stack of the process.
{trace, InternalTraceFlags}: InternalTraceFlags is an integer representing internal trace flag for this process. This InfoTuple may be changed or removed without prior notice.
{trap_exit, Boolean}: Boolean is true if the process is trapping exits, otherwise it is false.

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

Failure: badarg if Pid is not a local process, or if Item is not a valid Item.

Returns a list of process identifiers corresponding to all the processes currently existing on the local node.

Note that a process that is exiting, exists but is not alive, i.e., is_process_alive/1 will return false for a process that is exiting, but its process identifier will be part of the result returned from processes/0.

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

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.

Warning

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.

Types:

Key = Val = 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.

Note

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}

Types:

Class = error | exit | throw

Reason = term()

Stacktrace = raise_stacktrace()

raise_stacktrace() = [{module(), atom(), arity() | [term()]} |
                      {function(), [term()]}]
                   | [{module(),
                       atom(),
                       arity() | [term()],
                       [{atom(), term()}]} |
                      {function(),
                       [term()],
                       [{atom(), term()}]}]

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

Warning

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 4-tuples {Module, Function, Arity | Args, Location} 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, Location} tuples where Fun is a local fun and Args is an argument list.

The Location element at the end is optional. Omitting it is equivalent to specifying an empty 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 error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

Types:

TimerRef = reference()

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).

Types:

Ref = reference()

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

Warning

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

Types:

RegName = atom()

PidOrPort = port() | pid()

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 PidOrPort 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.

Types:

RegName = atom()

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

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

Types:

Suspendee = pid()

Decreases the suspend count on the process identified by Suspendee. Suspendee should previously have been suspended via erlang:suspend_process/2, or erlang:suspend_process/1 by the process calling erlang:resume_process(Suspendee). When the suspend count on Suspendee reach zero, Suspendee will be resumed, i.e., the state of the Suspendee is changed from suspended into the state Suspendee was in before it was suspended.

Warning

This BIF is intended for debugging only.

Failures:

badarg: If Suspendee isn't a process identifier.
badarg: If the process calling erlang:resume_process/1 had not previously increased the suspend count on the process identified by Suspendee.
badarg: If the process identified by Suspendee is not alive.

Types:

Number = number()

Returns an integer by rounding Number.

> round(5.5).
6

Allowed in guard tests.

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

> self().
<0.26.0>

Allowed in guard tests.

Types:

Dest = dst()

Msg = term()

dst() = pid()
      | port()
      | (RegName :: atom())
      | {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.

Types:

Dest = dst()

Msg = term()

Options = [nosuspend | noconnect]

Res = ok | nosuspend | noconnect

dst() = pid()
      | port()
      | (RegName :: atom())
      | {RegName :: atom(), Node :: node()}

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 auto-connected before doing the send, noconnect is returned instead.

Warning

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

Types:

Time = integer() >= 0

0 <= Time <= 4294967295

Dest = pid() | atom()

Msg = term()

TimerRef = reference()

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

If Dest is a pid() it has to be a pid() of a local process, dead or alive.

The Time value can, in the current implementation, not be greater than 4294967295.

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.

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

Types:

Dest = dst()

Msg = term()

dst() = pid()
      | port()
      | (RegName :: atom())
      | {RegName :: atom(), Node :: node()}

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 queue to be over-full 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 non-responsive, 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 non-responsive 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.

Warning

Use with extreme care!

Types:

Dest = dst()

Msg = term()

Options = [noconnect]

dst() = pid()
      | port()
      | (RegName :: atom())
      | {RegName :: atom(), Node :: node()}

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.

Warning

Use with extreme care!

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.

Types:

Index = integer() >= 1

1..tuple_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}

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.

Types:

Fun = function()

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

Types:

Node = node()

Fun = function()

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.

Types:

Module = 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>

Types:

Node = node()

Module = 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.

Types:

Fun = function()

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 the new process, atomically. Otherwise works like spawn/3.

Types:

Node = node()

Fun = function()

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 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.

Types:

Module = 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 between the calling process and the new process, atomically. Otherwise works like spawn/3.

Types:

Node = node()

Module = 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 between 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.

Types:

Fun = function()

Returns the pid of a new process started by the application of Fun to the empty list [] and reference for a monitor created to the new process. Otherwise works like spawn/3.

Types:

Module = module()

Function = atom()

Args = [term()]

A new process is started by the application of Module:Function to Args, and the process is monitored at the same time. Returns the pid and a reference for the monitor. Otherwise works like spawn/3.

Types:

Fun = function()

Options = [Option]

Option = link
       | monitor
       | {priority, Level :: priority_level()}
       | {fullsweep_after, Number :: integer() >= 0}
       | {min_heap_size, Size :: integer() >= 0}
       | {min_bin_vheap_size, VSize :: integer() >= 0}

priority_level() = low | normal | high | max

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

If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

Types:

Node = node()

Fun = function()

Options = [Option]

Option = link
       | monitor
       | {priority, Level :: priority_level()}
       | {fullsweep_after, Number :: integer() >= 0}
       | {min_heap_size, Size :: integer() >= 0}
       | {min_bin_vheap_size, VSize :: integer() >= 0}

priority_level() = low | normal | high | max

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.

