path: root/docs/users_guide/runtime_control.xml

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<sect1 id="runtime-control">
  <title>Running a compiled program</title>

  <indexterm><primary>runtime control of Haskell programs</primary></indexterm>
  <indexterm><primary>running, compiled program</primary></indexterm>
  <indexterm><primary>RTS options</primary></indexterm>

  <para>To make an executable program, the GHC system compiles your
  code and then links it with a non-trivial runtime system (RTS),
  which handles storage management, profiling, etc.</para>

  <para>You have some control over the behaviour of the RTS, by giving
  special command-line arguments to your program.</para>

  <para>When your Haskell program starts up, its RTS extracts
  command-line arguments bracketed between
  <option>+RTS</option><indexterm><primary><option>+RTS</option></primary></indexterm>
  and
  <option>-RTS</option><indexterm><primary><option>-RTS</option></primary></indexterm>
  as its own.  For example:</para>

<screen>
% ./a.out -f +RTS -p -S -RTS -h foo bar
</screen>

  <para>The RTS will snaffle <option>-p</option> <option>-S</option>
  for itself, and the remaining arguments <literal>-f -h foo bar</literal>
  will be handed to your program if/when it calls
  <function>System.getArgs</function>.</para>

  <para>No <option>-RTS</option> option is required if the
  runtime-system options extend to the end of the command line, as in
  this example:</para>

<screen>
% hls -ltr /usr/etc +RTS -A5m
</screen>

  <para>If you absolutely positively want all the rest of the options
  in a command line to go to the program (and not the RTS), use a
  <option>&ndash;&ndash;RTS</option><indexterm><primary><option>--RTS</option></primary></indexterm>.</para>

  <para>As always, for RTS options that take
  <replaceable>size</replaceable>s: If the last character of
  <replaceable>size</replaceable> is a K or k, multiply by 1000; if an
  M or m, by 1,000,000; if a G or G, by 1,000,000,000.  (And any
  wraparound in the counters is <emphasis>your</emphasis>
  fault!)</para>

  <para>Giving a <literal>+RTS -f</literal>
  <indexterm><primary><option>-f</option></primary><secondary>RTS option</secondary></indexterm> option
  will print out the RTS options actually available in your program
  (which vary, depending on how you compiled).</para>

  <para>NOTE: since GHC is itself compiled by GHC, you can change RTS
  options in the compiler using the normal
  <literal>+RTS ... -RTS</literal>
  combination.  eg. to increase the maximum heap
  size for a compilation to 128M, you would add
  <literal>+RTS -M128m -RTS</literal>
  to the command line.</para>

  <sect2 id="rts-optinos-environment">
    <title>Setting global RTS options</title>

    <indexterm><primary>RTS options</primary><secondary>from the environment</secondary></indexterm>
    <indexterm><primary>environment variable</primary><secondary>for
    setting RTS options</secondary></indexterm>

    <para>RTS options are also taken from the environment variable
    <envar>GHCRTS</envar><indexterm><primary><envar>GHCRTS</envar></primary>
      </indexterm>.  For example, to set the maximum heap size
    to 128M for all GHC-compiled programs (using an
    <literal>sh</literal>-like shell):</para>

<screen>
   GHCRTS='-M128m'
   export GHCRTS
</screen>

    <para>RTS options taken from the <envar>GHCRTS</envar> environment
    variable can be overridden by options given on the command
    line.</para>

  </sect2>

  <sect2 id="rts-options-misc">
    <title>Miscellaneous RTS options</title>

    <variablelist>
     <varlistentry>
       <term><option>-V<replaceable>secs</replaceable></option>
       <indexterm><primary><option>-V</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>Sets the interval that the RTS clock ticks at.  The
         runtime uses a single timer signal to count ticks; this timer
         signal is used to control the context switch timer (<xref
         linkend="using-concurrent" />) and the heap profiling
         timer <xref linkend="rts-options-heap-prof" />.  Also, the
         time profiler uses the RTS timer signal directly to record
         time profiling samples.</para>

         <para>Normally, setting the <option>-V</option> option
         directly is not necessary: the resolution of the RTS timer is
         adjusted automatically if a short interval is requested with
         the <option>-C</option> or <option>-i</option> options.
         However, setting <option>-V</option> is required in order to
         increase the resolution of the time profiler.</para>

         <para>Using a value of zero disables the RTS clock
         completetly, and has the effect of disabling timers that
         depend on it: the context switch timer and the heap profiling
         timer.  Context switches will still happen, but
         deterministically and at a rate much faster than normal.
         Disabling the interval timer is useful for debugging, because
         it eliminates a source of non-determinism at runtime.</para>
       </listitem>
     </varlistentry>

     <varlistentry>
       <term><option>--install-signal-handlers=<replaceable>yes|no</replaceable></option>
       <indexterm><primary><option>--install-signal-handlers</option></primary><secondary>RTS
       option</secondary></indexterm></term>
       <listitem>
         <para>If yes (the default), the RTS installs signal handlers to catch
         things like ctrl-C. This option is primarily useful for when
         you are using the Haskell code as a DLL, and want to set your
         own signal handlers.</para>
       </listitem>
     </varlistentry>
    </variablelist>
  </sect2>

