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Erlang Run-Time System Application (ERTS)
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The Erlang Emulator


The erl program starts an Erlang runtime system. The exact details (for example, whether erl is a script or a program and which other programs it calls) are system-dependent.

Windows users probably wants to use the werl program instead, which runs in its own window with scrollbars and supports command-line editing. The erl program on Windows provides no line editing in its shell, and on Windows 95 there is no way to scroll back to text which has scrolled off the screen. The erl program must be used, however, in pipelines or if you want to redirect standard input or output.


Starts an Erlang runtime system.

The arguments can be divided into emulator flags, flags and plain arguments:

  • Any argument starting with the character + is interpreted as an emulator flag.

    As indicated by the name, emulator flags controls the behavior of the emulator.

  • Any argument starting with the character - (hyphen) is interpreted as a flag which should be passed to the Erlang part of the runtime system, more specifically to the init system process, see init(3).

    The init process itself interprets some of these flags, the init flags. It also stores any remaining flags, the user flags. The latter can be retrieved by calling init:get_argument/1.

    It can be noted that there are a small number of "-" flags which now actually are emulator flags, see the description below.

  • Plain arguments are not interpreted in any way. They are also stored by the init process and can be retrieved by calling init:get_plain_arguments/0. Plain arguments can occur before the first flag, or after a -- flag. Additionally, the flag -extra causes everything that follows to become plain arguments.


% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
(arnie@host)1> init:get_argument(sname).
(arnie@host)2> init:get_plain_arguments().

Here +W w and +R 9 are emulator flags. -s my_init is an init flag, interpreted by init. -sname arnie is a user flag, stored by init. It is read by Kernel and will cause the Erlang runtime system to become distributed. Finally, everything after -extra (that is, +bertie) is considered as plain arguments.

% erl -myflag 1
1> init:get_argument(myflag).
2> init:get_plain_arguments().

Here the user flag -myflag 1 is passed to and stored by the init process. It is a user defined flag, presumably used by some user defined application.


In the following list, init flags are marked (init flag). Unless otherwise specified, all other flags are user flags, for which the values can be retrieved by calling init:get_argument/1. Note that the list of user flags is not exhaustive, there may be additional, application specific flags which instead are documented in the corresponding application documentation.

--(init flag)

Everything following -- up to the next flag (-flag or +flag) is considered plain arguments and can be retrieved using init:get_plain_arguments/0.

-Application Par Val

Sets the application configuration parameter Par to the value Val for the application Application, see app(4) and application(3).

-args_file FileName

Command line arguments are read from the file FileName. The arguments read from the file replace the '-args_file FileName' flag on the resulting command line.

The file FileName should be a plain text file and may contain comments and command line arguments. A comment begins with a # character and continues until next end of line character. Backslash (\\) is used as quoting character. All command line arguments accepted by erl are allowed, also the -args_file FileName flag. Be careful not to cause circular dependencies between files containing the -args_file flag, though.

The -extra flag is treated specially. Its scope ends at the end of the file. Arguments following an -extra flag are moved on the command line into the -extra section, i.e. the end of the command line following after an -extra flag.


The initial Erlang shell does not read user input until the system boot procedure has been completed (Erlang 5.4 and later). This flag disables the start synchronization feature and lets the shell start in parallel with the rest of the system.

-boot File

Specifies the name of the boot file, File.boot, which is used to start the system. See init(3). Unless File contains an absolute path, the system searches for File.boot in the current and $ROOT/bin directories.

Defaults to $ROOT/bin/start.boot.

-boot_var Var Dir

If the boot script contains a path variable Var other than $ROOT, this variable is expanded to Dir. Used when applications are installed in another directory than $ROOT/lib, see systools:make_script/1,2.


Enables the code path cache of the code server, see code(3).

-compile Mod1 Mod2 ...

Compiles the specified modules and then terminates (with non-zero exit code if the compilation of some file did not succeed). Implies -noinput. Not recommended - use erlc instead.

-config Config

Specifies the name of a configuration file, Config.config, which is used to configure applications. See app(4) and application(3).

-connect_all false

If this flag is present, global will not maintain a fully connected network of distributed Erlang nodes, and then global name registration cannot be used. See global(3).

-cookie Cookie

Obsolete flag without any effect and common misspelling for -setcookie. Use -setcookie instead.