Types:

Module = module()

Function = atom()

Args = [term()]

Options = [Option]

Option = link
       | monitor
       | {priority, Level :: priority_level()}
       | {fullsweep_after, Number :: integer() >= 0}
       | {min_heap_size, Size :: integer() >= 0}
       | {min_bin_vheap_size, VSize :: integer() >= 0}

priority_level() = low | normal | high | max

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

If the option monitor is given, the newly created process will be monitored and both the pid and reference for the monitor will be returned.

link

Sets a link to the parent process (like spawn_link/3 does).

monitor

Monitor the new process (just like monitor/2 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 selected for execution for the first time. For more information on priorities see process_flag(priority, Level).

{fullsweep_after, Number}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

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}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

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.

{min_bin_vheap_size, VSize}

This option is only useful for performance tuning. In general, you should not use this option unless you know that there is problem with execution times and/or memory consumption, and you should measure to make sure that the option improved matters.

Gives a minimum binary virtual 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. Therefore, it is recommended to use this option only for fine-tuning an application and to measure the execution time with various VSize values.

Types:

Node = node()

Module = module()

Function = atom()

Args = [term()]

Options = [Option]

Option = link
       | monitor
       | {priority, Level :: priority_level()}
       | {fullsweep_after, Number :: integer() >= 0}
       | {min_heap_size, Size :: integer() >= 0}
       | {min_bin_vheap_size, VSize :: integer() >= 0}

priority_level() = low | normal | high | max

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.

Types:

Bin = binary()

Pos = integer() >= 0

0..byte_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").
<<"0123456789">>
> byte_size(B).
10
> {B1, B2} = split_binary(B,3).
{<<"012">>,<<"3456789">>}
> byte_size(B1).
3
> byte_size(B2).
7

Types:

Time = integer() >= 0

0 <= Time <= 4294967295

Dest = pid() | atom()

Msg = term()

TimerRef = reference()

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

If Dest is a pid() it has to be a pid() of a local process, dead or alive.

The Time value can, in the current implementation, not be greater than 4294967295.

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().

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

Types:

ContextSwitches = integer() >= 0

ContextSwitches is the total number of context switches since the system started.

Types:

Total_Exact_Reductions = Exact_Reductions_Since_Last_Call = integer() >= 0

Note

statistics(exact_reductions) is a more expensive operation than statistics(reductions) especially on an Erlang machine with SMP support.

Types:

Number_of_GCs = Words_Reclaimed = integer() >= 0

This information may not be valid for all implementations.

> statistics(garbage_collection).
{85,23961,0}

Types:

Input = Output = integer() >= 0

Input is the total number of bytes received through ports, and Output is the total number of bytes output to ports.

Types:

Total_Reductions = Reductions_Since_Last_Call = integer() >= 0

Note

Since erts-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).

> statistics(reductions).
{2046,11}

Returns the total length of the run queues, that is, the number of processes that are ready to run on all available run queues.

Types:

Total_Run_Time = Time_Since_Last_Call = integer() >= 0

Note that the run-time is the sum of the run-time for all threads in the Erlang run-time system and may therefore be greater than the wall-clock time.

> statistics(runtime).
{1690,1620}

Types:

SchedulerId = integer() >= 1

ActiveTime = TotalTime = integer() >= 0

Returns a list of tuples with {SchedulerId, ActiveTime, TotalTime}, where SchedulerId is an integer id of the scheduler, ActiveTime is the duration the scheduler has been busy, TotalTime is the total time duration since scheduler_wall_time activation. The time unit is not defined and may be subject to change between releases, operating systems and system restarts. scheduler_wall_time should only be used to calculate relative values for scheduler-utilization. ActiveTime can never exceed TotalTime.

The definition of a busy scheduler is when it is not idle or not scheduling (selecting) a process or port, meaning; executing process code, executing linked-in-driver or NIF code, executing built-in-functions or any other runtime handling, garbage collecting or handling any other memory management. Note, a scheduler may also be busy even if the operating system has scheduled out the scheduler thread.

Returns undefined if the system flag scheduler_wall_time is turned off.

The list of scheduler information is unsorted and may appear in different order between calls.

Using scheduler_wall_time to calculate scheduler utilization.

> erlang:system_flag(scheduler_wall_time, true).
false
> Ts0 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok

Some time later we will take another snapshot and calculate scheduler-utilization per scheduler.

> Ts1 = lists:sort(erlang:statistics(scheduler_wall_time)), ok.
ok
> lists:map(fun({{I, A0, T0}, {I, A1, T1}}) ->
	{I, (A1 - A0)/(T1 - T0)} end, lists:zip(Ts0,Ts1)).
[{1,0.9743474730177548},
 {2,0.9744843782751444},
 {3,0.9995902361669045},
 {4,0.9738012596572161},
 {5,0.9717956667018103},
 {6,0.9739235846420741},
 {7,0.973237033077876},
 {8,0.9741297293248656}]

Using the same snapshots to calculate a total scheduler-utilization.

