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<header>
<copyright>
<year>1996</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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<title>erl</title>
<prepared></prepared>
<docno></docno>
<date></date>
<rev></rev>
<file>erl.xml</file>
</header>
<com>erl</com>
<comsummary>The Erlang emulator.</comsummary>
<description>
<p>The <c><![CDATA[erl]]></c> program starts an Erlang runtime system.
The exact details (for example, whether <c><![CDATA[erl]]></c> is a
script or a program and which other programs it calls) are
system-dependent.</p>
<p>Windows users probably want to use the <c><![CDATA[werl]]></c> program
instead, which runs in its own window with scrollbars and supports
command-line editing. The <c><![CDATA[erl]]></c> program on Windows
provides no line editing in its shell, and on Windows 95 there is no way
to scroll back to text that has scrolled off the screen. The
<c><![CDATA[erl]]></c> program must be used, however, in pipelines or if
you want to redirect standard input or output.</p>
<note>
<p>As from ERTS 5.9 (Erlang/OTP R15B) the runtime system does by
default <em>not</em> bind schedulers to logical processors.
For more information, see system flag
<seealso marker="#+sbt"><c>+sbt</c></seealso>.</p>
</note>
</description>
<funcs>
<func>
<name>erl <arguments></name>
<fsummary>Start an Erlang runtime system.</fsummary>
<desc>
<p>Starts an Erlang runtime system.</p>
<p>The arguments can be divided into <em>emulator flags</em>,
<em>flags</em>, and <em>plain arguments</em>:</p>
<list type="bulleted">
<item>
<p>Any argument starting with character <c><![CDATA[+]]></c> is
interpreted as an
<seealso marker="#emu_flags">emulator flag</seealso>.</p>
<p>As indicated by the name, emulator flags control
the behavior of the emulator.</p>
</item>
<item>
<p>Any argument starting with character <c><![CDATA[-]]></c>
(hyphen) is interpreted as a
<seealso marker="#init_flags">flag</seealso>, which is to
be passed to the Erlang part of the runtime system, more
specifically to the <c><![CDATA[init]]></c> system process, see
<seealso marker="init"><c>init(3)</c></seealso>.</p>
<p>The <c><![CDATA[init]]></c> process itself interprets some of
these flags, the <em>init flags</em>. It also stores any
remaining flags, the <em>user flags</em>. The latter can be
retrieved by calling <c><![CDATA[init:get_argument/1]]></c>.</p>
<p>A small number of "-" flags exist, which now actually are
emulator flags, see the description below.</p>
</item>
<item>
<p>Plain arguments are not interpreted in any way. They are also
stored by the <c><![CDATA[init]]></c> process and can be retrieved
by calling <c><![CDATA[init:get_plain_arguments/0]]></c>.
Plain arguments can occur before the first flag, or after a
<c><![CDATA[--]]></c> flag. Also, the <c><![CDATA[-extra]]></c>
flag causes everything that follows to become plain arguments.</p>
</item>
</list>
<p><em>Examples:</em></p>
<pre>
% <input>erl +W w -sname arnie +R 9 -s my_init -extra +bertie</input>
(arnie@host)1> <input>init:get_argument(sname).</input>
{ok,[["arnie"]]}
(arnie@host)2> <input>init:get_plain_arguments().</input>
["+bertie"]</pre>
<p>Here <c><![CDATA[+W w]]></c> and <c><![CDATA[+R 9]]></c> are
emulator flags. <c><![CDATA[-s my_init]]></c> is an init flag,
interpreted by <c><![CDATA[init]]></c>.
<c><![CDATA[-sname arnie]]></c> is a user flag, stored by
<c><![CDATA[init]]></c>. It is read by Kernel and causes the
Erlang runtime system to become distributed. Finally, everything after
<c><![CDATA[-extra]]></c> (that is, <c><![CDATA[+bertie]]></c>) is
considered as plain arguments.</p>
<pre>
% <input>erl -myflag 1</input>
1> <input>init:get_argument(myflag).</input>
{ok,[["1"]]}
2> <input>init:get_plain_arguments().</input>
[]</pre>
<p>Here the user flag <c><![CDATA[-myflag 1]]></c> is passed to and
stored by the <c><![CDATA[init]]></c> process. It is a user-defined
flag, presumably used by some user-defined application.</p>
</desc>
</func>
</funcs>
<section>
<marker id="init_flags"></marker>
<title>Flags</title>
<p>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
<c><![CDATA[init:get_argument/1]]></c>. Notice that the list of user
flags is not exhaustive, there can be more application-specific
flags that instead are described in the corresponding
application documentation.</p>
<taglist>
<tag><c><![CDATA[--]]></c> (init flag)</tag>
<item>
<p>Everything following <c><![CDATA[--]]></c> up to the next flag
(<c><![CDATA[-flag]]></c> or <c><![CDATA[+flag]]></c>) is considered
plain arguments and can be retrieved using
<c><![CDATA[init:get_plain_arguments/0]]></c>.</p>
</item>
<tag><c><![CDATA[-Application Par Val]]></c></tag>
<item>
<p>Sets the application configuration parameter <c><![CDATA[Par]]></c>
to the value <c><![CDATA[Val]]></c> for the application
<c><![CDATA[Application]]></c>; see
<seealso marker="kernel:app"><c>app(4)</c></seealso> and
<seealso marker="kernel:application">
<c>application(3)</c></seealso>.</p>
</item>
<tag><marker id="args_file"/><c><![CDATA[-args_file FileName]]></c></tag>
<item>
<p>Command-line arguments are read from the file
<c><![CDATA[FileName]]></c>. The arguments read from the file replace
flag '<c><![CDATA[-args_file FileName]]></c>' on the resulting
command line.</p>
<p>The file <c><![CDATA[FileName]]></c> is to be a plain text file and
can contain comments and command-line arguments. A comment begins
with a <c>#</c> character and continues until the next end of line
character. Backslash (\\) is used as quoting character. All
command-line arguments accepted by <c><![CDATA[erl]]></c> are allowed,
also flag <c><![CDATA[-args_file FileName]]></c>. Be careful not to
cause circular dependencies between files containing flag
<c><![CDATA[-args_file]]></c>, though.</p>
<p>The flag <c><![CDATA[-extra]]></c> is treated in special way. Its
scope ends at the end of the file. Arguments following an
<c><![CDATA[-extra]]></c> flag are moved on the command line into the
<c><![CDATA[-extra]]></c> section, that is, the end of the command
line following after an <c><![CDATA[-extra]]></c> flag.</p>
</item>
<tag><c><![CDATA[-async_shell_start]]></c></tag>
<item>
<p>The initial Erlang shell does not read user input until
the system boot procedure has been completed (Erlang/OTP 5.4 and
later). This flag disables the start synchronization feature
and lets the shell start in parallel with the rest of
the system.</p>
</item>
<tag><c><![CDATA[-boot File]]></c></tag>
<item>
<p>Specifies the name of the boot file, <c><![CDATA[File.boot]]></c>,
which is used to start the system; see
<seealso marker="init"><c>init(3)</c></seealso>. Unless
<c><![CDATA[File]]></c> contains an absolute path, the system searches
for <c><![CDATA[File.boot]]></c> in the current and
<c><![CDATA[$ROOT/bin]]></c> directories.</p>
<p>Defaults to <c><![CDATA[$ROOT/bin/start.boot]]></c>.</p>
</item>
<tag><c><![CDATA[-boot_var Var Dir]]></c></tag>
<item>
<p>If the boot script contains a path variable <c><![CDATA[Var]]></c>
other than <c><![CDATA[$ROOT]]></c>, this variable is expanded to
<c><![CDATA[Dir]]></c>. Used when applications are installed in
another directory than <c><![CDATA[$ROOT/lib]]></c>; see
<seealso marker="sasl:systools#make_script/1">
<c>systools:make_script/1,2</c></seealso> in SASL.</p>
</item>
<tag><c><![CDATA[-code_path_cache]]></c></tag>
<item>
<p>Enables the code path cache of the code server; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-compile Mod1 Mod2 ...]]></c></tag>
<item>
<p>Compiles the specified modules and then terminates (with
non-zero exit code if the compilation of some file did not
succeed). Implies <c><![CDATA[-noinput]]></c>.</p>
<p>Not recommended; use <seealso marker="erlc"><c>erlc</c></seealso>
instead.</p>
</item>
<tag><c><![CDATA[-config Config]]></c></tag>
<item>
<p>Specifies the name of a configuration file,
<c><![CDATA[Config.config]]></c>, which is used to configure
applications; see
<seealso marker="kernel:app"><c>app(4)</c></seealso> and
<seealso marker="kernel:application">
<c>application(3)</c></seealso>.</p>
</item>
<tag><marker id="connect_all"/><c><![CDATA[-connect_all false]]></c></tag>
<item>
<p>If this flag is present, <c><![CDATA[global]]></c> does not maintain
a fully connected network of distributed Erlang nodes, and then
global name registration cannot be used; see
<seealso marker="kernel:global"><c>global(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-cookie Cookie]]></c></tag>
<item>
<p>Obsolete flag without any effect and common misspelling for
<c><![CDATA[-setcookie]]></c>. Use <c><![CDATA[-setcookie]]></c>
instead.</p>
</item>
<tag><c><![CDATA[-detached]]></c></tag>
<item>
<p>Starts the Erlang runtime system detached from the system
console. Useful for running daemons and backgrounds processes. Implies
<c><![CDATA[-noinput]]></c>.</p>
</item>
<tag><c><![CDATA[-emu_args]]></c></tag>
<item>
<p>Useful for debugging. Prints the arguments sent to the emulator.</p>
</item>
<tag><c><![CDATA[-env Variable Value]]></c></tag>
<item>
<p>Sets the host OS environment variable <c><![CDATA[Variable]]></c> to
the value <c><![CDATA[Value]]></c> for the Erlang runtime system.
