Erlang -- erlang

erlang

The Erlang BIFs.

By convention, most Built-In Functions (BIFs) are seen as being in this module. Some 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. For example, the calls atom_to_list(Erlang) and erlang:atom_to_list(Erlang) are identical.

Auto-imported BIFs are listed without module prefix. BIFs listed with module prefix are not auto-imported.

BIFs can fail for various reasons. All BIFs fail with reason badarg if they are called with arguments of an incorrect type. The other reasons are described in the description of each individual BIF.

Some BIFs can be used in guard tests and are marked with "Allowed in guard tests".

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

See erlang:timestamp/0.

Supported time unit representations:

PartsPerSecond :: integer() >= 1

Time unit expressed in parts per second. That is, the time unit equals 1/PartsPerSecond second.

seconds

Symbolic representation of the time unit represented by the integer 1.

milli_seconds

Symbolic representation of the time unit represented by the integer 1000.

micro_seconds

Symbolic representation of the time unit represented by the integer 1000000.

nano_seconds

Symbolic representation of the time unit represented by the integer 1000000000.

native

Symbolic representation of the native time unit used by the Erlang runtime system.

The native time unit is determined at runtime system start, and remains the same until the runtime system terminates. If a runtime system is stopped and then started again (even on the same machine), the native time unit of the new runtime system instance can differ from the native time unit of the old runtime system instance.

One can get an approximation of the native time unit by calling erlang:convert_time_unit(1, seconds, native). The result equals the number of whole native time units per second. In case the number of native time units per second does not add up to a whole number, the result is rounded downwards.

Note

The value of the native time unit gives you more or less no information at all about the quality of time values. It sets a limit for the resolution as well as for the precision of time values, but it gives absolutely no information at all about the accuracy of time values. The resolution of the native time unit and the resolution of time values can differ significantly.

The time_unit/0 type may be extended. Use erlang:convert_time_unit/3 in order to convert time values between time units.

Types:

Int = integer()

Types:

Float = float()

Int = integer()

Returns an integer or float that is the arithmetical absolute value of Float or Int, for example:

> 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()

Continues 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).

assigns the same value to Y as this:

        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).

assigns the same value to Z as this:

        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 that 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.

Example:

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

Types:

Fun = function()

Args = [term()]

Calls a fun, passing the elements in Args as arguments.

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 use is deprecated and will stop working in a future release.

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.

Example:

> apply(lists, reverse, [[a, b, c]]).
[c,b,a]
> apply(erlang, atom_to_list, ['Erlang']).
"Erlang"

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 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 corresponding to the text representation of Atom. If Encoding is latin1, there is one byte for each character in the text representation. If Encoding is utf8 or unicode, the characters are encoded using UTF-8 (that is, characters from 128 through 255 are encoded in two bytes).

Note

atom_to_binary(Atom, latin1) never fails because the text representation of an atom can only contain characters from 0 through 255. In a future release, the text representation of atoms can be allowed to contain any Unicode character and atom_to_binary(Atom, latin1) will then fail if the text representation for Atom contains a Unicode character greater than 255.

Example:

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

Types:

Atom = atom()

Returns a string corresponding to the text representation of Atom, for example:

> 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, for example:

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>>

Failure: badarg if PosLen in any way references outside the binary.

Start is zero-based, that is:

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

For details about the PosLen semantics, see the binary manual page in STDLIB.

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. Only Unicode characters up to 255 are allowed.

Note

binary_to_atom(Binary, utf8) fails if the binary contains Unicode characters greater than 255. In a future release, such Unicode characters can be allowed and binary_to_atom(Binary, utf8) does then not fail. For more information on Unicode support in atoms, see the note on UTF-8 encoded atoms in Section "External Term Format" in the User's Guide.

Examples:

> 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

As binary_to_atom/2, but the atom must exist.

Failure: badarg if the atom does not exist.

Types:

Binary = binary()

Returns the float whose text representation is Binary, for example:

> 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, for example:

> 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, for example:

> 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 corresponding 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. The positions in the binary are numbered starting from 1.

Note

The one-based indexing for binaries used by this function is deprecated. New code is to use binary:bin_to_list/3 in STDLIB instead. All functions in module binary consistently use zero-based indexing.

Types:

Bitstring = bitstring()

Returns a list of integers corresponding to the bytes of Bitstring. If the number of bits in the binary is not divisible by 8, the last element of the list is a bitstring containing the remaining 1-7 bits.

Types:

Binary = ext_binary()

Returns an Erlang term that is the result of decoding 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 can be used to attack the Erlang system. In the event of receiving unsafe data, decoding fails with a badarg error.

This prevents creation of new atoms directly, creation of new atoms indirectly (as they are embedded in certain structures, such as process identifiers, refs, and funs), and creation of new external function references. None of those resources are 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 that is the size in bits of Bitstring, for example:

> 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. A context switch is forced when the counter reaches the maximum number of reductions for a process (2000 reductions in OTP R12B).

Warning

This BIF can 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 that 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 is rounded up.

Examples:

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

Allowed in guard tests.

Types:

TimerRef = reference()

Async = Info = boolean()

Option = {async, Async} | {info, Info}

Options = [Option]

Time = integer() >= 0

Result = Time | false

Cancels a timer that has been created by erlang:start_timer(), or erlang:send_after(). TimerRef identifies the timer, and was returned by the BIF that created the timer.

Available Options:

{async, Async}

Asynchronous request for cancellation. Async defaults to false which will cause the cancellation to be performed synchronously. When Async is set to true, the cancel operation is performed asynchronously. That is, erlang:cancel_timer() will send an asynchronous request for cancellation to the timer service that manages the timer, and then return ok.