> {A, T} = lists:foldl(fun({{_, A0, T0}, {_, A1, T1}}, {Ai,Ti}) ->
	{Ai + (A1 - A0), Ti + (T1 - T0)} end, {0, 0}, lists:zip(Ts0,Ts1)), A/T.
0.9769136803764825

Note

scheduler_wall_time is by default disabled. Use erlang:system_flag(scheduler_wall_time, true) to enable it.

Types:

Total_Wallclock_Time = Wallclock_Time_Since_Last_Call = integer() >= 0

wall_clock can be used in the same manner as runtime, except that real time is measured as opposed to runtime or CPU time.

Types:

Suspendee = pid()

OptList = [Opt]

Opt = unless_suspending | asynchronous

Increases the suspend count on the process identified by Suspendee and puts it in the suspended state if it isn't already in the suspended state. A suspended process will not be scheduled for execution until the process has been resumed.

A process can be suspended by multiple processes and can be suspended multiple times by a single process. A suspended process will not leave the suspended state until its suspend count reach zero. The suspend count of Suspendee is decreased when erlang:resume_process(Suspendee) is called by the same process that called erlang:suspend_process(Suspendee). All increased suspend counts on other processes acquired by a process will automatically be decreased when the process terminates.

Currently the following options (Opts) are available:

asynchronous: A suspend request is sent to the process identified by Suspendee. Suspendee will eventually suspend unless it is resumed before it was able to suspend. The caller of erlang:suspend_process/2 will return immediately, regardless of whether the Suspendee has suspended yet or not. Note that the point in time when the Suspendee will actually suspend cannot be deduced from other events in the system. The only guarantee given is that the Suspendee will eventually suspend (unless it is resumed). If the asynchronous option has not been passed, the caller of erlang:suspend_process/2 will be blocked until the Suspendee has actually suspended.
unless_suspending: The process identified by Suspendee will be suspended unless the calling process already is suspending the Suspendee. If unless_suspending is combined with the asynchronous option, a suspend request will be sent unless the calling process already is suspending the Suspendee or if a suspend request already has been sent and is in transit. If the calling process already is suspending the Suspendee, or if combined with the asynchronous option and a send request already is in transit, false is returned and the suspend count on Suspendee will remain unchanged.

If the suspend count on the process identified by Suspendee was increased, true is returned; otherwise, false is returned.

Warning

This BIF is intended for debugging only.

Failures:

badarg: If Suspendee isn't a process identifier.
badarg: If the process identified by Suspendee is same the process as the process calling erlang:suspend_process/2.
badarg: If the process identified by Suspendee is not alive.
badarg: If the process identified by Suspendee resides on another node.
badarg: If OptList isn't a proper list of valid Opts.
system_limit: If the process identified by Suspendee has been suspended more times by the calling process than can be represented by the currently used internal data structures. The current system limit is larger than 2 000 000 000 suspends, and it will never be less than that.

Types:

Suspendee = pid()

Suspends the process identified by Suspendee. The same as calling erlang:suspend_process(Suspendee, []). For more information see the documentation of erlang:suspend_process/2.

Warning

This BIF is intended for debugging only.

Types:

Depth = OldDepth = integer() >= 0

Sets the maximum depth of call stack back-traces in the exit reason element of 'EXIT' tuples.

Returns the old value of the flag.

Types:

CpuTopology = OldCpuTopology = cpu_topology()

cpu_topology() = [LevelEntry :: level_entry()] | undefined

level_entry() = {LevelTag :: level_tag(),
                 SubLevel :: sub_level()}
              | {LevelTag :: level_tag(),
                 InfoList :: info_list(),
                 SubLevel :: sub_level()}

level_tag() = core | node | processor | thread

sub_level() = [LevelEntry :: level_entry()]
| (LogicalCpuId :: {logical, integer() >= 0})

info_list() = []

Warning

This argument is deprecated and scheduled for removal in erts-5.10/OTP-R16. Instead of using this argument you are advised to use the erl command line argument +sct. When this argument has been removed a final CPU topology to use will be determined at emulator boot time.

Sets the user defined CpuTopology. The user defined CPU topology will override any automatically detected CPU topology. By passing undefined as CpuTopology the system will revert back to the CPU topology automatically detected. The returned value equals the value returned from erlang:system_info(cpu_topology) before the change was made.

Returns the old value of the flag.

The CPU topology is used when binding schedulers to logical processors. If schedulers are already bound when the CPU topology is changed, the schedulers will be sent a request to rebind according to the new CPU topology.

The user defined CPU topology can also be set by passing the +sct command line argument to erl.

For information on the CpuTopology type and more, see the documentation of erlang:system_info(cpu_topology), and the erl +sct and +sbt command line flags.