Example:</p>
<pre>
% <input>erl -env DISPLAY gin:0</input></pre>
<p>In this example, an Erlang runtime system is started with
environment variable <c><![CDATA[DISPLAY]]></c> set to
<c><![CDATA[gin:0]]></c>.</p>
</item>
<tag><c><![CDATA[-eval Expr]]></c> (init flag)</tag>
<item>
<p>Makes <c><![CDATA[init]]></c> evaluate the expression
<c><![CDATA[Expr]]></c>; see
<seealso marker="init"><c>init(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-extra]]></c> (init flag)</tag>
<item>
<p>Everything following <c><![CDATA[-extra]]></c> is considered plain
arguments and can be retrieved using
<c><![CDATA[init:get_plain_arguments/0]]></c>.</p>
</item>
<tag><c><![CDATA[-heart]]></c></tag>
<item>
<p>Starts heartbeat monitoring of the Erlang runtime system;
see <seealso marker="kernel:heart">
<c>heart(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-hidden]]></c></tag>
<item>
<p>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
any of the connected nodes, that is, none of the connected
nodes are part of the result from <c><![CDATA[nodes/0]]></c> on the
other node. See also hidden global groups;
<seealso marker="kernel:global_group">
<c>global_group(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-hosts Hosts]]></c></tag>
<item>
<p>Specifies the IP addresses for the hosts on which Erlang boot servers
are running, see <seealso marker="kernel:erl_boot_server">
<c>erl_boot_server(3)</c></seealso>. This flag
is mandatory if flag <c><![CDATA[-loader inet]]></c> is present.</p>
<p>The IP addresses must be specified in the standard form (four
decimal numbers separated by periods, for example,
<c><![CDATA["150.236.20.74"]]></c>. Hosts names are not acceptable,
but a broadcast address (preferably limited to the local network)
is.</p>
</item>
<tag><c><![CDATA[-id Id]]></c></tag>
<item>
<p>Specifies the identity of the Erlang runtime system. If it is
run as a distributed node, <c><![CDATA[Id]]></c> must be identical to
the name supplied together with flag <c><![CDATA[-sname]]></c> or
<c><![CDATA[-name]]></c>.</p>
</item>
<tag><c><![CDATA[-init_debug]]></c></tag>
<item>
<p>Makes <c><![CDATA[init]]></c> write some debug information while
interpreting the boot script.</p>
</item>
<tag><marker id="instr"/><c><![CDATA[-instr]]></c> (emulator flag)</tag>
<item>
<p>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 analyzed using the <c><![CDATA[instrument]]></c> module.
Functionally, it behaves exactly like an ordinary Erlang
runtime system.</p>
</item>
<tag><c><![CDATA[-loader Loader]]></c></tag>
<item>
<p>Specifies the method used by <c><![CDATA[erl_prim_loader]]></c> to
load Erlang modules into the system; see
<seealso marker="erl_prim_loader"><c>erl_prim_loader(3)</c></seealso>.
Two <c><![CDATA[Loader]]></c> methods are supported:</p>
<list type="bulleted">
<item>
<p><c><![CDATA[efile]]></c>, which means use the local file system,
this is the default.</p>
</item>
<item>
<p><c><![CDATA[inet]]></c>, which means use a boot server on
another machine. The flags <c><![CDATA[-id]]></c>,
<c><![CDATA[-hosts]]></c> and <c><![CDATA[-setcookie]]></c> must
also be specified.</p>
</item>
</list>
<p>If <c><![CDATA[Loader]]></c> is something else, the user-supplied
<c><![CDATA[Loader]]></c> port program is started.</p>
</item>
<tag><c><![CDATA[-make]]></c></tag>
<item>
<p>Makes the Erlang runtime system invoke <c><![CDATA[make:all()]]></c>
in the current working directory and then terminate; see
<seealso marker="tools:make"><c>make(3)</c></seealso>. Implies
<c><![CDATA[-noinput]]></c>.</p>
</item>
<tag><c><![CDATA[-man Module]]></c></tag>
<item>
<p>Displays the manual page for the Erlang module
<c><![CDATA[Module]]></c>. Only supported on Unix.</p>
</item>
<tag><c><![CDATA[-mode interactive | embedded]]></c></tag>
<item>
<p>Indicates if the system is to load code dynamically
(<c><![CDATA[interactive]]></c>), or if all code is to be loaded
during system initialization (<c><![CDATA[embedded]]></c>); see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.
Defaults to <c><![CDATA[interactive]]></c>.</p>
</item>
<tag><c><![CDATA[-name Name]]></c></tag>
<item>
<p>Makes the Erlang runtime system into a distributed node.
This flag invokes all network servers necessary for a node to
become distributed; see <seealso marker="kernel:net_kernel">
<c>net_kernel(3)</c></seealso>. It is also ensured that
<c><![CDATA[epmd]]></c> runs on the current host before Erlang is
started; see <seealso marker="epmd"><c>epmd(1)</c></seealso>.and the
<seealso marker="#start_epmd"><c>-start_epmd</c></seealso> option.</p>
<p>The node name will be <c><![CDATA[Name@Host]]></c>, where
<c><![CDATA[Host]]></c> is the fully qualified host name of the
current host. For short names, use flag <c><![CDATA[-sname]]></c>
instead.</p>
</item>
<tag><c><![CDATA[-noinput]]></c></tag>
<item>
<p>Ensures that the Erlang runtime system never tries to read
any input. Implies <c><![CDATA[-noshell]]></c>.</p>
</item>
<tag><c><![CDATA[-noshell]]></c></tag>
<item>
<p>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.</p>
</item>
<tag><c><![CDATA[-nostick]]></c></tag>
<item>
<p>Disables the sticky directory facility of the Erlang code
server; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-oldshell]]></c></tag>
<item>
<p>Invokes the old Erlang shell from Erlang/OTP 3.3. The old shell
can still be used.</p>
</item>
<tag><c><![CDATA[-pa Dir1 Dir2 ...]]></c></tag>
<item>
<p>Adds the specified directories to the beginning of the code
path, similar to <c><![CDATA[code:add_pathsa/1]]></c>; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.
As an alternative to <c>-pa</c>, if several directories are
to be prepended to the code path and the directories have a
common parent directory, that parent directory can be
specified in environment variable <c>ERL_LIBS</c>; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-pz Dir1 Dir2 ...]]></c></tag>
<item>
<p>Adds the specified directories to the end of the code path,
similar to <c><![CDATA[code:add_pathsz/1]]></c>; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-path Dir1 Dir2 ...]]></c></tag>
<item>
<p>Replaces the path specified in the boot script; see
<seealso marker="sasl:script"><c>script(4)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-proto_dist Proto]]></c></tag>
<item>
<p>Specifies a protocol for Erlang distribution:</p>
<taglist>
<tag><c>inet_tcp</c></tag>
<item>TCP over IPv4 (the default)</item>
<tag><c>inet_tls</c></tag>
<item>Distribution over TLS/SSL</item>
<tag><c>inet6_tcp</c></tag>
<item>TCP over IPv6</item>
</taglist>
<p>For example, to start up IPv6 distributed nodes:</p>
<pre>
% <input>erl -name [email protected] -proto_dist inet6_tcp</input></pre>
</item>
<tag><c><![CDATA[-remsh Node]]></c></tag>
<item>
<p>Starts Erlang with a remote shell connected to
<c><![CDATA[Node]]></c>.</p>
</item>
<tag><c><![CDATA[-rsh Program]]></c></tag>
<item>
<p>Specifies an alternative to <c><![CDATA[rsh]]></c> for starting a
slave node on a remote host; see
<seealso marker="stdlib:slave"><c>slave(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-run Mod [Func [Arg1, Arg2, ...]]]]></c> (init
flag)</tag>
<item>
<p>Makes <c><![CDATA[init]]></c> call the specified function.
<c><![CDATA[Func]]></c> defaults to <c><![CDATA[start]]></c>.