Starts the Erlang runtime system detached from the system console. Useful for running daemons and backgrounds processes.


Useful for debugging. Prints out the actual arguments sent to the emulator.

-env Variable Value

Sets the host OS environment variable Variable to the value Value for the Erlang runtime system. Example:

% erl -env DISPLAY gin:0

In this example, an Erlang runtime system is started with the DISPLAY environment variable set to gin:0.

-eval Expr(init flag)

Makes init evaluate the expression Expr, see init(3).

-extra(init flag)

Everything following -extra is considered plain arguments and can be retrieved using init:get_plain_arguments/0.


Starts heart beat monitoring of the Erlang runtime system. See heart(3).


Starts the Erlang runtime system as a hidden node, if it is run as a distributed node. Hidden nodes always establish hidden connections to all other nodes except for nodes in the same global group. Hidden connections are not published on neither of the connected nodes, i.e. neither of the connected nodes are part of the result from nodes/0 on the other node. See also hidden global groups, global_group(3).

-hosts Hosts

Specifies the IP addresses for the hosts on which Erlang boot servers are running, see erl_boot_server(3). This flag is mandatory if the -loader inet flag is present.

The IP addresses must be given in the standard form (four decimal numbers separated by periods, for example "". Hosts names are not acceptable, but a broadcast address (preferably limited to the local network) is.

-id Id

Specifies the identity of the Erlang runtime system. If it is run as a distributed node, Id must be identical to the name supplied together with the -sname or -name flag.


Makes init write some debug information while interpreting the boot script.

-instr(emulator flag)

Selects an instrumented Erlang runtime system (virtual machine) to run, instead of the ordinary one. When running an instrumented runtime system, some resource usage data can be obtained and analysed using the module instrument. Functionally, it behaves exactly like an ordinary Erlang runtime system.

-loader Loader

Specifies the method used by erl_prim_loader to load Erlang modules into the system. See erl_prim_loader(3). Two Loader methods are supported, efile and inet. efile means use the local file system, this is the default. inet means use a boot server on another machine, and the -id, -hosts and -setcookie flags must be specified as well. If Loader is something else, the user supplied Loader port program is started.


Makes the Erlang runtime system invoke make:all() in the current working directory and then terminate. See make(3). Implies -noinput.

-man Module

Displays the manual page for the Erlang module Module. Only supported on Unix.

-mode interactive | embedded

Indicates if the system should load code dynamically (interactive), or if all code should be loaded during system initialization (embedded), see code(3). Defaults to interactive.

-name Name

Makes the Erlang runtime system into a distributed node. This flag invokes all network servers necessary for a node to become distributed. See net_kernel(3). It is also ensured that epmd runs on the current host before Erlang is started. See epmd(1).

The name of the node will be Name@Host, where Host is the fully qualified host name of the current host. For short names, use the -sname flag instead.


Ensures that the Erlang runtime system never tries to read any input. Implies -noshell.


Starts an Erlang runtime system with no shell. This flag makes it possible to have the Erlang runtime system as a component in a series of UNIX pipes.


Disables the sticky directory facility of the Erlang code server, see code(3).


Invokes the old Erlang shell from Erlang 3.3. The old shell can still be used.

-pa Dir1 Dir2 ...

Adds the specified directories to the beginning of the code path, similar to code:add_pathsa/1. See code(3). As an alternative to -pa, if several directories are to be prepended to the code and the directories have a common parent directory, that parent directory could be specified in the ERL_LIBS environment variable. See code(3).

-pz Dir1 Dir2 ...

Adds the specified directories to the end of the code path, similar to code:add_pathsz/1. See code(3).

-remsh Node

Starts Erlang with a remote shell connected to Node.

-rsh Program

Specifies an alternative to rsh for starting a slave node on a remote host. See slave(3).

-run Mod [Func [Arg1, Arg2, ...]](init flag)

Makes init call the specified function. Func defaults to start. If no arguments are provided, the function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list [Arg1,Arg2,...] as argument. All arguments are passed as strings. See init(3).

-s Mod [Func [Arg1, Arg2, ...]](init flag)

Makes init call the specified function. Func defaults to start. If no arguments are provided, the function is assumed to be of arity 0. Otherwise it is assumed to be of arity 1, taking the list [Arg1,Arg2,...] as argument. All arguments are passed as atoms. See init(3).