Types:

DirtyCPUSchedulersOnline = OldDirtyCPUSchedulersOnline = integer() >= 1

Sets the amount of dirty CPU schedulers online. Valid range is 1 <= DirtyCPUSchedulersOnline <= N where N is the lesser of the return values of erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(schedulers_online).

Returns the old value of the flag.

Note that the number of dirty CPU schedulers online may change if the number of schedulers online changes. For example, if there are 12 schedulers and all are online, and 6 dirty CPU schedulers, all online as well, and system_flag/2 is used to set the number of schedulers online to 6, then the number of dirty CPU schedulers online is automatically decreased by half as well, down to 3. Similarly, the number of dirty CPU schedulers online increases proportionally to increases in the number of schedulers online.

Note that the dirty schedulers functionality is experimental, and that you have to enable support for dirty schedulers when building OTP in order to try out the functionality.

For more information see erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(dirty_cpu_schedulers_online).

Types:

Number = OldNumber = integer() >= 0

Number is a non-negative integer which indicates how many times generational garbage collections can be done without forcing a fullsweep collection. The value applies to new processes; processes already running are not affected.

Returns the old value of the flag.

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.

Types:

MinHeapSize = OldMinHeapSize = integer() >= 0

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.

Returns the old value of the flag.

Types:

MinBinVHeapSize = OldMinBinVHeapSize = integer() >= 0

Sets the default minimum binary virtual heap size for processes. The size is given in words. The new min_bin_vhheap_size only effects processes spawned after the change of min_bin_vhheap_size has been made. The min_bin_vheap_size can be set for individual processes by use of spawn_opt/N or process_flag/2.

Returns the old value of the flag.

Types:

BlockState = block | unblock

OldBlockState = block | unblock | enabled

If multi-scheduling is enabled, more than one scheduler thread is used by the emulator. Multi-scheduling can be blocked. When multi-scheduling has been blocked, only one scheduler thread will schedule Erlang processes.

If BlockState =:= block, multi-scheduling will be blocked. If BlockState =:= unblock and no-one else is blocking multi-scheduling and this process has only blocked one time, multi-scheduling will be unblocked. One process can block multi-scheduling multiple times. If a process has blocked multiple times, it has to unblock exactly as many times as it has blocked before it has released its multi-scheduling block. If a process that has blocked multi-scheduling exits, it will release its blocking of multi-scheduling.

The return values are disabled, blocked, or enabled. The returned value describes the state just after the call to erlang:system_flag(multi_scheduling, BlockState) has been made. The return values are described in the documentation of erlang:system_info(multi_scheduling).

NOTE: Blocking of multi-scheduling should normally not be needed. If you feel that you need to block multi-scheduling, think through the problem at least a couple of times again. Blocking multi-scheduling should only be used as a last resort since it will most likely be a very inefficient way to solve the problem.

Types:

How = scheduler_bind_type() | default_bind

OldBindType = scheduler_bind_type()

Warning

This argument is deprecated and scheduled for removal in erts-5.10/OTP-R16. Instead of using this argument you are advised to use the erl command line argument +sbt. When this argument has been removed a final scheduler bind type to use will be determined at emulator boot time.

Controls if and how schedulers are bound to logical processors.

When erlang:system_flag(scheduler_bind_type, How) is called, an asynchronous signal is sent to all schedulers online which causes them to try to bind or unbind as requested. NOTE: If a scheduler fails to bind, this will often be silently ignored. This since it isn't always possible to verify valid logical processor identifiers. If an error is reported, it will be reported to the error_logger. If you want to verify that the schedulers actually have bound as requested, call erlang:system_info(scheduler_bindings).

Schedulers can currently only be bound on newer Linux, Solaris, FreeBSD, and Windows systems, but more systems will be supported in the future.

In order for the runtime system to be able to bind schedulers, the CPU topology needs to be known. If the runtime system fails to automatically detect the CPU topology, it can be defined. For more information on how to define the CPU topology, see the erl +sct command line flag.

The runtime system will by default not bind schedulers to logical processors.

NOTE: If the Erlang runtime system is the only operating system process that binds threads to logical processors, this improves the performance of the runtime system. However, if other operating system processes (as for example another Erlang runtime system) also bind threads to logical processors, there might be a performance penalty instead. In some cases this performance penalty might be severe. If this is the case, you are advised to not bind the schedulers.

Schedulers can be bound in different ways. The How argument determines how schedulers are bound. How can currently be one of:

unbound: Same as the erl command line argument +sbt u.
no_spread: Same as the erl command line argument +sbt ns.
thread_spread: Same as the erl command line argument +sbt ts.
processor_spread: Same as the erl command line argument +sbt ps.
spread: Same as the erl command line argument +sbt s.
no_node_thread_spread: Same as the erl command line argument +sbt nnts.
no_node_processor_spread: Same as the erl command line argument +sbt nnps.
thread_no_node_processor_spread: Same as the erl command line argument +sbt tnnps.
default_bind: Same as the erl command line argument +sbt db.

The value returned equals How before the scheduler_bind_type flag was changed.