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
<c><![CDATA[[Arg1,Arg2,...]]]></c> as argument. All arguments are
passed as strings. See <seealso marker="init">
<c>init(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-s Mod [Func [Arg1, Arg2, ...]]]]></c> (init flag)</tag>
<item>
<p>Makes <c><![CDATA[init]]></c> call the specified function.
<c><![CDATA[Func]]></c> defaults to <c><![CDATA[start]]></c>.
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
<c><![CDATA[[Arg1,Arg2,...]]]></c> as argument. All arguments are
passed as atoms. See <seealso marker="init">
<c>init(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[-setcookie Cookie]]></c></tag>
<item>
<p>Sets the magic cookie of the node to <c><![CDATA[Cookie]]></c>; see
<seealso marker="erlang#set_cookie/2">
<c>erlang:set_cookie/2</c></seealso>.</p>
</item>
<tag><c><![CDATA[-shutdown_time Time]]></c></tag>
<item>
<p>Specifies how long time (in milliseconds) the <c><![CDATA[init]]></c>
process is allowed to spend shutting down the system. If
<c><![CDATA[Time]]></c> milliseconds have elapsed, all processes still
existing are killed. Defaults to <c><![CDATA[infinity]]></c>.</p>
</item>
<tag><c><![CDATA[-sname Name]]></c></tag>
<item>
<p>Makes the Erlang runtime system into a distributed node, similar to
<c><![CDATA[-name]]></c>, but the host name portion of the node
name <c><![CDATA[Name@Host]]></c> will be the short name, not fully
qualified.</p>
<p>This is sometimes the only way to run distributed Erlang if
the Domain Name System (DNS) is not running. No communication can
exist between nodes running with flag <c><![CDATA[-sname]]></c>
and those running with flag <c><![CDATA[-name]]></c>, as node
names must be unique in distributed Erlang systems.</p>
</item>
<tag><marker id="start_epmd"/><c>-start_epmd true | false</c></tag>
<item>
<p>Specifies whether Erlang should start
<seealso marker="epmd">epmd</seealso> on startup. By default
this is <c>true</c>, but if you prefer to start epmd
manually, set this to <c>false</c>.</p>
<p>This only applies if Erlang is started as a distributed node,
i.e. if <c>-name</c> or <c>-sname</c> is specified. Otherwise,
epmd is not started even if <c>-start_epmd true</c> is given.</p>
<p>Note that a distributed node will fail to start if epmd is
not running.</p>
</item>
<tag><marker id="smp"/><c><![CDATA[-smp [enable|auto|disable]]]></c></tag>
<item>
<p><c>-smp enable</c> and <c>-smp</c> start the Erlang runtime
system with SMP support enabled. This can fail if no runtime
system with SMP support is available. <c>-smp auto</c> starts
the Erlang runtime system with SMP support enabled if it is
available and more than one logical processor is detected.
<c>-smp disable</c> starts a runtime system without SMP support.</p>
<note>
<p>The runtime system with SMP support is not available on all
supported platforms. See also flag
<seealso marker="#+S"><c>+S</c></seealso>.</p>
</note>
</item>
<tag><c><![CDATA[-version]]></c> (emulator flag)</tag>
<item>
<p>Makes the emulator print its version number. The same
as <c><![CDATA[erl +V]]></c>.</p>
</item>
</taglist>
</section>
<section>
<marker id="emu_flags"></marker>
<title>Emulator Flags</title>
<p><c><![CDATA[erl]]></c> invokes the code for the Erlang emulator (virtual
machine), which supports the following flags:</p>
<taglist>
<tag><marker id="async_thread_stack_size"/>
<c><![CDATA[+a size]]></c></tag>
<item>
<p>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, that is, 64
kilobyte on 32-bit architectures. This small default size
has been chosen because the number of async threads can
be large. The default size is enough for drivers
delivered with Erlang/OTP, but might not be large
enough for other dynamically linked-in drivers that use the
<seealso marker="erl_driver#driver_async">
<c>driver_async()</c></seealso> functionality.
Notice that the value passed is only a suggestion,
and it can even be ignored on some platforms.</p>
</item>
<tag><marker id="async_thread_pool_size"/><c><![CDATA[+A size]]></c></tag>
<item>
<p>Sets the number of threads in async thread pool. Valid range
is 0-1024. Defaults to 10 if thread support is available.</p>
</item>
<tag><c><![CDATA[+B [c | d | i]]]></c></tag>
<item>
<p>Option <c><![CDATA[c]]></c> makes <c><![CDATA[Ctrl-C]]></c>
interrupt the current shell instead of invoking the emulator break
handler. Option <c><![CDATA[d]]></c> (same as specifying
<c><![CDATA[+B]]></c> without an extra option) disables the break
handler. Option <c><![CDATA[i]]></c> makes the emulator ignore any
break signal.</p>
<p>If option <c><![CDATA[c]]></c> is used with
<c><![CDATA[oldshell]]></c> on Unix, <c><![CDATA[Ctrl-C]]></c> will
restart the shell process rather than interrupt it.</p>
<p>Notice that on Windows, this flag is only applicable for
<c><![CDATA[werl]]></c>, not <c><![CDATA[erl]]></c>
(<c><![CDATA[oldshell]]></c>). Notice also that
<c><![CDATA[Ctrl-Break]]></c> is used instead of
<c><![CDATA[Ctrl-C]]></c> on Windows.</p>
</item>
<tag><marker id="+c"/><c><![CDATA[+c true | false]]></c></tag>
<item>
<p>Enables or disables
<seealso marker="time_correction#Time_Correction">time
correction</seealso>:</p>
<taglist>
<tag><c>true</c></tag>
<item>Enables time correction. This is the default if
time correction is supported on the specific platform.</item>
<tag><c>false</c></tag>
<item>Disables time correction.</item>
</taglist>
<p>For backward compatibility, the boolean value can be omitted.
This is interpreted as <c>+c false</c>.</p>
</item>
<tag><marker id="+C_"/><c><![CDATA[+C no_time_warp | single_time_warp |
multi_time_warp]]></c></tag>
<item>
<p>Sets <seealso marker="time_correction#Time_Warp_Modes">time warp
mode</seealso>:</p>
<taglist>
<tag><c>no_time_warp</c></tag>
<item><seealso marker="time_correction#No_Time_Warp_Mode">
No time warp mode</seealso> (the default)</item>
<tag><c>single_time_warp</c></tag>
<item><seealso marker="time_correction#Single_Time_Warp_Mode">
Single time warp mode</seealso></item>
<tag><c>multi_time_warp</c></tag>
<item><seealso marker="time_correction#Multi_Time_Warp_Mode">
Multi-time warp mode</seealso></item>
</taglist>
</item>
<tag><c><![CDATA[+d]]></c></tag>
<item>
<p>If the emulator detects an internal error (or runs out of memory),
it, by default, generates both a crash dump and a core dump.
The core dump is, however, not very useful as the content
of process heaps is destroyed by the crash dump generation.</p>
<p>Option <c>+d</c> instructs the emulator to produce only a
core dump and no crash dump if an internal error is detected.</p>
<p>Calling <seealso marker="erlang:halt/1">
<c>erlang:halt/1</c></seealso> with a string argument still
produces a crash dump. On Unix systems, sending an emulator process
a <c>SIGUSR1</c> signal also forces a crash dump.</p>
</item>
<tag><marker id="+e"/><c><![CDATA[+e Number]]></c></tag>
<item>
<p>Sets the maximum number of ETS tables.</p>
</item>
<tag><c><![CDATA[+ec]]></c></tag>
<item>
<p>Forces option <c>compressed</c> on all ETS tables.