{info, Info}

Request information about the Result of the cancellation. Info defaults to true which means the Result is given. When Info is set to false, no information about the result of the cancellation is given. When the operation is performed

synchronously: If Info is true, the Result is returned by erlang:cancel_timer(); otherwise, ok is returned.
asynchronously: If Info is true, a message on the form {cancel_timer, TimerRef, Result} is sent to the caller of erlang:cancel_timer() when the cancellation operation has been performed; otherwise, no message is sent.

More Options may be added in the future.

If Result is an integer, it represents the time in milli-seconds left until the canceled timer would have expired.

If Result is false, a timer corresponding to TimerRef could not be found. This can be either because the timer had expired, already had been canceled, or because TimerRef never corresponded to a timer. Even if the timer had expired, it does not tell you whether or not the timeout message has arrived at its destination yet.

Note

The timer service that manages the timer may be co-located with another scheduler than the scheduler that the calling process is executing on. If this is the case, communication with the timer service takes much longer time than if it is located locally. If the calling process is in critical path, and can do other things while waiting for the result of this operation, or is not interested in the result of the operation, you want to use option {async, true}. If using option {async, false}, the calling process blocks until the operation has been performed.

Types:

TimerRef = reference()

Time = integer() >= 0

Result = Time | false

Cancels a timer. The same as calling erlang:cancel_timer(TimerRef, []).

Types:

Module = module()

Returns true if Module has old code, otherwise false.

See also exit/2.

Types:

Module = OldModule = atom()

Used by a process to redefine the error handler for undefined function calls and undefined registered processes. Inexperienced users are not to use this flag, as code auto-loading depends on the correct operation of the error handling module.

Returns the old value of the flag.

Types:

MinHeapSize = OldMinHeapSize = integer() >= 0

Changes the minimum heap size for the calling process.

Returns the old value of the flag.

Types:

MinBinVHeapSize = OldMinBinVHeapSize = integer() >= 0

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

Sets the process priority. Level is an atom. There are four priority levels: low, normal, high, and max. Default is normal.

Note

Priority level max is reserved for internal use in the Erlang runtime system, and is not to be used by others.

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

Execution of processes on priority normal and low are interleaved. Processes on priority low are 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 are selected for execution. Notice however, that this does not mean that no processes on priority low or normal can run when there are processes running on priority high. On the runtime system with SMP support, more processes can be running in parallel than processes on priority high, that is, a low and a high priority process can execute at the same time.

When there are runnable processes on priority max, no processes on priority low, normal, or high are selected for execution. As with priority high, processes on lower priorities can 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 number of reductions since the last time it was selected for execution.

Note

Do not depend on the scheduling to remain exactly as it is today. Scheduling, at least on the runtime system with SMP support, is likely to be changed in a future release to use available processor cores better.

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

Making calls from a high priority process into code that you have no control over can cause the high priority process to wait for a process with lower priority. That is, effectively decreasing the priority of the high priority process during the call. Even if this is not the case with one version of the code that you have no control over, it can be the case in a future version of it. This can, for example, occur if a high priority process triggers code loading, as the code server runs on priority normal.

Other priorities than normal are normally not needed. When other priorities are used, use them with care, especially priority high. A process on priority high is only to perform work for short periods. Busy looping for long periods in a high priority process does most likely cause problems, as important OTP servers run 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 is greater than 0, call saving is made active for the process. This 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, as follows:

A tuple {Module, Function, Arity} for function calls
The 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()

Sets or clears flag sensitive for the current process. When a process has been marked as sensitive by calling process_flag(sensitive, true), features in the runtime system that can be used for examining the data 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 are generated. (If flag sensitive is turned off, trace messages are again generated if any trace flags are set.)
Sequential tracing: The sequential trace token is propagated as usual, but no sequential trace messages are generated.

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

Stack back-traces cannot be displayed for the process.

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

If {save_calls,N} has been set for the process, no function calls are saved to the call saving list. (The call saving list is not cleared. Furthermore, send, receive, and timeout events are still 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 valid 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 undefined and all InfoTuples are not mandatory. The InfoTuples part of the result can be changed without prior notice.

The 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
garbage_collection

If the process identified by Pid has a registered name, also an InfoTuple with item registered_name appears.

For information about specific InfoTuples, see process_info/2.

Warning

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

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 Item or ItemList. Returns 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 because of historical reasons, and is kept for backward compatibility.

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

Note

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

The following InfoTuples with corresponding Items are valid:

{backtrace, Bin}

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 can be changed or removed without prior notice.

{catchlevel, CatchLevel}

CatchLevel is the number of currently active catches in this process. This InfoTuple can 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 describing the location in the source code.

{current_stacktrace, Stack}

Returns 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 process dictionary.

{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 containing miscellaneous information about garbage collection for this process. The content of GCInfo can be changed without prior notice.

{group_leader, GroupLeader}

GroupLeader is group leader for the I/O of the process.

{heap_size, Size}

Size is the size in words of the youngest heap generation of the process. This generation includes the process stack. This information is highly implementation-dependent, and can change if the implementation changes.

{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 process identifiers 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 information item messages (see the following).

{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 process identifiers 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 a process identifier, the list item is {process, Pid}. 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 is the sequential trace token for the process. This InfoTuple can be changed or removed without prior notice.

{stack_size, Size}

Size is the stack size, in words, of the process.