Failure:

notsup: If binding of schedulers is not supported.
badarg: If How isn't one of the documented alternatives.
badarg: If no CPU topology information is available.

The scheduler bind type can also be set by passing the +sbt command line argument to erl.

For more information, see erlang:system_info(scheduler_bind_type), erlang:system_info(scheduler_bindings), the erl +sbt and +sct command line flags.

Types:

Boolean = OldBoolean = boolean()

Turns on/off scheduler wall time measurements.

For more information see, erlang:statistics(scheduler_wall_time).

Types:

SchedulersOnline = OldSchedulersOnline = integer() >= 1

Sets the amount of schedulers online. Valid range is 1 <= SchedulersOnline <= erlang:system_info(schedulers).

Returns the old value of the flag.

Note that if the emulator was built with support for dirty schedulers, changing the number of schedulers online can also change the number of dirty CPU schedulers online. For example, if there are 12 schedulers and all are online, and 6 dirty CPU schedulers, all online as well, and system_flag/2 is used to set the number of schedulers online to 6, then the number of dirty CPU schedulers online is automatically decreased by half as well, down to 3. Similarly, the number of dirty CPU schedulers online increases proportionally to increases in the number of schedulers online.

For more information see, erlang:system_info(schedulers), and erlang:system_info(schedulers_online).

Types:

TCW = OldTCW = integer() >= 0

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.

Returns the old value of the flag.

Types:

Allocator = undefined | glibc

Version = [integer() >= 0]

Features = [atom()]

Settings =
[{Subsystem :: atom(),
[{Parameter :: atom(), Value :: term()}]}]

Alloc = atom()

Returns various information about the allocators of the current system (emulator) as specified by Item:

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}.

Explanation:

Allocator corresponds to the malloc() implementation used. If Allocator equals undefined, the malloc() implementation used could not be identified. Currently 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).

alloc_util_allocators

Returns a list of the names of all allocators using the ERTS internal alloc_util framework as atoms. For more information see the "the alloc_util framework" section in the erts_alloc(3) documentation.

{allocator, Alloc}

Returns information about the specified allocator. As of erts version 5.6.1 the return value is a list of {instance, InstanceNo, InstanceInfo} tuples where InstanceInfo contains information about a specific instance of the allocator. As of erts version 5.10.4 the returned list when calling erlang:system_info({allocator, mseg_alloc}) also include an {erts_mmap, _} tuple as one element in the list. 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). After reading the erts_alloc(3) documentation, 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, multi-block carriers, and single-block 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.

{allocator_sizes, Alloc}

Returns various size information for the specified allocator. The information returned is a subset of the information returned by erlang:system_info({allocator, Alloc}).

Types:

CpuTopology = cpu_topology()

cpu_topology() = [LevelEntry :: level_entry()] | undefined

All LevelEntrys of a list must contain the same LevelTag, except on the top level where both node and processor LevelTags may co-exist.

level_entry() = {LevelTag :: level_tag(),
                 SubLevel :: sub_level()}
              | {LevelTag :: level_tag(),
                 InfoList :: info_list(),
                 SubLevel :: sub_level()}

{LevelTag, SubLevel} == {LevelTag, [], SubLevel}

level_tag() = core | node | processor | thread

More LevelTags may be introduced in the future.

sub_level() = [LevelEntry :: level_entry()]
| (LogicalCpuId :: {logical, integer() >= 0})

info_list() = []

The info_list() may be extended in the future.

Returns various information about the CPU topology of the current system (emulator) as specified by Item:

cpu_topology

Returns the CpuTopology which currently is used by the emulator. The CPU topology is used when binding schedulers to logical processors. The CPU topology used is the user defined CPU topology if such exists; otherwise, the automatically detected CPU topology if such exists. If no CPU topology exists, undefined is returned.

node refers to NUMA (non-uniform memory access) nodes, and thread refers to hardware threads (e.g. Intels hyper-threads).

A level in the CpuTopology term can be omitted if only one entry exists and the InfoList is empty.

thread can only be a sub level to core. core can be a sub level to either processor or node. processor can either be on the top level or a sub level to node. node can either be on the top level or a sub level to processor. That is, NUMA nodes can be processor internal or processor external. A CPU topology can consist of a mix of processor internal and external NUMA nodes, as long as each logical CPU belongs to one and only one NUMA node. Cache hierarchy is not part of the CpuTopology type yet, but will be in the future. Other things may also make it into the CPU topology in the future. In other words, expect the CpuTopology type to change.

{cpu_topology, defined}

Returns the user defined CpuTopology. For more information see the documentation of the erl +sct command line flag, and the documentation of the cpu_topology argument.

{cpu_topology, detected}

Returns the automatically detected CpuTopology. The emulator currently only detects the CPU topology on some newer Linux, Solaris, FreeBSD, and Windows systems. On Windows system with more than 32 logical processors the CPU topology is not detected.

For more information see the documentation of the cpu_topology argument.

{cpu_topology, used}

Returns the CpuTopology which is used by the emulator. For more information see the documentation of the cpu_topology argument.