Only intended for test and evaluation.</p>
</item>
<tag><marker id="file_name_encoding"></marker>
<c><![CDATA[+fnl]]></c></tag>
<item>
<p>The virtual machine works with filenames as if they are encoded
using the ISO Latin-1 encoding, disallowing Unicode characters with
code points > 255.</p>
<p>For more information about Unicode filenames, see section
<seealso marker="stdlib:unicode_usage#unicode_file_names">Unicode
Filenames</seealso> in the STDLIB User's Guide. Notice that
this value also applies to command-line parameters and environment
variables (see section <seealso
marker="stdlib:unicode_usage#unicode_in_environment_and_parameters">
Unicode in Enviroment and Parameters</seealso> in the STDLIB
User's Guide).</p>
</item>
<tag><c><![CDATA[+fnu[{w|i|e}]]]></c></tag>
<item>
<p>The virtual machine works with filenames as if they are encoded
using UTF-8 (or some other system-specific Unicode encoding). This is
the default on operating systems that enforce Unicode encoding, that
is, Windows and MacOS X.</p>
<p>The <c>+fnu</c> switch can be followed by <c>w</c>, <c>i</c>, or
<c>e</c> to control how wrongly encoded filenames are to be
reported:</p>
<list type="bulleted">
<item>
<p><c>w</c> means that a warning is sent to the <c>error_logger</c>
whenever a wrongly encoded filename is "skipped" in directory
listings. This is the default.</p>
</item>
<item>
<p><c>i</c> means that those wrongly encoded filenames are silently
ignored.</p>
</item>
<item>
<p><c>e</c> means that the API function returns an error whenever a
wrongly encoded filename (or directory name) is encountered.</p>
</item>
</list>
<p>Notice that <seealso marker="kernel:file#read_link/1">
<c>file:read_link/1</c></seealso> always returns an error if the link
points to an invalid filename.</p>
<p>For more information about Unicode filenames, see section
<seealso marker="stdlib:unicode_usage#unicode_file_names">Unicode
Filenames</seealso> in the STDLIB User's Guide. Notice that
this value also applies to command-line parameters and environment
variables (see section <seealso
marker="stdlib:unicode_usage#unicode_in_environment_and_parameters">
Unicode in Enviroment and Parameters</seealso> in the STDLIB
User's Guide).</p>
</item>
<tag><c><![CDATA[+fna[{w|i|e}]]]></c></tag>
<item>
<p>Selection between <c>+fnl</c> and <c>+fnu</c> is done based
on the current locale settings in the OS. This means that if you
have set your terminal for UTF-8 encoding, the filesystem is
expected to use the same encoding for filenames. This is
default on all operating systems, except MacOS X and Windows.</p>
<p>The <c>+fna</c> switch can be followed by <c>w</c>, <c>i</c>, or
<c>e</c>. This has effect if the locale settings cause the behavior
of <c>+fnu</c> to be selected; see the description of <c>+fnu</c>
above. If the locale settings cause the behavior of <c>+fnl</c> to be
selected, then <c>w</c>, <c>i</c>, or <c>e</c> have no effect.</p>
<p>For more information about Unicode filenames, see section
<seealso marker="stdlib:unicode_usage#unicode_file_names">Unicode
Filenames</seealso> in the STDLIB User's Guide. Notice that
this value also applies to command-line parameters and environment
variables (see section <seealso
marker="stdlib:unicode_usage#unicode_in_environment_and_parameters">
Unicode in Enviroment and Parameters</seealso> in the STDLIB
User's Guide).</p>
</item>
<tag><c><![CDATA[+hms Size]]></c></tag>
<item>
<p>Sets the default heap size of processes to the size
<c><![CDATA[Size]]></c>.</p>
</item>
<tag><c><![CDATA[+hmbs Size]]></c></tag>
<item>
<p>Sets the default binary virtual heap size of processes to the size
<c><![CDATA[Size]]></c>.</p>
</item>
<tag><marker id="+hmax"/><c><![CDATA[+hmax Size]]></c></tag>
<item>
<p>Sets the default maximum heap size of processes to the size
<c><![CDATA[Size]]></c>. Defaults to <c>0</c>, which means that no
maximum heap size is used. For more information, see
<seealso marker="erlang#process_flag_max_heap_size">
<c>process_flag(max_heap_size, MaxHeapSize)</c></seealso>.</p>
</item>
<tag><marker id="+hmaxel"/><c><![CDATA[+hmaxel true|false]]></c></tag>
<item>
<p>Sets whether to send an error logger message or not for processes
reaching the maximum heap size. Defaults to <c>true</c>.
For more information, see
<seealso marker="erlang#process_flag_max_heap_size">
<c>process_flag(max_heap_size, MaxHeapSize)</c></seealso>.</p>
</item>
<tag><marker id="+hmaxk"/><c><![CDATA[+hmaxk true|false]]></c></tag>
<item>
<p>Sets whether to kill processes reaching the maximum heap size or not.
Default to <c>true</c>. For more information, see
<seealso marker="erlang#process_flag_max_heap_size">
<c>process_flag(max_heap_size, MaxHeapSize)</c></seealso>.</p>
</item>
<tag><c><![CDATA[+hpds Size]]></c></tag>
<item>
<p>Sets the initial process dictionary size of processes to the size
<c><![CDATA[Size]]></c>.</p>
</item>
<tag><marker id="+hmqd"/><c>+hmqd off_heap|on_heap</c></tag>
<item>
<p>Sets the default value for process flag <c>message_queue_data</c>.
Defaults to <c>on_heap</c>. If <c>+hmqd</c> is not
passed, <c>on_heap</c> will be the default. For more information, see
<seealso marker="erlang#process_flag_message_queue_data">
<c>process_flag(message_queue_data, MQD)</c></seealso>.</p>
</item>
<tag><c><![CDATA[+K true | false]]></c></tag>
<item>
<p>Enables or disables the kernel poll functionality if supported by
the emulator. Defaults to <c><![CDATA[false]]></c> (disabled).
If the emulator does not support kernel poll, and flag
<c><![CDATA[+K]]></c> is passed to the emulator, a warning is
issued at startup.</p>
</item>
<tag><c><![CDATA[+l]]></c></tag>
<item>
<p>Enables autoload tracing, displaying information while loading
code.</p>
</item>
<tag><c><![CDATA[+L]]></c></tag>
<item>
<p>Prevents loading information about source filenames and line
numbers. This saves some memory, but exceptions do not contain
information about the filenames and line numbers.</p>
</item>
<tag><marker id="erts_alloc"/><c><![CDATA[+MFlag Value]]></c></tag>
<item>
<p>Memory allocator-specific flags. For more information, see
<seealso marker="erts_alloc"><c>erts_alloc(3)</c></seealso>.</p>
</item>
<tag><marker id="+pc"/><marker id="printable_character_range"/>
<c><![CDATA[+pc Range]]></c></tag>
<item>
<p>Sets the range of characters that the system considers printable in
heuristic detection of strings. This typically affects the shell,
debugger, and <c>io:format</c> functions (when <c>~tp</c> is used in
the format string).</p>
<p>Two values are supported for <c>Range</c>:</p>
<taglist>
<tag><c>latin1</c></tag>
<item>The default. Only characters in the ISO Latin-1 range can be
considered printable. This means that a character with a code point
> 255 is never considered printable and that lists containing
such characters are displayed as lists of integers rather than text
strings by tools.</item>
<tag><c>unicode</c></tag>
<item>All printable Unicode characters are considered when
determining if a list of integers is to be displayed in
string syntax. This can give unexpected results if, for
example, your font does not cover all Unicode characters.</item>
</taglist>
<p>See also <seealso marker="stdlib:io#printable_range/0">
<c>io:printable_range/0</c></seealso> in STDLIB.</p>
</item>
<tag><marker id="+P"/><marker id="max_processes"/><c><![CDATA[+P Number]]></c></tag>
<item>
<p>Sets the maximum number of simultaneously existing processes for this
system if a <c>Number</c> is passed as value. Valid range for
<c>Number</c> is <c>[1024-134217727]</c></p>
<p><em>NOTE</em>: The actual maximum chosen may be much larger than
the <c>Number</c> passed. Currently the runtime system often,
but not always, chooses a value that is a power of 2. This might,
however, be changed in the future. The actual value chosen can be
checked by calling
<seealso marker="erlang#system_info_process_limit">erlang:system_info(process_limit)</seealso>.</p>
<p>The default value is <c>262144</c></p>
</item>
<tag><marker id="+Q"/><marker id="max_ports"/><c><![CDATA[+Q Number]]></c></tag>
<item>
<p>Sets the maximum number of simultaneously existing ports for this
system if a Number is passed as value. Valid range for <c>Number</c>
is <c>[1024-134217727]</c></p>
<p><em>NOTE</em>: The actual maximum chosen may be much larger than
the actual <c>Number</c> passed. Currently the runtime system often,
but not always, chooses a value that is a power of 2. This might,
however, be changed in the future. The actual value chosen can be
checked by calling
<seealso marker="erlang#system_info_port_limit">erlang:system_info(port_limit)</seealso>.</p>
<p>The default value used is normally <c>65536</c>. However, if
the runtime system is able to determine maximum amount of file
descriptors that it is allowed to open and this value is larger
than <c>65536</c>, the chosen value will increased to a value
larger or equal to the maximum amount of file descriptors that
can be opened.</p>
<p>On Windows the default value is set to <c>8196</c> because the
normal OS limitations are set higher than most machines can handle.</p>
</item>
<tag><marker id="compat_rel"/><c><![CDATA[+R ReleaseNumber]]></c></tag>
<item>
<p>Sets the compatibility mode.</p>
<p>The distribution mechanism is not backward compatible by
default. This flag sets the emulator in compatibility mode
with an earlier Erlang/OTP release <c><![CDATA[ReleaseNumber]]></c>.