{status, Status}

Status is the status of the process and is one of the following:

exiting
garbage_collecting
waiting (for a message)
running
runnable (ready to run, but another process is running)
suspended (suspended on a "busy" port or by the BIF erlang:suspend_process/[1,2])

{suspending, SuspendeeList}

SuspendeeList is a list of {Suspendee, ActiveSuspendCount, OutstandingSuspendCount} tuples. Suspendee is the process identifier of a process that has been, or is to be, suspended by the process identified by Pid through one of the following BIFs:

erlang:suspend_process/2
erlang:suspend_process/1

ActiveSuspendCount is the number of times 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.
If OutstandingSuspendCount =/= 0, option asynchronous of erlang:suspend_process/2 has been used and the suspendee has not yet been suspended by Pid.

Notice that 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 includes the process stack.

{trace, InternalTraceFlags}

InternalTraceFlags is an integer representing the internal trace flag for this process. This InfoTuple can be changed or removed without prior notice.

{trap_exit, Boolean}

Boolean is true if the process is trapping exits, otherwise false.

Notice that not all implementations support all these Items.

Failures:

badarg: If Pid is not a local process.
badarg: If Item is an invalid item.

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

Notice that an exiting process exists, but is not alive. That is, is_process_alive/1 returns false for an exiting process, but its process identifier is part of the result returned from processes/0.

Example:

> 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 is to be called to check that no processes execute old code in the module.

Warning

This BIF is intended for the code server (see code(3)) and is not to 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 exists, the old value is deleted and replaced by Val, and the function returns the old value.

Example:

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

Note

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

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. Avoid to use it in applications, unless you really know what you are doing.

Class is 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. Stacktrace is a list as returned from get_stacktrace(), that is, a list of four-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 can also contain {Fun, Args, Location} tuples, where Fun is a local fun and Args is an argument list.

Element Location 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 is truncated to the current maximum stacktrace depth.

Since 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 badarg. If you want to be sure not to return, you can call error(erlang:raise(Class, Reason, Stacktrace)) and hope to distinguish exceptions later.

Types:

TimerRef = reference()

Async = boolean()

Option = {async, Async}

Options = [Option]

Time = integer() >= 0

Result = Time | false

Read the state of a timer that has been created by either erlang:start_timer(), or erlang:send_after(). TimerRef identifies the timer, and was returned by the BIF that created the timer.

Available Options:

{async, Async}: Asynchronous request for state information. Async defaults to false which will cause the operation to be performed synchronously. In this case, the Result is returned by erlang:read_timer(). When Async is true, erlang:read_timer() sends an asynchronous request for the state information to the timer service that manages the timer, and then returns ok. A message on the format {read_timer, TimerRef, Result} is sent to the caller of erlang:read_timer() when the operation has been processed.

More Options may be added in the future.

If Result is an integer, it represents the time in milli-seconds left until the timer expires.

If Result is false, a timer corresponding to TimerRef could not be found. This can be because the timer had expired, it had been canceled, or because TimerRef never has corresponded to a timer. Even if the timer has expired, it does not tell you whether or not the timeout message has arrived at its destination yet.

Note

The timer service that manages the timer may be co-located with another scheduler than the scheduler that the calling process is executing on. If this is the case, communication with the timer service takes much longer time than if it is located locally. If the calling process is in critical path, and can do other things while waiting for the result of this operation, you want to use option {async, true}. If using option {async, false}, the calling process will be blocked until the operation has been performed.

Types:

TimerRef = reference()

Time = integer() >= 0

Result = Time | false

Read the state of a timer. The same as calling erlang:read_timer(TimerRef, []).

Types:

Ref = reference()

Returns a string corresponding to the text representation of Ref.

Warning

This BIF is intended for debugging and is not to be used in application programs.

Types:

RegName = atom()

PidOrPort = port() | pid()

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

Example:

> register(db, Pid).
true

Failures:

badarg: If PidOrPort is not an existing local process or port.
badarg: If RegName is already in use.
badarg: If the process or port is already registered (already has a name).
badarg: If RegName is the atom undefined.

Types:

RegName = atom()

Returns a list of names that have been registered using register/2, for example:

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

Types:

Suspendee = pid()

Decreases the suspend count on the process identified by Suspendee. Suspendee is previously to have been suspended through erlang:suspend_process/2 or erlang:suspend_process/1 by the process calling erlang:resume_process(Suspendee). When the suspend count on Suspendee reaches zero, Suspendee is resumed, that is, its state is changed from suspended into the state it had before it was suspended.

Warning

This BIF is intended for debugging only.

Failures:

badarg: If Suspendee is not 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, for example:

round(5.5).
6

Allowed in guard tests.

Returns the process identifier of the calling process, for example:

> 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 can be a remote or local process identifier, 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()}

Either sends a message and returns ok, or does not send the message but returns something else (see the following). Otherwise the same as erlang:send/2. For more detailed explanation and warnings, see erlang:send_nosuspend/2,3.

The options are as follows:

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 to do the send, noconnect is returned instead.

Warning

As with erlang:send_nosuspend/2,3: use with extreme care.

Types:

Time = integer()

Dest = pid() | atom()

Msg = term()

Options = [Option]

Abs = boolean()

Option = {abs, Abs}

TimerRef = reference()

Starts a timer. When the timer expires, the message Msg is sent to the process identified by Dest. Apart from the format of the timeout message, erlang:send_after/4 works exactly as erlang:start_timer/4.

Types:

Time = integer() >= 0

Dest = pid() | atom()

Msg = term()

TimerRef = reference()

Starts a timer. The same as calling erlang:send_after(Time, Dest, Msg, []).

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 to 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 does not send the message and returns false.

The same occurs 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 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 is shut down (because of tick time-outs) by net_kernel. The normal reaction to take when this occurs is some kind of premature shutdown of the other node.