-setcookie Cookie

Sets the magic cookie of the node to Cookie, see erlang:set_cookie/2.

-shutdown_time Time

Specifies how long time (in milliseconds) the init process is allowed to spend shutting down the system. If Time ms have elapsed, all processes still existing are killed. Defaults to infinity.

-sname Name

Makes the Erlang runtime system into a distributed node, similar to -name, but the host name portion of the node name Name@Host will be the short name, not fully qualified.

This is sometimes the only way to run distributed Erlang if the DNS (Domain Name System) is not running. There can be no communication between nodes running with the -sname flag and those running with the -name flag, as node names must be unique in distributed Erlang systems.

-smp [enable|auto|disable]

-smp enable and -smp starts the Erlang runtime system with SMP support enabled. This may fail if no runtime system with SMP support is available. -smp auto starts the Erlang runtime system with SMP support enabled if it is available and more than one logical processor are detected. -smp disable starts a runtime system without SMP support. By default -smp auto will be used unless a conflicting parameter has been passed, then -smp disable will be used. Currently only the -hybrid parameter conflicts with -smp auto.

NOTE: The runtime system with SMP support will not be available on all supported platforms. See also the +S flag.

-version(emulator flag)

Makes the emulator print out its version number. The same as erl +V.

Emulator Flags

erl invokes the code for the Erlang emulator (virtual machine), which supports the following flags:

+a size

Suggested stack size, in kilowords, for threads in the async-thread pool. Valid range is 16-8192 kilowords. The default suggested stack size is 16 kilowords, i.e, 64 kilobyte on 32-bit architectures. This small default size has been chosen since the amount of async-threads might be quite large. The default size is enough for drivers delivered with Erlang/OTP, but might not be sufficiently large for other dynamically linked in drivers that use the driver_async() functionality. Note that the value passed is only a suggestion, and it might even be ignored on some platforms.

+A size

Sets the number of threads in async thread pool, valid range is 0-1024. Default is 0.

+B [c | d | i]

The c option makes Ctrl-C interrupt the current shell instead of invoking the emulator break handler. The d option (same as specifying +B without an extra option) disables the break handler. The i option makes the emulator ignore any break signal.

If the c option is used with oldshell on Unix, Ctrl-C will restart the shell process rather than interrupt it.

Note that on Windows, this flag is only applicable for werl, not erl (oldshell). Note also that Ctrl-Break is used instead of Ctrl-C on Windows.


Disable compensation for sudden changes of system time.

Normally, erlang:now/0 will not immediately reflect sudden changes in the system time, in order to keep timers (including receive-after) working. Instead, the time maintained by erlang:now/0 is slowly adjusted towards the new system time. (Slowly means in one percent adjustments; if the time is off by one minute, the time will be adjusted in 100 minutes.)

When the +c option is given, this slow adjustment will not take place. Instead erlang:now/0 will always reflect the current system time. Note that timers are based on erlang:now/0. If the system time jumps, timers then time out at the wrong time.


If the emulator detects an internal error (or runs out of memory), it will by default generate both a crash dump and a core dump. The core dump will, however, not be very useful since the content of process heaps is destroyed by the crash dump generation.

The +d option instructs the emulator to only produce a core dump and no crash dump if an internal error is detected.

Calling erlang:halt/1 with a string argument will still produce a crash dump.

+h Size

Sets the default heap size of processes to the size Size.

+K true | false

Enables or disables the kernel poll functionality if the emulator supports it. Default is false (disabled). If the emulator does not support kernel poll, and the +K flag is passed to the emulator, a warning is issued at startup.


Enables auto load tracing, displaying info while loading code.

+MFlag Value

Memory allocator specific flags, see erts_alloc(3) for further information.

+P Number

Sets the maximum number of concurrent processes for this system. Number must be in the range 16..134217727. Default is 32768.

+R ReleaseNumber

Sets the compatibility mode.

The distribution mechanism is not backwards compatible by default. This flags sets the emulator in compatibility mode with an earlier Erlang/OTP release ReleaseNumber. The release number must be in the range 7..<current release>. This limits the emulator, making it possible for it to communicate with Erlang nodes (as well as C- and Java nodes) running that earlier release.

For example, an R10 node is not automatically compatible with an R9 node, but R10 nodes started with the +R 9 flag can co-exist with R9 nodes in the same distributed Erlang system, they are R9-compatible.