The release number must be in the range
<c><![CDATA[<current release>-2..<current release>]]></c>. 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.</p>
<note>
<p>Ensure that 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
<em>all</em> Y nodes have compatibility mode X.</p>
</note>
</item>
<tag><c><![CDATA[+r]]></c></tag>
<item>
<p>Forces ETS memory block to be moved on realloc.</p>
</item>
<tag><marker id="+rg"/><c><![CDATA[+rg ReaderGroupsLimit]]></c></tag>
<item>
<p>Limits the number of reader groups used by read/write locks
optimized for read operations in the Erlang runtime system. By
default the reader groups limit is 64.</p>
<p>When the number of schedulers is less than or equal to the reader
groups limit, each scheduler has its own reader group. When the
number of schedulers is larger than the reader groups limit,
schedulers share reader groups. Shared reader groups degrade
read lock and read unlock performance while many
reader groups degrade write lock performance. So, the limit is a
tradeoff between performance for read operations and performance
for write operations. Each reader group consumes 64 byte
in each read/write lock.</p>
<p>Notice that a runtime system using shared reader groups benefits from
<seealso marker="#+sbt">binding schedulers to logical
processors</seealso>, as the reader groups are distributed better
between schedulers.</p>
</item>
<tag><marker id="+S"/>
<c><![CDATA[+S Schedulers:SchedulerOnline]]></c></tag>
<item>
<p>Sets the number of scheduler threads to create and scheduler threads
to set online when SMP support has been enabled. The maximum for both
values is 1024. If the Erlang runtime system is able to determine the
number of logical processors configured and logical processors
available, <c>Schedulers</c> defaults to logical processors
configured, and <c>SchedulersOnline</c> defaults to logical processors
available; otherwise the default values are 1. <c>Schedulers</c> can
be omitted if <c>:SchedulerOnline</c> is not and conversely. The
number of schedulers online can be changed at runtime through
<seealso marker="erlang#system_flag_schedulers_online">
<c>erlang:system_flag(schedulers_online,
SchedulersOnline)</c></seealso>.</p>
<p>If <c>Schedulers</c> or <c>SchedulersOnline</c> is specified as a
negative number, the value is subtracted from the default number of
logical processors configured or logical processors available,
respectively.</p>
<p>Specifying value <c>0</c> for <c>Schedulers</c> or
<c>SchedulersOnline</c> resets the number of scheduler threads or
scheduler threads online, respectively, to its default value.</p>
<p>This option is ignored if the emulator does not have SMP support
enabled (see flag <seealso marker="#smp"><c>-smp</c></seealso>).</p>
</item>
<tag><marker id="+SP"/><c><![CDATA[+SP
SchedulersPercentage:SchedulersOnlinePercentage]]></c></tag>
<item>
<p>Similar to <seealso marker="#+S"><c>+S</c></seealso> but uses
percentages to set the number of scheduler threads to create, based
on logical processors configured, and scheduler threads to set online,
based on logical processors available, when SMP support has been
enabled. Specified values must be > 0. For example,
<c>+SP 50:25</c> sets the number of scheduler threads to 50% of the
logical processors configured, and the number of scheduler threads
online to 25% of the logical processors available.
<c>SchedulersPercentage</c> can be omitted if
<c>:SchedulersOnlinePercentage</c> is not and conversely. The number
of schedulers online can be changed at runtime through
<seealso marker="erlang#system_flag_schedulers_online">
<c>erlang:system_flag(schedulers_online,
SchedulersOnline)</c></seealso>.</p>
<p>This option interacts with <seealso marker="#+S"><c>+S</c></seealso>
settings. For example, on a system with 8 logical cores configured
and 8 logical cores available, the combination of the options
<c>+S 4:4 +SP 50:25</c> (in either order) results in 2 scheduler
threads (50% of 4) and 1 scheduler thread online (25% of 4).</p>
<p>This option is ignored if the emulator does not have SMP support
enabled (see flag <seealso marker="#smp"><c>-smp</c></seealso>).</p>
</item>
<tag><marker id="+SDcpu"/><c><![CDATA[+SDcpu
DirtyCPUSchedulers:DirtyCPUSchedulersOnline]]></c></tag>
<item>
<p>Sets the number of dirty CPU scheduler threads to create and dirty
CPU scheduler threads to set online when threading support has been
enabled. The maximum for both values is 1024, and each value is
further limited by the settings for normal schedulers:</p>
<list type="bulleted">
<item>The number of dirty CPU scheduler threads created cannot exceed
the number of normal scheduler threads created.</item>
<item>The number of dirty CPU scheduler threads online cannot exceed
the number of normal scheduler threads online.</item>
</list>
<p>For details, see the <seealso marker="#+S"><c>+S</c></seealso> and
<seealso marker="#+SP"><c>+SP</c></seealso>. By default, the number
of dirty CPU scheduler threads created equals the number of normal
scheduler threads created, and the number of dirty CPU scheduler
threads online equals the number of normal scheduler threads online.
<c>DirtyCPUSchedulers</c> can be omitted if
<c>:DirtyCPUSchedulersOnline</c> is not and conversely. The number of
dirty CPU schedulers online can be changed at runtime through
<seealso marker="erlang#system_flag_dirty_cpu_schedulers_online">
<c>erlang:system_flag(dirty_cpu_schedulers_online,
DirtyCPUSchedulersOnline)</c></seealso>.</p>
<p>The amount of dirty CPU schedulers is limited by the amount of
normal schedulers in order to limit the effect on processes
executing on ordinary schedulers. If the amount of dirty CPU
schedulers was allowed to be unlimited, dirty CPU bound jobs would
potentially starve normal jobs.</p>
<p>This option is ignored if the emulator does not have threading
support enabled. <em>This option is experimental</em> and
is supported only if the emulator was configured and built with
support for dirty schedulers enabled (it is disabled by default).</p>
</item>
<tag><marker id="+SDPcpu"/><c><![CDATA[+SDPcpu
DirtyCPUSchedulersPercentage:DirtyCPUSchedulersOnlinePercentage]]></c></tag>
<item>
<p>Similar to <seealso marker="#+SDcpu"><c>+SDcpu</c></seealso> but
uses percentages to set the number of dirty CPU scheduler threads to
create and the number of dirty CPU scheduler threads to set online
when threading support has been enabled. Specified values must be
> 0. For example, <c>+SDPcpu 50:25</c> sets the number of dirty
CPU scheduler threads to 50% of the logical processors configured
and the number of dirty CPU scheduler threads online to 25% of the
logical processors available. <c>DirtyCPUSchedulersPercentage</c> can
be omitted if <c>:DirtyCPUSchedulersOnlinePercentage</c> is not and
conversely. The number of dirty CPU schedulers online can be changed
at runtime through
<seealso marker="erlang#system_flag_dirty_cpu_schedulers_online">
<c>erlang:system_flag(dirty_cpu_schedulers_online,
DirtyCPUSchedulersOnline)</c></seealso>.</p>
<p>This option interacts with <seealso
marker="#+SDcpu"><c>+SDcpu</c></seealso> settings. For example, on a
system with 8 logical cores configured and 8 logical cores available,
the combination of the options <c>+SDcpu 4:4 +SDPcpu 50:25</c> (in
either order) results in 2 dirty CPU scheduler threads (50% of 4) and
1 dirty CPU scheduler thread online (25% of 4).</p>
<p>This option is ignored if the emulator does not have threading
support enabled. <em>This option is experimental</em> and
is supported only if the emulator was configured and built with
support for dirty schedulers enabled (it is disabled by default).</p>
</item>
<tag><marker id="+SDio"/><c><![CDATA[+SDio DirtyIOSchedulers]]></c></tag>
<item>
<p>Sets the number of dirty I/O scheduler threads to create when
threading support has been enabled. Valid range is 0-1024. By
default, the number of dirty I/O scheduler threads created is 10,
same as the default number of threads in the <seealso
marker="#async_thread_pool_size">async thread pool</seealso>.</p>
<p>The amount of dirty IO schedulers is not limited by the amount of
normal schedulers <seealso marker="#+SDcpu">like the amount of
dirty CPU schedulers</seealso>. This since only I/O bound work is
expected to execute on dirty I/O schedulers. If the user should schedule CPU
bound jobs on dirty I/O schedulers, these jobs might starve ordinary
jobs executing on ordinary schedulers.</p>
<p>This option is ignored if the emulator does not have threading
support enabled. <em>This option is experimental</em> and
is supported only if the emulator was configured and built with
support for dirty schedulers enabled (it is disabled by default).</p>
</item>
<tag><c><![CDATA[+sFlag Value]]></c></tag>
<item>
<p>Scheduling specific flags.</p>
<taglist>
<tag><marker id="+sbt"/><c>+sbt BindType</c></tag>
<item>
<p>Sets scheduler bind type.</p>
<p>Schedulers can also be bound using flag
<seealso marker="#+stbt"><c>+stbt</c></seealso>. The only
difference between these two flags is how the following errors
are handled:</p>
<list type="bulleted">
<item>Binding of schedulers is not supported on the specific
platform.</item>
<item>No available CPU topology. That is, the runtime system was
not able to detect the CPU topology automatically, and no
<seealso marker="#+sct">user-defined CPU topology</seealso>
was set.</item>
</list>
<p>If any of these errors occur when <c>+sbt</c> has been passed,
the runtime system prints an error message, and refuses to
start. If any of these errors occur when <c>+stbt</c> has been
passed, the runtime system silently ignores the error, and
start up using unbound schedulers.</p>
<p>Valid <c>BindType</c>s:</p>
<taglist>
<tag><c>u</c></tag>
<item><c>unbound</c> - Schedulers are not bound to logical
processors, that is, the operating system decides where the
scheduler threads execute, and when to migrate them. This is
the default.