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

In many systems, transient states of overloaded queues are normal. The fact that this function returns false does not 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 can be sent on the (TCP) channel without blocking, the message is not guaranteed to arrive at the remote node. For a disconnected non-responsive node, the return value is true (mimics the behavior of operator !). The expected behavior and 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 a Boolean return value.

This function behaves like erlang:send_nosuspend/2, but takes a third parameter, a list of options. The only option is noconnect, which makes the function return false if the remote node is not currently reachable by the local node. The normal behavior is to try to connect to the node, which can stall the process during a short period. The use of option noconnect makes it possible to be sure not to get the slightest delay when sending to a remote process. This is especially useful when communicating with nodes that expect to always be the connecting part (that is, 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 Section Distributed Erlang in the Erlang Reference Manual in System Documentation).

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 that is a copy of argument Tuple1 with the element given by integer argument Index (the first element is the element with index 1) replaced by argument Value, for example:

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

Types:

Item = tuple() | binary()

Returns the number of elements in a tuple or the number of bytes in a binary or bitstring, for example:

> size({morni, mulle, bwange}).
3
> size(<<11, 22, 33>>).
3

For bitstrings the number of whole bytes is returned. That is, if the number of bits in the bitstring is not divisible by 8, the resulting number of bytes is rounded down.

Allowed in guard tests.

See also tuple_size/1, byte_size/1 and bit_size/1.

Types:

Fun = function()

Returns the process identifier 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 process identifier 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 process identifier of a new process started by the application of Module:Function to Args.

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 and its replacement is undefined, a failure with reason undef occurs.

Example:

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

Types:

Node = node()

Module = module()

Function = atom()

Args = [term()]

Returns the process identifier (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/3.

Types:

Fun = function()

Returns the process identifier 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 process identifier (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 an exit signal with reason noconnection is sent to the calling process. Otherwise works like spawn/3.

Types:

Module = module()

Function = atom()

Args = [term()]

Returns the process identifier 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 process identifier (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 an exit signal with reason noconnection is sent to the calling process. Otherwise works like spawn/3.

Types:

Fun = function()

Returns the process identifier of a new process, started by the application of Fun to the empty list [], and a 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. The process is monitored at the same time. Returns the process identifier 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 process identifier (pid) of a new process started by the application of Fun to the empty list []. Otherwise works like spawn_opt/4.

If option monitor is given, the newly created process is monitored, and both the pid and reference for the monitor is 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 process identifier (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 as spawn/3, except that an extra option list is given when creating the process.

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

The options are as follows:

link

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

monitor

Monitors the new process (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 is 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}

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves 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 is done.

Option fullsweep_after makes it possible to specify the maximum number of generational collections before forcing a fullsweep, even if there is room on the old heap. Setting the number to zero disables the general collection algorithm, that is, all live data is copied at every garbage collection.

A few cases when it can be useful to change fullsweep_after:

If binaries that are no longer used are to be thrown away as soon as possible. (Set Number to zero.)
A process that mostly have short-lived data is fullsweeped seldom or never, that is, the old heap contains mostly garbage. To ensure a fullsweep occasionally, set Number to a suitable value, such as 10 or 20.
In embedded systems with a limited amount of RAM and no virtual memory, you might want to preserve memory by setting Number to zero. (The value can be set globally, see erlang:system_flag/2.)

{min_heap_size, Size}

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves matters.

Gives a minimum heap size, in words. Setting this value higher than the system default can speed up some processes because less garbage collection is done. However, setting a too high value can waste memory and slow down the system because of worse data locality. Therefore, use this option only for fine-tuning an application and to measure the execution time with various Size values.

{min_bin_vheap_size, VSize}

Useful only for performance tuning. Do not use this option unless you know that there is problem with execution times or memory consumption, and ensure that the option improves matters.

Gives a minimum binary virtual heap size, in words. Setting this value higher than the system default can speed up some processes because less garbage collection is done. However, setting a too high value can waste memory. Therefore, 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 process identifier (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.

Note

Option monitor is not supported by spawn_opt/5.

Types:

Bin = binary()

Pos = integer() >= 0

0..byte_size(Bin)

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

Example:

> 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()

Dest = pid() | atom()

Msg = term()

Options = [Option]

Abs = boolean()

Option = {abs, Abs}

TimerRef = reference()

Starts a timer. When the timer expires, the message {timeout, TimerRef, Msg} is sent to the process identified by Dest.

Available Options:

{abs, false}: This is the default. It means the Time value is interpreted as a time in milli-seconds relative current Erlang monotonic time.
{abs, true}: Absolute Time value. The Time value is interpreted as an absolute Erlang monotonic time in milli-seconds.

More Options may be added in the future.

The absolute point in time, the timer is set to expire on, has to be in the interval [erlang:system_info(start_time), erlang:system_info(end_time)]. Further, if a relative time is specified, the Time value is not allowed to be negative.

If Dest is a pid(), it must be a pid() of a process created on the current runtime system instance. This process may or may not have terminated. If Dest is an atom(), it is interpreted as the name of a locally registered process. The process referred to by the name is looked up at the time of timer expiration. No error is given if the name does not refer to a process.

If Dest is a pid(), the timer is automatically canceled if the process referred to by the pid() is not alive, or if the process exits. This feature was introduced in ERTS version 5.4.11. Notice that timers are not automatically canceled when Dest is an atom().

Failure: badarg if the arguments do not satisfy the requirements specified here.

Types:

Time = integer() >= 0

Dest = pid() | atom()

Msg = term()

TimerRef = reference()

Starts a timer. The same as calling erlang:start_timer(Time, Dest, Msg, []).