Note: Make sure all nodes (Erlang-, C-, and Java nodes) of a distributed Erlang system is of the same Erlang/OTP release, or from two different Erlang/OTP releases X and Y, where all Y nodes have compatibility mode X.

For example: A distributed Erlang system can consist of R10 nodes, or of R9 nodes and R9-compatible R10 nodes, but not of R9 nodes, R9-compatible R10 nodes and "regular" R10 nodes, as R9 and "regular" R10 nodes are not compatible.


Force ets memory block to be moved on realloc.

+S Schedulers:SchedulerOnline

Sets the amount of scheduler threads to create and scheduler threads to set online when SMP support has been enabled. Valid range for both values are 1-1024. If the Erlang runtime system is able to determine the amount of logical processors configured and logical processors available, Schedulers will default to logical processors configured, and SchedulersOnline will default to logical processors available; otherwise, the default values will be 1. Schedulers may be omitted if :SchedulerOnline is not and vice versa. The amount of schedulers online can be changed at run time via erlang:system_flag(schedulers_online, SchedulersOnline).

This flag will be ignored if the emulator doesn't have SMP support enabled (see the -smp flag).

+sFlag Value

Scheduling specific flags.

+sbt BindType

Set scheduler bind type. Currently valid BindTypes:


Same as erlang:system_flag(scheduler_bind_type, unbound).


Same as erlang:system_flag(scheduler_bind_type, no_spread).


Same as erlang:system_flag(scheduler_bind_type, thread_spread).


Same as erlang:system_flag(scheduler_bind_type, processor_spread).


Same as erlang:system_flag(scheduler_bind_type, spread).


Same as erlang:system_flag(scheduler_bind_type, no_node_thread_spread).


Same as erlang:system_flag(scheduler_bind_type, no_node_processor_spread).


Same as erlang:system_flag(scheduler_bind_type, thread_no_node_processor_spread).


Same as erlang:system_flag(scheduler_bind_type, default_bind).

Binding of schedulers are currently only supported on newer Linux and Solaris systems.

If no CPU topology is available when the +sbt flag is processed and BindType is any other type than u, the runtime system will fail to start. CPU topology can be defined using the +sct flag. Note that the +sct flag may have to be passed before the +sbt flag on the command line (in case no CPU topology has been automatically detected).

For more information, see erlang:system_flag(scheduler_bind_type, SchedulerBindType).

+sct CpuTopology
  • <Id> = integer(); when 0 =< <Id> =< 65535
  • <IdRange> = <Id>-<Id>
  • <IdOrIdRange> = <Id> | <IdRange>
  • <IdList> = <IdOrIdRange>,<IdOrIdRange> | <IdOrIdRange>
  • <LogicalIds> = L<IdList>
  • <ThreadIds> = T<IdList> | t<IdList>
  • <CoreIds> = C<IdList> | c<IdList>
  • <ProcessorIds> = P<IdList> | p<IdList>
  • <NodeIds> = N<IdList> | n<IdList>
  • <IdDefs> = <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds> | <LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>
  • CpuTopology = <IdDefs>:<IdDefs> | <IdDefs>

Upper-case letters signify real identifiers and lower-case letters signify fake identifiers only used for description of the topology. Identifiers passed as real identifiers may be used by the runtime system when trying to access specific hardware and if they are not correct the behavior is undefined. Faked logical CPU identifiers are not accepted since there is no point in defining the CPU topology without real logical CPU identifiers. Thread, core, processor, and node identifiers may be left out. If left out, thread id defaults to t0, core id defaults to c0, processor id defaults to p0, and node id will be left undefined. Either each logical processor must belong to one and only one NUMA node, or no logical processors must belong to any NUMA nodes.

Both increasing and decreasing <IdRange>s are allowed.

NUMA node identifiers are system wide. That is, each NUMA node on the system have to have a unique identifier. Processor identifiers are also system wide. Core identifiers are processor wide. Thread identifiers are core wide.

The order of the identifier types imply the hierarchy of the CPU topology. Valid orders are either <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds>, or <LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>. That is, thread is part of a core which is part of a processor which is part of a NUMA node, or thread is part of a core which is part of a NUMA node which is part of a processor. A cpu topology can consist of both processor external, and processor internal NUMA nodes as long as each logical processor belongs to one and only one NUMA node. If <ProcessorIds> is left out, its default position will be before <NodeIds>. That is, the default is processor external NUMA nodes.