</item>
<tag><c>ns</c></tag>
<item><c>no_spread</c> - Schedulers with close scheduler
identifiers are bound as close as possible in hardware.
</item>
<tag><c>ts</c></tag>
<item><c>thread_spread</c> - Thread refers to hardware threads
(such as Intel's hyper-threads). Schedulers with low scheduler
identifiers, are bound to the first hardware thread of
each core, then schedulers with higher scheduler identifiers
are bound to the second hardware thread of each core,and so on.
</item>
<tag><c>ps</c></tag>
<item><c>processor_spread</c> - Schedulers are spread like
<c>thread_spread</c>, but also over physical processor chips.
</item>
<tag><c>s</c></tag>
<item><c>spread</c> - Schedulers are spread as much as possible.
</item>
<tag><c>nnts</c></tag>
<item><c>no_node_thread_spread</c> - Like <c>thread_spread</c>,
but if multiple Non-Uniform Memory Access (NUMA) nodes exist,
schedulers are spread over one NUMA node at a time,
that is, all logical processors of one NUMA node are bound
to schedulers in sequence.
</item>
<tag><c>nnps</c></tag>
<item><c>no_node_processor_spread</c> - Like
<c>processor_spread</c>, but if multiple NUMA nodes exist,
schedulers are spread over one NUMA node at a time, that is,
all logical processors of one NUMA node are bound to
schedulers in sequence.
</item>
<tag><c>tnnps</c></tag>
<item><c>thread_no_node_processor_spread</c> - A combination of
<c>thread_spread</c>, and <c>no_node_processor_spread</c>.
Schedulers are spread over hardware threads across NUMA
nodes, but schedulers are only spread over processors
internally in one NUMA node at a time.
</item>
<tag><c>db</c></tag>
<item><c>default_bind</c> - Binds schedulers the default way.
Defaults to <c>thread_no_node_processor_spread</c>
(which can change in the future).
</item>
</taglist>
<p>Binding of schedulers is only supported on newer
Linux, Solaris, FreeBSD, and Windows systems.</p>
<p>If no CPU topology is available when flag <c>+sbt</c>
is processed and <c>BindType</c> is any other type than
<c>u</c>, the runtime system fails to start. CPU
topology can be defined using flag
<seealso marker="#+sct"><c>+sct</c></seealso>. Notice
that flag <c>+sct</c> can have to be passed before flag
<c>+sbt</c> on the command line (if no CPU topology
has been automatically detected).</p>
<p>The runtime system does by default <em>not</em> bind schedulers
to logical processors.</p>
<note>
<p>If the Erlang runtime system is the only operating system
process that binds threads to logical processors, this
improves the performance of the runtime system. However,
if other operating system processes (for example
another Erlang runtime system) also bind threads to
logical processors, there can be a performance penalty
instead. This performance penalty can sometimes be
severe. If so, you are advised not to
bind the schedulers.</p>
</note>
<p>How schedulers are bound matters. For example, in
situations when there are fewer running processes than
schedulers online, the runtime system tries to migrate
processes to schedulers with low scheduler identifiers.
The more the schedulers are spread over the hardware,
the more resources are available to the runtime
system in such situations.</p>
<note>
<p>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 the
<c>error_logger</c>. If you want to verify that the
schedulers have bound as requested, call
<seealso marker="erlang#system_info_scheduler_bindings">
<c>erlang:system_info(scheduler_bindings)</c></seealso>.</p>
</note>
</item>
<tag><marker id="+sbwt"/>
<c>+sbwt none|very_short|short|medium|long|very_long</c></tag>
<item>
<p>Sets scheduler busy wait threshold. Defaults to <c>medium</c>.
The threshold determines how long schedulers are to busy
wait when running out of work before going to sleep.</p>
<note>
<p>This flag can be removed or changed at any time
without prior notice.</p>
</note>
</item>
<tag><marker id="+scl"/><c>+scl true|false</c></tag>
<item>
<p>Enables or disables scheduler compaction of load. By default
scheduler compaction of load is enabled. When enabled, load
balancing strives for a load distribution, which causes
as many scheduler threads as possible to be fully loaded (that is,
not run out of work). This is accomplished by migrating load
(for example, runnable processes) into a smaller set of schedulers
when schedulers frequently run out of work. When disabled,
the frequency with which schedulers run out of work is
not taken into account by the load balancing logic.</p>
<p><c>+scl false</c> is similar to
<seealso marker="#+sub"><c>+sub true</c></seealso>, but
<c>+sub true</c> also balances scheduler utilization
between schedulers.</p>
</item>
<tag><marker id="+sct"/><c>+sct CpuTopology</c></tag>
<item>
<list type="bulleted">
<item><c><![CDATA[<Id> = integer(); when 0 =< <Id> =< 65535]]></c>
</item>
<item><c><![CDATA[<IdRange> = <Id>-<Id>]]></c></item>
<item><c><![CDATA[<IdOrIdRange> = <Id> | <IdRange>]]></c></item>
<item><c><![CDATA[<IdList> = <IdOrIdRange>,<IdOrIdRange> |
<IdOrIdRange>]]></c></item>
<item><c><![CDATA[<LogicalIds> = L<IdList>]]></c></item>
<item><c><![CDATA[<ThreadIds> = T<IdList> | t<IdList>]]></c>
</item>
<item><c><![CDATA[<CoreIds> = C<IdList> | c<IdList>]]></c></item>
<item><c><![CDATA[<ProcessorIds> = P<IdList> | p<IdList>]]></c>
</item>
<item><c><![CDATA[<NodeIds> = N<IdList> | n<IdList>]]></c></item>
<item><c><![CDATA[<IdDefs> =
<LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds> |
<LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>]]></c>
</item>
<item><c><![CDATA[CpuTopology = <IdDefs>:<IdDefs> |
<IdDefs>]]></c></item>
</list>
<p>Sets a user-defined CPU topology. The user-defined
CPU topology overrides any automatically detected
CPU topology. The CPU topology is used when
<seealso marker="#+sbt">binding schedulers to logical
processors</seealso>.</p>
<p>Uppercase letters signify real identifiers and lowercase
letters signify fake identifiers only used for description
of the topology. Identifiers passed as real identifiers can
be used by the runtime system when trying to access specific
hardware; if they are incorrect the behavior is
undefined. Faked logical CPU identifiers are not accepted,
as there is no point in defining the CPU topology without
real logical CPU identifiers. Thread, core, processor, and
node identifiers can be omitted. If omitted, the thread ID
defaults to <c>t0</c>, the core ID defaults to <c>c0</c>,
the processor ID defaults to <c>p0</c>, and the node ID is
left undefined. Either each logical processor must
belong to only one NUMA node, or no logical
processors must belong to any NUMA nodes.</p>
<p>Both increasing and decreasing <c><![CDATA[<IdRange>]]></c>s
are allowed.</p>
<p>NUMA node identifiers are system wide. That is, each NUMA
node on the system must have a unique identifier. Processor
identifiers are also system wide. Core identifiers are
processor wide. Thread identifiers are core wide.</p>
<p>The order of the identifier types implies the hierarchy of the
CPU topology. The valid orders are as follows:</p>
<list type="bulleted">
<item>
<p><c><![CDATA[<LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds>]]></c>,
that is, thread is part of a core that is part of a processor,
which is part of a NUMA node.</p>
</item>
<item>
<p><c><![CDATA[<LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>]]></c>,
that is, thread is part of a core that is part of a NUMA node,
which is part of a processor.</p>
</item>
</list>
<p>A CPU topology can consist of both processor external, and
processor internal NUMA nodes as long as each logical processor
belongs to only one NUMA node. If
<c><![CDATA[<ProcessorIds>]]></c> is omitted, its default position
is before <c><![CDATA[<NodeIds>]]></c>. That is, the default is
processor external NUMA nodes.</p>
<p>If a list of identifiers is used in an
<c><![CDATA[<IdDefs>]]></c>:</p>
<list type="bulleted">
<item><c><![CDATA[<LogicalIds>]]></c> must be a list
of identifiers.</item>
<item>At least one other identifier type besides
<c><![CDATA[<LogicalIds>]]></c> must also have a
list of identifiers.</item>
<item>All lists of identifiers must produce the
same number of identifiers.</item>
</list>
<p>A simple example. A single quad core processor can be
described as follows:</p>
<pre>
% <input>erl +sct L0-3c0-3</input>
1> <input>erlang:system_info(cpu_topology).</input>
[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,2}},
{core,{logical,3}}]}]</pre>
<p>A more complicated example with two quad core
processors, each processor in its own NUMA node.