Types:

ContextSwitches = integer() >= 0

Returns the total number of context switches since the system started.

Types:

Total_Exact_Reductions = Exact_Reductions_Since_Last_Call = integer() >= 0

Returns the number of exact reductions.

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

Returns information about garbage collection, for example:

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

This information can be invalid for some implementations.

Types:

Input = Output = integer() >= 0

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

Types:

Total_Reductions = Reductions_Since_Last_Call = integer() >= 0

Returns information about reductions, for example:

> statistics(reductions).
{2046,11}

Note

As from ERTS 5.5 (OTP 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).

Returns the total length of 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

Returns information about runtime, in milliseconds.

This is the sum of the runtime for all threads in the Erlang runtime system and can therefore be greater than the wall clock time.

Example:

> 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, and TotalTime is the total time duration since scheduler_wall_time activation. The time unit is undefined and can be subject to change between releases, OSs, and system restarts. scheduler_wall_time is only to 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 and is not scheduling (selecting) a process or port, that is:

Executing process code
Executing linked-in-driver or NIF code
Executing built-in-functions, or any other runtime handling
Garbage collecting
Handling any other memory management

Notice that a scheduler can also be busy even if the OS has scheduled out the scheduler thread.

Returns undefined if system flag scheduler_wall_time is turned off.

The list of scheduler information is unsorted and can 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 the user takes another snapshot and calculates scheduler-utilization per scheduler, for example:

> 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. To enable it, use erlang:system_flag(scheduler_wall_time, true).

Types:

Total_Wallclock_Time = Wallclock_Time_Since_Last_Call = integer() >= 0

Returns information about wall clock. 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 is not already in that 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 does not leave the suspended state until its suspend count reaches 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 are automatically decreased when the process terminates.

The options (Opts) are as follows:

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

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

Warning

This BIF is intended for debugging only.

Failures:

badarg: If Suspendee is not a process identifier.
badarg: If the process identified by Suspendee is the same 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 is not 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 system limit is higher than 2,000,000,000 suspends and will never be lower.

Types:

Suspendee = pid()

Suspends the process identified by Suspendee. The same as calling erlang:suspend_process(Suspendee, []).

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, use command-line argument +sct in erl(1).

When this argument is removed, a final CPU topology to use is determined at emulator boot time.

Sets the user-defined CpuTopology. The user-defined CPU topology overrides any automatically detected CPU topology. By passing undefined as CpuTopology, the system reverts 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 are sent a request to rebind according to the new CPU topology.

The user-defined CPU topology can also be set by passing command-line argument +sct to erl(1).

For information on type CpuTopology and more, see erlang:system_info(cpu_topology) as well as the command-line flags +sct and +sbt in erl(1).

Types:

DirtyCPUSchedulersOnline = OldDirtyCPUSchedulersOnline = integer() >= 1

Sets the number of dirty CPU schedulers online. Range is 1 <= DirtyCPUSchedulersOnline <= N, where N is the smallest of the return values of erlang:system_info(dirty_cpu_schedulers) and erlang:system_info(schedulers_online).

Returns the old value of the flag.

The number of dirty CPU schedulers online can change if the number of schedulers online changes. For example, if 12 schedulers and 6 dirty CPU schedulers are online, 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

The dirty schedulers functionality is experimental. Enable support for dirty schedulers when building OTP 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

Sets system flag fullsweep_after. Number is a non-negative integer indicating how many times generational garbage collections can be done without forcing a fullsweep collection. The value applies to new processes, while 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.

This value can also be set through (OS) 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 effects only processes spawned after the change of min_heap_size has been made. min_heap_size can be set for individual processes by using 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 effects only processes spawned after the change of min_bin_vhheap_size has been made. min_bin_vheap_size can be set for individual processes by using 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 is blocked, only one scheduler thread schedules Erlang processes.

If BlockState =:= block, multi-scheduling is blocked. If BlockState =:= unblock and no one else blocks multi-scheduling, and this process has blocked only once, multi-scheduling is unblocked.

One process can block multi-scheduling multiple times. If a process has blocked multiple times, it must 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 releases 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. For information about the return values, see erlang:system_info(multi_scheduling).

Note

Blocking of multi-scheduling is normally not needed. If you feel that you need to block multi-scheduling, consider it a few more times again. Blocking multi-scheduling is only to be used as a last resort, as it is most likely 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, use command-line argument +sbt in erl(1). When this argument is removed, a final scheduler bind type to use is 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, causing them to try to bind or unbind as requested.

Note

If a scheduler fails to bind, this is often silently ignored, as it is not always possible to verify valid logical processor identifiers. If an error is reported, it is reported to error_logger. To verify that the schedulers have bound as requested, call erlang:system_info(scheduler_bindings).

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

In order for the runtime system to be able to bind schedulers, the CPU topology must be known. If the runtime system fails to detect the CPU topology automatically, it can be defined. For more information on how to define the CPU topology, see command-line flag +sct in erl(1).

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

Note

If the Erlang runtime system is the only OS process binding threads to logical processors, this improves the performance of the runtime system. However, if other OS processes (for example, another Erlang runtime system) also bind threads to logical processors, there can be a performance penalty instead. Sometimes this performance penalty can be severe. If so, it is recommended to not bind the schedulers.