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

allocated_areas, allocator, alloc_util_allocators, allocator_sizes

See above.

build_type

Returns an atom describing the build type of the runtime system. This is normally the atom opt for optimized. Other possible return values are debug, purify, quantify, purecov, gcov, valgrind, gprof, and lcnt. Possible return values may be added and/or removed at any time without prior notice.

c_compiler_used

Returns a two-tuple describing the C compiler used when compiling the runtime system. The first element is an atom describing the name of the compiler, or undefined if unknown. The second element is a term describing the version of the compiler, or undefined if unknown.

check_io

Returns a list containing miscellaneous information regarding the emulators internal I/O checking. Note, the content of the returned list may vary between platforms and over time. The only thing guaranteed is that a list is returned.

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).

cpu_topology

See above.

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.

debug_compiled

Returns true if the emulator has been debug compiled; otherwise, false.

dirty_cpu_schedulers

Returns the number of dirty CPU scheduler threads used by the emulator. Dirty CPU schedulers execute CPU-bound native functions such as NIFs, linked-in driver code, and BIFs that cannot be managed cleanly by the emulator's normal schedulers.

The number of dirty CPU scheduler threads is determined at emulator boot time and cannot be changed after that. The number of dirty CPU scheduler threads online can however be changed at any time. The number of dirty CPU schedulers can be set on startup by passing the +SDcpu or +SDPcpu command line flags, see erl(1).

Note that the dirty schedulers functionality is experimental, and that you have to enable support for dirty schedulers when building OTP in order to try out the functionality.

dirty_cpu_schedulers_online

Returns the number of dirty CPU schedulers online. The return value satisfies the following relationship: 1 <= DirtyCPUSchedulersOnline <= N, where N is the lesser of the return values of erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(schedulers_online).

The number of dirty CPU schedulers online can be set on startup by passing the +SDcpu command line flag, see erl(1).

Note that the dirty schedulers functionality is experimental, and that you have to enable support for dirty schedulers when building OTP in order to try out the functionality.

For more information, see erlang:system_info(dirty_cpu_schedulers), erlang:system_info(dirty_io_schedulers), erlang:system_info(schedulers_online), and erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).

dirty_io_schedulers

Returns the number of dirty I/O schedulers as an integer. Dirty I/O schedulers execute I/O-bound native functions such as NIFs and linked-in driver code that cannot be managed cleanly by the emulator's normal schedulers.

This value can be set on startup by passing the +SDio command line flag, see erl(1).

Note that the dirty schedulers functionality is experimental, and that you have to enable support for dirty schedulers when building OTP in order to try out the functionality.

For more information, see erlang:system_info(dirty_cpu_schedulers), erlang:system_info(dirty_cpu_schedulers_online), and erlang:system_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).

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_buf_busy_limit

Returns the value of the distribution buffer busy limit in bytes. This limit can be set on startup by passing the +zdbbl command line flag to erl.

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.

driver_version

Returns a string containing the erlang driver version used by the runtime system. It will be on the form "<major ver>.<minor ver>".

dynamic_trace

Returns an atom describing the dynamic trace framework compiled into the virtual machine. It can currently be either dtrace, systemtap or none. For a commercial or standard build, this is always none, the other return values indicate a custom configuration (e.g. ./configure --with-dynamic-trace=dtrace). See the dyntrace manual page and the README.dtrace/README.systemtap files in the Erlang source code top directory for more information about dynamic tracing.

dynamic_trace_probes

Returns a boolean() indicating if dynamic trace probes (either dtrace or systemtap) are built into the emulator. This can only be true if the virtual machine was built for dynamic tracing (i.e. system_info(dynamic_trace) returns dtrace or systemtap).

elib_malloc

This option will be removed in a future release. The return value will always be false since the elib_malloc allocator has been removed.

ets_limit

Returns the maximum number of ETS tables allowed. This limit can be increased on startup by passing the +e command line flag to erl or by setting the environment variable ERL_MAX_ETS_TABLES before starting the Erlang runtime system.

fullsweep_after

Returns {fullsweep_after, integer() >= 0} 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.

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 only the following heap type exists:

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.

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.

kernel_poll

Returns true if the emulator uses some kind of kernel-poll implementation; otherwise, false.