The ordering of logical processors is a bit weird.
This to give a better example of identifier lists:</p>
<pre>
% <input>erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1</input>
1> <input>erlang:system_info(cpu_topology).</input>
[{node,[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,3}},
{core,{logical,2}}]}]},
{node,[{processor,[{core,{logical,7}},
{core,{logical,4}},
{core,{logical,6}},
{core,{logical,5}}]}]}]</pre>
<p>As long as real identifiers are correct, it is OK
to pass a CPU topology that is not a correct
description of the CPU topology. When used with
care this can be very useful. This
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 number 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:</p>
<pre>
% <input>erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one</input>
% <input>erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two</input></pre>
<p>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.</p>
<p>Notice 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.</p>
<p>For more information, see
<seealso marker="erlang#system_info_cpu_topology">
<c>erlang:system_info(cpu_topology)</c></seealso>.</p>
</item>
<tag><marker id="+secio"/><c>+secio true|false</c></tag>
<item>
<p>Enables or disables eager check I/O scheduling. Defaults
to <c>true</c>. The default was changed from <c>false</c>
as from ERTS 7.0. The behavior before this
flag was introduced corresponds to <c>+secio false</c>.</p>
<p>The flag effects when schedulers will check for I/O
operations possible to execute, and when such I/O operations
will execute. As the parameter name implies,
schedulers are more eager to check for I/O when
<c>true</c> is passed. This, however, also implies that
execution of outstanding I/O operation is not
prioritized to the same extent as when <c>false</c> is
passed.</p>
<p><seealso marker="erlang#system_info_eager_check_io">
<c>erlang:system_info(eager_check_io)</c></seealso>
returns the value of this parameter used when starting
the virtual machine.</p>
</item>
<tag><marker id="+sfwi"/><c>+sfwi Interval</c></tag>
<item>
<p>Sets scheduler-forced wakeup interval. All run queues are
scanned each <c>Interval</c> milliseconds. While there are
sleeping schedulers in the system, one scheduler is woken
for each non-empty run queue found. <c>Interval</c> default
to <c>0</c>, meaning this feature is disabled.</p>
<note>
<p>This feature has been introduced as a temporary workaround
for long-executing native code, and native code that does not
bump reductions properly in OTP. When these bugs have be fixed,
this flag will be removed.</p>
</note>
</item>
<tag><marker id="+spp"/><c>+spp Bool</c></tag>
<item>
<p>Sets default scheduler hint for port parallelism. If set to
<c>true</c>, the virtual machine schedules port tasks when it
improves parallelism in the system. If set to <c>false</c>, the
virtual machine tries to perform port tasks immediately,
improving latency at the expense of parallelism. Default to
<c>false</c>. The default used can be inspected in runtime by
calling <seealso marker="erlang#system_info_port_parallelism">
<c>erlang:system_info(port_parallelism)</c></seealso>.
The default can be overridden on port creation by passing option
<seealso marker="erlang#open_port_parallelism">
<c>parallelism</c></seealso> to
<seealso marker="erlang#open_port/2">
<c>erlang:open_port/2</c></seealso></p>.
</item>
<tag><marker id="sched_thread_stack_size"/>
<c><![CDATA[+sss size]]></c></tag>
<item>
<p>Suggested stack size, in kilowords, for scheduler threads.
Valid range is 4-8192 kilowords. The default stack size is
OS-dependent.</p>
</item>
<tag><marker id="+stbt"/><c>+stbt BindType</c></tag>
<item>
<p>Tries to set the scheduler bind type. The same as flag
<seealso marker="#+sbt"><c>+sbt</c></seealso> except
how some errors are handled. For more information, see
<seealso marker="#+sbt"><c>+sbt</c></seealso>.</p>
</item>
<tag><marker id="+sub"/><c>+sub true|false</c></tag>
<item>
<p>Enables or disables
<seealso marker="erts:erlang#statistics_scheduler_wall_time">
scheduler utilization</seealso> balancing of load. By default
scheduler utilization balancing is disabled and instead scheduler
compaction of load is enabled, which strives for a load
distribution that causes as many scheduler threads as possible
to be fully loaded (that is, not run out of work). When scheduler
utilization balancing is enabled, the system instead tries to
balance scheduler utilization between schedulers. That is,
strive for equal scheduler utilization on all schedulers.</p>
<p><c>+sub true</c> is only supported on systems where the runtime
system detects and uses a monotonically increasing high-resolution
clock. On other systems, the runtime system fails to start.</p>
<p><c>+sub true</c> implies <seealso marker="#+scl">
<c>+scl false</c></seealso>. The difference between
<c>+sub true</c> and <c>+scl false</c> is that <c>+scl false</c>
does not try to balance the scheduler utilization.</p>
</item>
<tag><marker id="+swct"/>
<c>+swct very_eager|eager|medium|lazy|very_lazy</c></tag>
<item>
<p>Sets scheduler wake cleanup threshold. Defaults to <c>medium</c>.
Controls how eager schedulers are to be requesting
wakeup because of certain cleanup operations. When a lazy setting
is used, more outstanding cleanup operations can be left undone
while a scheduler is idling. When an eager setting is used,
schedulers are more frequently woken, potentially increasing
CPU-utilization.</p>
<note>
<p>This flag can be removed or changed at any time without prior
notice.</p>
</note>
</item>
<tag><marker id="+sws"/><c>+sws default|legacy</c></tag>
<item>
<p>Sets scheduler wakeup strategy. Default strategy changed in
ERTS 5.10 (Erlang/OTP R16A). This strategy was known as
<c>proposal</c> in Erlang/OTP R15. The <c>legacy</c> strategy
was used as default from R13 up to and including R15.</p>
<note>
<p>This flag can be removed or changed at any time without prior
notice.</p>
</note>
</item>
<tag><marker id="+swt"/>
<c>+swt very_low|low|medium|high|very_high</c></tag>
<item>
<p>Sets scheduler wakeup threshold. Defaults to <c>medium</c>.
The threshold determines when to wake up sleeping schedulers
when more work than can be handled by currently awake schedulers
exists. A low threshold causes earlier wakeups, and a high
threshold causes later wakeups. Early wakeups distribute work
over multiple schedulers faster, but work does more easily bounce
between schedulers.</p>
<note>
<p>This flag can be removed or changed at any time without prior
notice.</p>
</note>
</item>
</taglist>
</item>
<tag><marker id="+t"/><c><![CDATA[+t size]]></c></tag>
<item>
<p>Sets the maximum number of atoms the virtual machine can handle.
Defaults to 1,048,576.</p>
</item>
<tag><marker id="+T"/><c><![CDATA[+T Level]]></c></tag>
<item>
<p>Enables modified timing and sets the modified timing level. Valid
range is 0-9. The timing of the runtime system is changed. A high
level usually means a greater change than a low level. Changing the
timing can be very useful for finding timing-related bugs.</p>
<p>Modified timing affects the following:</p>
<taglist>
<tag>Process spawning</tag>
<item>A process calling <c><![CDATA[spawn]]></c>,
<c><![CDATA[spawn_link]]></c>, <c><![CDATA[spawn_monitor]]></c>,
or <c><![CDATA[spawn_opt]]></c> is scheduled out immediately
after completing the call. When higher modified timing levels are
used, the caller also sleeps for a while after it is scheduled out.
</item>
<tag>Context reductions</tag>
<item>The number of reductions a process is allowed to use before it
is scheduled out is increased or reduced.
</item>
<tag>Input reductions</tag>
<item>The number of reductions performed before checking I/O is
increased or reduced.
</item>
</taglist>
<note>
<p>Performance suffers when modified timing is enabled. This flag is
<em>only</em> intended for testing and debugging.</p>
<p><c><![CDATA[return_to]]></c> and <c><![CDATA[return_from]]></c>
trace messages are lost when tracing on the spawn BIFs.</p>
<p>This flag can be removed or changed at any time without prior
notice.</p>
</note>
</item>
<tag><c><![CDATA[+v]]></c></tag>
<item>
<p>Verbose.</p>
</item>
<tag><c><![CDATA[+V]]></c></tag>
<item>
<p>Makes the emulator print its version number.</p>
</item>
<tag><c><![CDATA[+W w | i | e]]></c></tag>
<item>
<p>Sets the mapping of warning messages for
<c><![CDATA[error_logger]]></c>. Messages sent to the error logger
using one of the warning routines can be mapped to errors
(<c><![CDATA[+W e]]></c>), warnings (<c><![CDATA[+W w]]></c>), or
information reports (<c><![CDATA[+W i]]></c>). Defaults to warnings.