Schedulers can be bound in different ways. Argument How determines how schedulers are bound and can be any of the following:

unbound: Same as command-line argument +sbt u in erl(1).
no_spread: Same as command-line argument +sbt ns in erl(1).
thread_spread: Same as command-line argument +sbt ts in erl(1).
processor_spread: Same as command-line argument +sbt ps in erl(1).
spread: Same as command-line argument +sbt s in erl(1).
no_node_thread_spread: Same as command-line argument +sbt nnts in erl(1).
no_node_processor_spread: Same as command-line argument +sbt nnps in erl(1).
thread_no_node_processor_spread: Same as command-line argument +sbt tnnps in erl(1).
default_bind: Same as command-line argument +sbt db in erl(1).

The returned value equals How before flag scheduler_bind_type was changed.

Failures:

notsup: If binding of schedulers is not supported.
badarg: If How is not one of the documented alternatives.
badarg: If CPU topology information is unavailable.

The scheduler bind type can also be set by passing command-line argument +sbt to erl(1).

For more information, see erlang:system_info(scheduler_bind_type), erlang:system_info(scheduler_bindings), as well as command-line flags +sbt and +sct in erl(1).

Types:

Boolean = OldBoolean = boolean()

Turns on or off scheduler wall time measurements.

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

Types:

SchedulersOnline = OldSchedulersOnline = integer() >= 1

Sets the number of schedulers online. Range is 1 <= SchedulersOnline <= erlang:system_info(schedulers).

Returns the old value of the flag.

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 12 schedulers and 6 dirty CPU schedulers are online, 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 trace control word to TCW, which is to be an unsigned integer. For more information, see the function set_tcw in Section "Match Specifications in Erlang" in the User's Guide.

Returns the old value of the flag.

Types:

OldState = preliminary | final | volatile

Finalizes the time offset when single time warp mode is used. If another time warp mode is used, the time offset state is left unchanged.

Returns the old state identifier. That is:

If preliminary is returned, finalization was performed and the time offset is now final.
If final is returned, the time offset was already in the final state. This either because another erlang:system_flag(time_offset, finalize) call, or because no time warp mode is used.
If volatile is returned, the time offset cannot be finalized because multi time warp mode is used.

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 the type of memory as first element and the amount of allocated memory in bytes as second element. When information about allocated and used memory is present, also a third element is present, containing 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 therefore changes when needed without prior notice.

Notice that 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. For information about the total amount of memory allocated by the emulator, see erlang:memory/0,1.

allocator

Returns {Allocator, Version, Features, Settings, where:

Allocator corresponds to the malloc() implementation used. If Allocator equals undefined, the malloc() implementation used cannot be identified. 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 the allocation features used.
Settings is a list of subsystems, their configurable parameters, and used values. Settings can 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 Section "The alloc_util framework" in erts_alloc(3).

{allocator, Alloc}

Returns information about the specified allocator. As from ERTS 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 from ERTS 5.10.4, the returned list when calling erlang:system_info({allocator, mseg_alloc}) also includes 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.

Notice that the information returned is highly implementation-dependent and can be changed or removed at any time without prior notice. It was initially intended as a tool when developing new allocators, but as it can 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 more or less speaks 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 denote multi-block carriers, and single-block carriers, respectively. Sizes are presented in bytes. When a size is not presented, it is the amount of something. Sizes and amounts are often presented by three values:

The first is the current value.
The second is the maximum value since the last call to erlang:system_info({allocator, Alloc}).
The third is the 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 the memory pool size and the second value is the 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 can coexist.

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 can be introduced in a future release.

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

info_list() = []

The info_list() can be extended in a future release.

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

cpu_topology

Returns the CpuTopology currently 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 Non-Uniform Memory Access (NUMA) nodes. thread refers to hardware threads (for example, Intel hyper-threads).

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

thread can only be a sub level to core. core can be a sub level to processor or node. processor can be on the top level or a sub level to node. node can 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 NUMA node. Cache hierarchy is not part of the CpuTopology type, but will be in a future release. Other things can also make it into the CPU topology in a future release. In other words, expect the CpuTopology type to change.

{cpu_topology, defined}

Returns the user-defined CpuTopology. For more information, see command-line flag +sct in erl(1) and argument cpu_topology.

{cpu_topology, detected}

Returns the automatically detected CpuTopologyy. The emulator 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 argument cpu_topology.

{cpu_topology, used}

Returns CpuTopology used by the emulator. For more information, see argument cpu_topology.

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 can be added 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 about the emulators internal I/O checking. Notice that the content of the returned list can vary between platforms and over time. It is only guaranteed 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 that the current emulator has been set to be backward compatible with. The compatibility mode can be configured at startup by using command-line flag +R in 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. The valid creations are integers in the range 1..3, but this will probably change in a future release. If the node is not alive, 0 is returned.

debug_compiled

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

delayed_node_table_gc

Returns the amount of time in seconds garbage collection of an entry in a node table is delayed. This limit can be set on startup by passing the command line flag +zdntgc to erl. For more information see the documentation of the command line flag.

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 normal emulator schedulers.

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

Notice that the dirty schedulers functionality is experimental. Enable support for dirty schedulers when building OTP to try out the functionality.

dirty_cpu_schedulers_online

Returns the number of dirty CPU schedulers online. The return value satisfies 1 <= DirtyCPUSchedulersOnline <= N, where N is the smallest 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 at startup by passing command-line flag +SDcpu in erl(1).

Notice that the dirty schedulers functionality is experimental. Enable support for dirty schedulers when building OTP 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, which cannot be managed cleanly by the normal emulator schedulers.

This value can be set at startup by passing command-line argument +SDio in erl(1).