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.

logical_processors

Returns the detected number of logical processors configured on the system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors configured.

logical_processors_available

Returns the detected number of logical processors available to the Erlang runtime system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors available. The number of logical processors available is less than or equal to the number of logical processors online.

logical_processors_online

Returns the detected number of logical processors online on the system. The return value is either an integer, or the atom unknown if the emulator wasn't able to detect logical processors online. The number of logical processors online is less than or equal to the number of logical processors configured.

machine

Returns a string containing the Erlang machine name.

min_heap_size

Returns {min_heap_size, MinHeapSize} where MinHeapSize is the current system wide minimum heap size for spawned processes.

min_bin_vheap_size

Returns {min_bin_vheap_size, MinBinVHeapSize} where MinBinVHeapSize is the current system wide minimum binary virtual heap size for spawned processes.

modified_timing_level

Returns the modified timing level (an integer) if modified timing has been enabled; otherwise, undefined. See the +T command line flag in the documentation of the erl(1) command for more information on modified timing.

multi_scheduling

Returns disabled, blocked, or enabled. A description of the return values:

disabled: The emulator has only one scheduler thread. The emulator does not have SMP support, or have been started with only one scheduler thread.
blocked: The emulator has more than one scheduler thread, but all scheduler threads but one have been blocked, i.e., only one scheduler thread will schedule Erlang processes and execute Erlang code.
enabled: The emulator has more than one scheduler thread, and no scheduler threads have been blocked, i.e., all available scheduler threads will schedule Erlang processes and execute Erlang code.

multi_scheduling_blockers

Returns a list of PIDs when multi-scheduling is blocked; otherwise, the empty list. The PIDs in the list is PIDs of the processes currently blocking multi-scheduling. A PID will only be present once in the list, even if the corresponding process has blocked multiple times.

otp_release

Returns a string containing the OTP release number of the OTP release that the currently executing ERTS application is part of.

As of OTP release 17, the OTP release number corresponds to the major OTP version number. There is no erlang:system_info() argument giving the exact OTP version. This since the exact OTP version in the general case is hard to determine. For more information see the documentation of versions in the system principles guide.

port_parallelism

Returns the default port parallelism scheduling hint used. For more information see the +spp command line argument of erl(1).

port_count

Returns the number of ports currently existing at the local node as an integer. The same value as length(erlang:ports()) returns, but more efficient.

port_limit

Returns the maximum number of simultaneously existing ports at the local node as an integer. This limit can be configured at startup by using the +Q command line flag of erl(1).

process_count

Returns the number of processes currently existing at the local node as an integer. The same value as length(processes()) returns, but more efficient.

process_limit

Returns the maximum number of simultaneously existing processes at the local node as an integer. This limit can be configured at startup by using the +P command line flag of 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_bind_type

Returns information on how user has requested schedulers to be bound or not bound.

NOTE: Even though user has requested schedulers to be bound, they might have silently failed to bind. In order to inspect actual scheduler bindings call erlang:system_info(scheduler_bindings).

For more information, see the erl +sbt command line argument, and erlang:system_info(scheduler_bindings).

scheduler_bindings

Returns information on currently used scheduler bindings.

A tuple of a size equal to erlang:system_info(schedulers) is returned. The elements of the tuple are integers or the atom unbound. Logical processor identifiers are represented as integers. The Nth element of the tuple equals the current binding for the scheduler with the scheduler identifier equal to N. E.g., if the schedulers have been bound, element(erlang:system_info(scheduler_id), erlang:system_info(scheduler_bindings)) will return the identifier of the logical processor that the calling process is executing on.

Note that only schedulers online can be bound to logical processors.

For more information, see the erl +sbt command line argument, erlang:system_info(schedulers_online).

scheduler_id

Returns the scheduler id (SchedulerId) of the scheduler thread that the calling process is executing on. SchedulerId is a positive integer; where 1 <= SchedulerId <= erlang:system_info(schedulers). See also erlang:system_info(schedulers).

schedulers

Returns the number of scheduler threads used by the emulator. Scheduler threads online schedules Erlang processes and Erlang ports, and execute Erlang code and Erlang linked in driver code.

The number of scheduler threads is determined at emulator boot time and cannot be changed after that. The amount of schedulers online can however be changed at any time.

schedulers_online

Returns the amount of schedulers online. The scheduler identifiers of schedulers online satisfy the following relationship: 1 <= SchedulerId <= erlang:system_info(schedulers_online).

For more information, see erlang:system_info(schedulers), and erlang:system_flag(schedulers_online, SchedulersOnline).

erlang:system_info(Item :: schedulers | schedulers_online) ->
integer() >= 1

smp_support

Returns true if the emulator has been compiled with smp support; otherwise, false.

system_version

Returns a string containing version number and some important properties such as the number of schedulers.

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 async threads in the async thread pool used for asynchronous driver calls (driver_async()) 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.

update_cpu_info

The runtime system rereads the CPU information available and updates its internally stored information about the detected CPU topology and the amount of logical processors configured, online, and available. If the CPU information has changed since the last time it was read, the atom changed is returned; otherwise, the atom unchanged is returned. If the CPU information has changed you probably want to adjust the amount of schedulers online. You typically want to have as many schedulers online as logical processors available.

version

Returns a string containing the version number of the emulator.

wordsize

Same as {wordsize, internal}.

{wordsize, internal}

Returns the size of Erlang term words in bytes as an integer, i.e. on a 32-bit architecture 4 is returned, and on a pure 64-bit architecture 8 is returned. On a halfword 64-bit emulator, 4 is returned, as the Erlang terms are stored using a virtual wordsize of half the system's wordsize.