The current mapping can be retrieved using
<c><![CDATA[error_logger:warning_map/0]]></c>. For more information,
see <seealso marker="kernel:error_logger#warning_map/0">
<c>error_logger:warning_map/0</c></seealso> in Kernel.</p>
</item>
<tag><c><![CDATA[+zFlag Value]]></c></tag>
<item>
<p>Miscellaneous flags:</p>
<taglist>
<tag><marker id="+zdbbl"/><c>+zdbbl size</c></tag>
<item>
<p>Sets the distribution buffer busy limit
(<seealso marker="erlang#system_info_dist_buf_busy_limit">
<c>dist_buf_busy_limit</c></seealso>)
in kilobytes. Valid range is 1-2097151. Defaults to 1024.</p>
<p>A larger buffer limit allows processes to buffer
more outgoing messages over the distribution. When the
buffer limit has been reached, sending processes will be
suspended until the buffer size has shrunk. The buffer
limit is per distribution channel. A higher limit
gives lower latency and higher throughput at the expense
of higher memory use.</p>
</item>
<tag><marker id="+zdntgc"/><c>+zdntgc time</c></tag>
<item>
<p>Sets the delayed node table garbage collection time
(<seealso marker="erlang#system_info_delayed_node_table_gc">
<c>delayed_node_table_gc</c></seealso>)
in seconds. Valid values are either <c>infinity</c> or
an integer in the range 0-100000000. Defaults to 60.</p>
<p>Node table entries that are not referred linger
in the table for at least the amount of time that this
parameter determines. The lingering prevents repeated
deletions and insertions in the tables from occurring.</p>
</item>
</taglist>
</item>
</taglist>
</section>
<section>
<marker id="environment_variables"></marker>
<title>Environment Variables</title>
<taglist>
<tag><c><![CDATA[ERL_CRASH_DUMP]]></c></tag>
<item>
<p>If the emulator needs to write a crash dump, the value of this
variable is the filename of the crash dump file.
If the variable is not set, the name of the crash dump file is
<c><![CDATA[erl_crash.dump]]></c> in the current directory.</p>
</item>
<tag><c><![CDATA[ERL_CRASH_DUMP_NICE]]></c></tag>
<item>
<p><em>Unix systems</em>: If the emulator needs to write a crash dump,
it uses the value of this variable to set the nice value
for the process, thus lowering its priority. Valid range is
1-39 (higher values are replaced with 39). The highest
value, 39, gives the process the lowest priority.</p>
</item>
<tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS]]></c></tag>
<item>
<p><em>Unix systems</em>: This variable gives the number of seconds
that the emulator is allowed to spend writing a crash dump. When the
given number of seconds have elapsed, the emulator is terminated by a
<c>SIGALRM</c> signal.</p>
<p>If the variable is <em>not</em> set or set to <c>0</c> seconds
(<c><![CDATA[ERL_CRASH_DUMP_SECONDS=0]]></c>), the runtime system does
not even attempt to write the crash dump file. It only terminates.</p>
<p>If the variable is set to negative value, such as
<c><![CDATA[ERL_CRASH_DUMP_SECONDS=-1]]></c>, the runtime system
waits indefinitely for the crash dump file to be written.</p>
<p>This variable is used with <seealso marker="kernel:heart">
<c>heart(3)</c></seealso> if <c>heart</c> is running:</p>
<taglist>
<tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=0]]></c></tag>
<item>Suppresses the writing a crash dump file entirely, thus
rebooting the runtime system immediately. This is the same as not
setting the environment variable.
</item>
<tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=-1]]></c></tag>
<item>Setting the environment variable to a negative value causes the
termination of the runtime system to wait until the crash dump file
has been completly written.
</item>
<tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=S]]></c></tag>
<item>Waits for <c>S</c> seconds to complete the crash dump file and
then terminates the runtime system.
</item>
</taglist>
</item>
<tag><c><![CDATA[ERL_CRASH_DUMP_BYTES]]></c></tag>
<item>
<p>This variable sets the maximum size of a crash dump file in bytes.
The crash dump will be truncated if this limit is exceeded. If the
variable is not set, no size limit is enforced by default. If the
variable is set to <c>0</c>, the runtime system does not even attempt
to write a crash dump file.</p>
<p>Introduced in ERTS 8.1.2 (Erlang/OTP 19.2).</p>
</item>
<tag><marker id="ERL_AFLAGS"/><c><![CDATA[ERL_AFLAGS]]></c></tag>
<item>
<p>The content of this variable is added to the beginning of the
command line for <c><![CDATA[erl]]></c>.</p>
<p>Flag <c><![CDATA[-extra]]></c> is treated in a special way. Its
scope ends at the end of the environment variable content. Arguments
following an <c><![CDATA[-extra]]></c> flag are moved on the command
line into section <c><![CDATA[-extra]]></c>, that is, the end of the
command line following an <c><![CDATA[-extra]]></c> flag.</p>
</item>
<tag><marker id="ERL_ZFLAGS"/><c><![CDATA[ERL_ZFLAGS]]></c> and
<marker id="ERL_FLAGS"/><c><![CDATA[ERL_FLAGS]]></c></tag>
<item>
<p>The content of these variables are added to the end of the command
line for <c><![CDATA[erl]]></c>.</p>
<p>Flag <c><![CDATA[-extra]]></c> is treated in a special way. Its
scope ends at the end of the environment variable content. Arguments
following an <c><![CDATA[-extra]]></c> flag are moved on the command
line into section <c><![CDATA[-extra]]></c>, that is, the end of the
command line following an <c><![CDATA[-extra]]></c> flag.</p>
</item>
<tag><c><![CDATA[ERL_LIBS]]></c></tag>
<item>
<p>Contains a list of additional library directories that the code
server searches for applications and adds to the code path; see
<seealso marker="kernel:code"><c>code(3)</c></seealso>.</p>
</item>
<tag><c><![CDATA[ERL_EPMD_ADDRESS]]></c></tag>
<item>
<p>Can be set to a comma-separated list of IP addresses, in which case
the <seealso marker="epmd"><c>epmd</c></seealso> daemon listens only
on the specified address(es) and on the loopback address (which is
implicitly added to the list if it has not been specified).</p>
</item>
<tag><c><![CDATA[ERL_EPMD_PORT]]></c></tag>
<item>
<p>Can contain the port number to use when communicating with
<seealso marker="epmd"><c>epmd</c></seealso>. The default port works
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 <c>epmd</c> port number.</p>
</item>
</taglist>
</section>
<section>
<marker id="configuration"></marker>
<title>Configuration</title>
<p>The standard Erlang/OTP system can be reconfigured to change the default
behavior on startup.</p>
<taglist>
<tag>The <c>.erlang</c> startup file</tag>
<item>
<p>When Erlang/OTP is started, the system searches for a file named
<c>.erlang</c> in the directory where Erlang/OTP is started. If not
found, the user's home directory is searched for an <c>.erlang</c>
file.</p>
<p>If an <c>.erlang</c> file is found, it is assumed to contain valid
Erlang expressions. These expressions are evaluated as if they were
input to the shell.</p>
<p>A typical <c>.erlang</c> file contains a set of search paths, for
example:</p>
<code type="none"><![CDATA[
io:format("executing user profile in HOME/.erlang\n",[]).
code:add_path("/home/calvin/test/ebin").
code:add_path("/home/hobbes/bigappl-1.2/ebin").
io:format(".erlang rc finished\n",[]). ]]></code>
</item>
<tag>user_default and shell_default</tag>
<item>
<p>Functions in the shell that are not prefixed by a module name are
assumed to be functional objects (funs), built-in functions (BIFs),
or belong to the module <c>user_default</c> or
<c>shell_default</c>.</p>
<p>To include private shell commands, define them in a module
<c>user_default</c> and add the following argument as the first line
in the <c>.erlang</c> file:</p>
<code type="none"><![CDATA[
code:load_abs("..../user_default"). ]]></code>
</item>
<tag>erl</tag>
<item>
<p>If the contents of <c>.erlang</c> are changed and a private version
of <c>user_default</c> is defined, the Erlang/OTP environment can be
customized. More powerful changes can be made by supplying
command-line arguments in the startup script <c>erl</c>. For more
information, see <seealso marker="init"><c>init(3)</c></seealso>.</p>
</item>
</taglist>
</section>
<section>
<title>See Also</title>
<p><seealso marker="epmd"><c>epmd(1)</c></seealso>,
<seealso marker="erl_prim_loader"><c>erl_prim_loader(3)</c></seealso>,
<seealso marker="erts_alloc"><c>erts_alloc(3)</c></seealso>,
<seealso marker="init"><c>init(3)</c></seealso>,
<seealso marker="kernel:application">
<c>application(3)</c></seealso>,
<seealso marker="kernel:auth"><c>auth(3)</c></seealso>,
<seealso marker="kernel:code"><c>code(3)</c></seealso>,
<seealso marker="kernel:erl_boot_server">
<c>erl_boot_server(3)</c></seealso>,
<seealso marker="kernel:heart"><c>heart(3)</c></seealso>,
<seealso marker="kernel:net_kernel"><c>net_kernel(3)</c></seealso>,
<seealso marker="tools:make"><c>make(3)</c></seealso></p>
</section>
</comref>