Notice that the dirty schedulers functionality is experimental. Enable support for dirty schedulers when building OTP 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 Section "How to interpret the Erlang crash dumps" in the User's Guide.

dist_buf_busy_limit

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

dist_ctrl

Returns a list of tuples {Node, ControllingEntity}, one entry for each connected remote node. Node is the node name and ControllingEntity is the port or process identifier responsible for the communication to that node. More specifically, ControllingEntity for nodes connected through TCP/IP (the normal case) is the socket used in communication with the specific node.

driver_version

Returns a string containing the Erlang driver version used by the runtime system. It has the form "<major ver>.<minor ver>".

dynamic_trace

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

dynamic_trace_probes

Returns a boolean() indicating if dynamic trace probes (dtrace or systemtap) are built into the emulator. This can only be true if the Virtual Machine was built for dynamic tracing (that is, system_info(dynamic_trace) returns dtrace or systemtap).

end_time

The last Erlang monotonic time in native time unit that can be represented internally in the current Erlang runtime system instance. The time between the start time and the end time is at least a quarter of a millennium.

elib_malloc

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

eager_check_io

Returns the value of the erl command line flag +secio which is either true or false. See the documentation of the command line flag for information about the different values.

ets_limit

Returns the maximum number of ETS tables allowed. This limit can be increased at startup by passing command-line flag +e to erl(1) or by setting 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 in the following.

garbage_collection

Returns a list describing the default garbage collection settings. A process spawned on the local node by a spawn or spawn_link uses these garbage collection settings. The default settings can be changed by using 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. One 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 Section "How to interpret the Erlang crash dumps" in the 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 Section "How to interpret the Erlang crash dumps" in the User's Guide.

logical_processors

Returns the detected number of logical processors configured in the system. The return value is either an integer, or the atom unknown if the emulator cannot detect the configured logical processors.

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 cannot detect the available logical processors. The number of available logical processors 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 cannot 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 is enabled, otherwise, undefined. For more information about modified timing, see command-line flag +T in erl(1)

multi_scheduling

Returns disabled, blocked, or enabled:

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 except one are blocked, that is, only one scheduler thread schedules Erlang processes and executes Erlang code.
enabled: The emulator has more than one scheduler thread, and no scheduler threads are blocked, that is, all available scheduler threads schedule Erlang processes and execute Erlang code.

multi_scheduling_blockers

Returns a list of Pids when multi-scheduling is blocked, otherwise the empty list is returned. The Pids in the list represent all the processes currently blocking multi-scheduling. A Pid occurs only once in the list, even if the corresponding process has blocked multiple times.

nif_version

Returns a string containing the version of the Erlang NIF interface used by the runtime system. It is on the form "<major ver>.<minor ver>".

otp_release

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

As from OTP 17, the OTP release number corresponds to the major OTP version number. No erlang:system_info() argument gives the exact OTP version. This is because the exact OTP version in the general case is difficult to determine. For more information, see the description of versions in System principles in System Documentation.

os_monotonic_time_source

Returns a list containing information about the source of OS monotonic time that is used by the runtime system.

If [] is returned, no OS monotonic time is available. The list contains two-tuples with Keys as first element, and Values as second element. The order of these tuples is undefined. The following tuples can be part of the list, but more tuples can be introduced in the future:

{function, Function}: Function is the name of the function used. This tuple always exist if OS monotonic time is available to the runtime system.
{clock_id, ClockId}: This tuple only exist if Function can be used with different clocks. ClockId corresponds to the clock identifier used when calling Function.
{resolution, OsMonotonicTimeResolution}: Highest possible resolution of current OS monotonic time source as parts per second. If no resolution information can be retrieved from the OS, OsMonotonicTimeResolution is set to the resolution of the time unit of Functions return value. That is, the actual resolution can be lower than OsMonotonicTimeResolution. Also note that the resolution does not say anything about the accuracy, and whether the precision do align with the resolution. You do, however, know that the precision is not better than OsMonotonicTimeResolution.
{extended, Extended}: Extended equals yes if the range of time values has been extended; otherwise, Extended equals no. The range needs to be extended if Function returns values that wrap fast. This typically is the case when the return value is a 32-bit value.
{parallel, Parallel}: Parallel equals yes if Function is called in parallel from multiple threads. If it is not called in parallel, because calls needs to be serialized, Parallel equals no.
{time, OsMonotonicTime}: OsMonotonicTime equals current OS monotonic time in native time unit.

os_system_time_source

Returns a list containing information about the source of OS system time that is used by the runtime system.

The list contains two-tuples with Keys as first element, and Values as second element. The order if these tuples is undefined. The following tuples can be part of the list, but more tuples can be introduced in the future:

{function, Function}: Function is the name of the funcion used.
{clock_id, ClockId}: This tuple only exist if Function can be used with different clocks. ClockId corresponds to the clock identifier used when calling Function.
{resolution, OsSystemTimeResolution}: Highest possible resolution of current OS system time source as parts per second. If no resolution information can be retrieved from the OS, OsSystemTimeResolution is set to the resolution of the time unit of Functions return value. That is, the actual resolution may be lower than OsSystemTimeResolution. Also note that the resolution does not say anything about the accuracy, and whether the precision do align with the resolution. You do, however, know that the precision is not better than OsSystemTimeResolution.
{parallel, Parallel}: Parallel equals yes if Function is called in parallel from multiple threads. If it is not called in parallel, because calls needs to be serialized, Parallel equals no.
{time, OsSystemTime}: OsSystemTime equals current OS system time in native time unit.

port_parallelism

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

port_count

Returns the number of ports currently existing at the local node. The value is given as an integer. This is the same value as returned by length(erlang:ports()), 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 command-line flag +Q in erl(1).

process_count

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

process_limit

Returns the maximum number of simultaneously existing processes at the local node. The value is given as an integer. This limit can be configured at startup by using command-line flag +P in erl(1).

procs

Returns a binary containing a string of process and port information formatted as in Erlang crash dumps. For more information, see Section "How to interpret the Erlang crash dumps" in the User's Guide.

scheduler_bind_type

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

Notice that even though a user has requested schedulers to be bound, they can silently have failed to bind. To inspect the scheduler bindings, call erlang:system_info(scheduler_bindings).