{wordsize, external}

Returns the true wordsize of the emulator, i.e. the size of a pointer, in bytes as an integer. On a pure 32-bit architecture 4 is returned, on both a halfword and pure 64-bit architecture, 8 is returned.

Note

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.

Types:

MonSettings = undefined | {MonitorPid, Options}

MonitorPid = pid()

Options = [system_monitor_option()]

system_monitor_option() = busy_port
                        | busy_dist_port
                        | {long_gc, integer() >= 0}
                        | {long_schedule, integer() >= 0}
                        | {large_heap, integer() >= 0}

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.

Types:

Arg = MonSettings = undefined | {MonitorPid, Options}

MonitorPid = pid()

Options = [system_monitor_option()]

system_monitor_option() = busy_port
                        | busy_dist_port
                        | {long_gc, integer() >= 0}
                        | {long_schedule, integer() >= 0}
                        | {large_heap, integer() >= 0}

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.

Types:

MonitorPid = pid()

Options = [system_monitor_option()]

MonSettings = undefined | {OldMonitorPid, OldOptions}

OldMonitorPid = pid()

OldOptions = [system_monitor_option()]

system_monitor_option() = busy_port
                        | busy_dist_port
                        | {long_gc, integer() >= 0}
                        | {long_schedule, integer() >= 0}
                        | {large_heap, integer() >= 0}

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 tuples are tagged with heap_size, heap_block_size, stack_size, mbuf_size, old_heap_size, and old_heap_block_size. These tuples are explained in the documentation of the gc_start trace message (see erlang:trace/3). New tuples may be added, and the order of the tuples in the Info list may be changed at any time without prior notice.

{long_schedule, Time}

If a process or port in the system runs uninterrupted for at least Time wall clock milliseconds, a message {monitor, PidOrPort, long_schedule, Info} is sent to MonitorPid. PidOrPort is the process or port that was running and Info is a list of two-element tuples describing the event. In case of a pid(), the tuples {timeout, Millis}, {in, Location} and {out, Location} will be present, where Location is either an MFA ({Module, Function, Arity}) describing the function where the process was scheduled in/out, or the atom undefined. In case of a port(), the tuples {timeout, Millis} and {port_op,Op} will be present. Op will be one of proc_sig, timeout, input, output, event or dist_cmd, depending on which driver callback was executing. proc_sig is an internal operation and should never appear, while the others represent the corresponding driver callbacks timeout, ready_input, ready_output, event and finally outputv (when the port is used by distribution). The Millis value in the timeout tuple will tell you the actual uninterrupted execution time of the process or port, which will always be >= the Time value supplied when starting the trace. New tuples may be added to the Info list in the future, and the order of the tuples in the list may be changed at any time without prior notice.

This can be used to detect problems with NIF's or drivers that take too long to execute. Generally, 1 ms is considered a good maximum time for a driver callback or a NIF. However, a time sharing system should usually consider everything below 100 ms as "possible" and fairly "normal". Schedule times above that might however indicate swapping or a NIF/driver that is misbehaving. Misbehaving NIF's and drivers could cause bad resource utilization and bad overall performance of the system.

{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. Note: As of erts version 5.6 the monitor message is sent if the sum of the sizes of all memory blocks allocated for all heap generations is equal to or larger than Size. Previously the monitor message was sent if the memory block allocated for the youngest generation was equal to or larger than Size.

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.

Note

If a monitoring process gets so large that it itself starts to cause system monitor messages when garbage collecting, the messages will enlarge 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 or is not a local process.

Types:

ProfilerSettings = undefined | {ProfilerPid, Options}

ProfilerPid = pid() | port()

Options = [system_profile_option()]

system_profile_option() = exclusive
                        | runnable_ports
                        | runnable_procs
                        | scheduler

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

Types:

ProfilerPid = pid() | port() | undefined

Options = [system_profile_option()]

ProfilerSettings = undefined
| {pid() | port(), [system_profile_option()]}

system_profile_option() = exclusive
                        | runnable_ports
                        | runnable_procs
                        | scheduler

Sets system profiler options. ProfilerPid is a local pid or port that will receive profiling messages. The receiver is excluded from all profiling. The second argument is a list of profiling options:

exclusive: If a synchronous call to a port from a process is done, the calling process is considered not runnable during the call runtime to the port. The calling process is notified as inactive and subsequently active when the port callback returns.
runnable_procs: If a process is put into or removed from the run queue a message, {profile, Pid, State, Mfa, Ts}, is sent to ProfilerPid. Running processes that is reinserted into the run queue after having been preemptively scheduled out will not trigger this message.
runnable_ports: If a port is put into or removed from the run queue a message, {profile, Port, State, 0, Ts}, is sent to ProfilerPid.
scheduler: If a scheduler is put to sleep or awoken a message, {profile, scheduler, Id, State, NoScheds, Ts}, is sent to ProfilerPid.

Note

erlang:system_profile is considered experimental and its behaviour may change in the future.

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.

erlang

MODULE

MODULE SUMMARY

DESCRIPTION

DATA TYPES

EXPORTS