If a list of identifiers is used in an <IdDefs>:

  • <LogicalIds> have to be a list of identifiers.
  • At least one other identifier type apart from <LogicalIds> also have to have a list of identifiers.
  • All lists of identifiers have to produce the same amount of identifiers.

A simple example. A single quad core processor may be described this way:

% erl +sct L0-3c0-3
1> erlang:system_info(cpu_topology).

A little more complicated example. Two quad core processors. Each processor in its own NUMA node. The ordering of logical processors is a little weird. This in order to give a better example of identifier lists:

% erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1
1> erlang:system_info(cpu_topology).

As long as real identifiers are correct it is okay to pass a CPU topology that is not a correct description of the CPU topology. When used with care this can actually be very useful. This in order to trick the emulator to bind its schedulers as you want. For example, if you want to run multiple Erlang runtime systems on the same machine, you want to reduce the amount of schedulers used and manipulate the CPU topology so that they bind to different logical CPUs. An example, with two Erlang runtime systems on a quad core machine:

% erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one
% erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two

In this example each runtime system have two schedulers each online, and all schedulers online will run on different cores. If we change to one scheduler online on one runtime system, and three schedulers online on the other, all schedulers online will still run on different cores.

Note that a faked CPU topology that does not reflect how the real CPU topology looks like is likely to decrease the performance of the runtime system.

For more information, see erlang:system_flag(cpu_topology, CpuTopology).

+sss size

Suggested stack size, in kilowords, for scheduler threads. Valid range is 4-8192 kilowords. The default stack size is OS dependent.

+T Level

Enables modified timing and sets the modified timing level. Currently valid range is 0-9. The timing of the runtime system will change. A high level usually means a greater change than a low level. Changing the timing can be very useful for finding timing related bugs.

Currently, modified timing affects the following:

Process spawning

A process calling spawn, spawn_link, spawn_monitor, or spawn_opt will be scheduled out immediately after completing the call. When higher modified timing levels are used, the caller will also sleep for a while after being scheduled out.

Context reductions
The amount of reductions a process is a allowed to use before being scheduled out is increased or reduced.
Input reductions
The amount of reductions performed before checking I/O is increased or reduced.

NOTE: Performance will suffer when modified timing is enabled. This flag is only intended for testing and debugging. Also note that return_to and return_from trace messages will be lost when tracing on the spawn BIFs. This flag may be removed or changed at any time without prior notice.


Makes the emulator print out its version number.



+W w | i

Sets the mapping of warning messages for error_logger. Messages sent to the error logger using one of the warning routines can be mapped either to errors (default), warnings (+W w), or info reports (+W i). The current mapping can be retrieved using error_logger:warning_map/0. See error_logger(3) for further information.

Environment variables


If the emulator needs to write a crash dump, the value of this variable will be the file name of the crash dump file. If the variable is not set, the name of the crash dump file will be erl_crash.dump in the current directory.


Unix systems: If the emulator needs to write a crash dump, it will use the value of this variable to set the nice value for the process, thus lowering its priority. The allowable range is 1 through 39 (higher values will be replaced with 39). The highest value, 39, will give the process the lowest priority.


Unix systems: This variable gives the number of seconds that the emulator will be allowed to spend writing a crash dump. When the given number of seconds have elapsed, the emulator will be terminated by a SIGALRM signal.


The content of this environment variable will be added to the beginning of the command line for erl.

The -extra flag is treated specially. Its scope ends at the end of the environment variable content. Arguments following an -extra flag are moved on the command line into the -extra section, i.e. the end of the command line following after an -extra flag.


The content of these environment variables will be added to the end of the command line for erl.

The -extra flag is treated specially. Its scope ends at the end of the environment variable content. Arguments following an -extra flag are moved on the command line into the -extra section, i.e. the end of the command line following after an -extra flag.


This environment variable contains a list of additional library directories that the code server will search for applications and add to the code path. See code(3).


This environment variable can contain the port number to use when communicating with epmd. The default port will work fine in most cases. A different port can be specified to allow nodes of independent clusters to co-exist on the same host. All nodes in a cluster must use the same epmd port number.