For more information, see command-line argument +sbt in erl(1) and erlang:system_info(scheduler_bindings).

scheduler_bindings

Returns information about the currently used scheduler bindings.

A tuple of a size equal to erlang:system_info(schedulers) is returned. The tuple elements 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. For example, if the schedulers are bound, element(erlang:system_info(scheduler_id), erlang:system_info(scheduler_bindings)) returns the identifier of the logical processor that the calling process is executing on.

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

For more information, see command-line argument +sbt in erl(1) and 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 later. However, the number of schedulers online can be changed at any time.

schedulers_online

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

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

smp_support

Returns true if the emulator has been compiled with SMP support, otherwise false is returned.

start_time

The Erlang monotonic time in native time unit at the time when current Erlang runtime system instance started. See also erlang:system_info(end_time).

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()). The value is given as an integer.

time_correction

Returns a boolean value indicating whether time correction is enabled or not.

time_offset

Returns the state of the time offset:

preliminary: The time offset is preliminary, and will be changed at a later time when being finalized. The preliminary time offset is used during the preliminary phase of the single time warp mode.
final: The time offset is final. This either because no time warp mode is used, or because the time offset have been finalized when single time warp mode is used.
volatile: The time offset is volatile. That is, it can change at any time. This is because multi time warp mode is used.

time_warp_mode

Returns a value identifying the time warp mode being used:

no_time_warp: The no time warp mode is used.
single_time_warp: The single time warp mode is used.
multi_time_warp: The multi time warp mode is used.

tolerant_timeofday

Returns whether a pre erts-7.0 backwards compatible compensation for sudden changes of system time is enabled or disabled. Such compensation is enabled when the time offset is final, and time correction is enabled.

trace_control_word

Returns the value of the node trace control word. For more information, see function get_tcw in Section Match Specifications in Erlang in the 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 number 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. If the CPU information has changed, you probably want to adjust the number 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, that is, 4 is returned on a 32-bit architecture, and 8 is returned on a pure 64-bit architecture. On a halfword 64-bit emulator, 4 is returned, as the Erlang terms are stored using a virtual word size of half the system word size.

{wordsize, external}

Returns the true word size of the emulator, that is, the size of a pointer. The value is given in bytes as an integer. On a pure 32-bit architecture, 4 is returned. On both a half word and on a pure 64-bit architecture, 8 is returned.

Note

Argument scheduler has changed name to scheduler_id to avoid mix up with argument schedulers. Argument scheduler was introduced in ERTS 5.5 and renamed in ERTS 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 can 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 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 the system performance monitoring options. MonitorPid is a local process identifier (pid) receiving system monitor messages. The second argument is a list of monitoring options:

{long_gc, Time}

If a garbage collection in the system takes at least Time wall clock milliseconds, a message {monitor, GcPid, long_gc, Info} is sent to MonitorPid. GcPid is the pid that was garbage collected. 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 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 description of trace message gc_start (see erlang:trace/3). New tuples can be added, and the order of the tuples in the Info list can 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. Info is a list of two-element tuples describing the event.

If a pid(), the tuples {timeout, Millis}, {in, Location}, and {out, Location} are present, where Location is either an MFA ({Module, Function, Arity}) describing the function where the process was scheduled in/out, or the atom undefined.

If a port(), the tuples {timeout, Millis} and {port_op,Op} are present. Op is 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 is never to appear, while the others represent the corresponding driver callbacks timeout, ready_input, ready_output, event, and outputv (when the port is used by distribution). Value Millis in the timeout tuple informs about the uninterrupted execution time of the process or port, which always is equal to or higher than the Time value supplied when starting the trace. New tuples can be added to the Info list in a future release. The order of the tuples in the list can be changed at any time without prior notice.

This can be used to detect problems with NIFs or drivers that take too long to execute. 1 ms is considered a good maximum time for a driver callback or a NIF. However, a time-sharing system is usually to consider everything below 100 ms as "possible" and fairly "normal". However, longer schedule times can indicate swapping or a misbehaving NIF/driver. Misbehaving NIFs and drivers can cause bad resource utilization and bad overall system performance.

{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 earlier, except that the tuple tagged with timeout is not present.

As of ERTS 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 higher than Size. Previously the monitor message was sent if the memory block allocated for the youngest generation was equal to or higher 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 enlarge the process message queue and probably make the problem worse.

Keep the monitoring process neat and do not set the system monitor limits too tight.

Failures:

badarg: If MonitorPid does not exist.
badarg: If MonitorPid 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 can 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 process identifier (pid) or port receiving 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 later 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 are reinserted into the run queue after having been preempted do 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 behavior can change in a future release.

Returns current Erlang system time in native time unit.

Calling erlang:system_time() is equivalent to: erlang:monotonic_time() + erlang:time_offset().

Note

This time is not a monotonically increasing time in the general case. For more information, see the documentation of time warp modes in the ERTS User's Guide.

Types:

Unit = time_unit()

Returns current Erlang system time converted into the Unit passed as argument.

Calling erlang:system_time(Unit) is equivalent to: erlang:convert_time_unit(erlang:system_time(), native, Unit).

Note

This time is not a monotonically increasing time in the general case. For more information, see the documentation of time warp modes in the ERTS User's Guide.

Types:

Term = term()

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

This can be used for various 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