  <sect2 id="rts-options-gc">
    <title>RTS options to control the garbage collector</title>

    <indexterm><primary>garbage collector</primary><secondary>options</secondary></indexterm>
    <indexterm><primary>RTS options</primary><secondary>garbage collection</secondary></indexterm>

    <para>There are several options to give you precise control over
    garbage collection.  Hopefully, you won't need any of these in
    normal operation, but there are several things that can be tweaked
    for maximum performance.</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-A</option><replaceable>size</replaceable>
          <indexterm><primary><option>-A</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>allocation area, size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 256k&rsqb; Set the allocation area size
          used by the garbage collector.  The allocation area
          (actually generation 0 step 0) is fixed and is never resized
          (unless you use <option>-H</option>, below).</para>

	  <para>Increasing the allocation area size may or may not
          give better performance (a bigger allocation area means
          worse cache behaviour but fewer garbage collections and less
          promotion).</para>

	  <para>With only 1 generation (<option>-G1</option>) the
          <option>-A</option> option specifies the minimum allocation
          area, since the actual size of the allocation area will be
          resized according to the amount of data in the heap (see
          <option>-F</option>, below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-c</option>
          <indexterm><primary><option>-c</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>garbage collection</primary><secondary>compacting</secondary></indexterm>
          <indexterm><primary>compacting garbage collection</primary></indexterm>
        </term>
	<listitem>
	  <para>Use a compacting algorithm for collecting the oldest
	  generation.  By default, the oldest generation is collected
	  using a copying algorithm; this option causes it to be
	  compacted in-place instead.  The compaction algorithm is
	  slower than the copying algorithm, but the savings in memory
	  use can be considerable.</para>

	  <para>For a given heap size (using the <option>-H</option>
	  option), compaction can in fact reduce the GC cost by
	  allowing fewer GCs to be performed.  This is more likely
	  when the ratio of live data to heap size is high, say
	  &gt;30&percnt;.</para>

	  <para>NOTE: compaction doesn't currently work when a single
	  generation is requested using the <option>-G1</option>
	  option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term><option>-c</option><replaceable>n</replaceable></term>

	<listitem>
	  <para>&lsqb;Default: 30&rsqb; Automatically enable
	  compacting collection when the live data exceeds
	  <replaceable>n</replaceable>&percnt; of the maximum heap size
	  (see the <option>-M</option> option).  Note that the maximum
	  heap size is unlimited by default, so this option has no
	  effect unless the maximum heap size is set with
	  <option>-M</option><replaceable>size</replaceable>. </para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-F</option><replaceable>factor</replaceable>
          <indexterm><primary><option>-F</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, factor</primary></indexterm>
        </term>
	<listitem>

	  <para>&lsqb;Default: 2&rsqb; This option controls the amount
          of memory reserved for the older generations (and in the
          case of a two space collector the size of the allocation
          area) as a factor of the amount of live data.  For example,
          if there was 2M of live data in the oldest generation when
          we last collected it, then by default we'll wait until it
          grows to 4M before collecting it again.</para>

	  <para>The default seems to work well here.  If you have
          plenty of memory, it is usually better to use
          <option>-H</option><replaceable>size</replaceable> than to
          increase
          <option>-F</option><replaceable>factor</replaceable>.</para>

	  <para>The <option>-F</option> setting will be automatically
          reduced by the garbage collector when the maximum heap size
          (the <option>-M</option><replaceable>size</replaceable>
          setting) is approaching.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-G</option><replaceable>generations</replaceable>
          <indexterm><primary><option>-G</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>generations, number of</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 2&rsqb; Set the number of generations
          used by the garbage collector.  The default of 2 seems to be
          good, but the garbage collector can support any number of
          generations.  Anything larger than about 4 is probably not a
          good idea unless your program runs for a
          <emphasis>long</emphasis> time, because the oldest
          generation will hardly ever get collected.</para>

	  <para>Specifying 1 generation with <option>+RTS -G1</option>
          gives you a simple 2-space collector, as you would expect.
          In a 2-space collector, the <option>-A</option> option (see
          above) specifies the <emphasis>minimum</emphasis> allocation
          area size, since the allocation area will grow with the
          amount of live data in the heap.  In a multi-generational
          collector the allocation area is a fixed size (unless you
          use the <option>-H</option> option, see below).</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-H</option><replaceable>size</replaceable>
          <indexterm><primary><option>-H</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, suggested</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 0&rsqb; This option provides a
          &ldquo;suggested heap size&rdquo; for the garbage collector.  The
          garbage collector will use about this much memory until the
          program residency grows and the heap size needs to be
          expanded to retain reasonable performance.</para>

	  <para>By default, the heap will start small, and grow and
          shrink as necessary.  This can be bad for performance, so if
          you have plenty of memory it's worthwhile supplying a big
          <option>-H</option><replaceable>size</replaceable>.  For
          improving GC performance, using
          <option>-H</option><replaceable>size</replaceable> is
          usually a better bet than
          <option>-A</option><replaceable>size</replaceable>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
	  <option>-I</option><replaceable>seconds</replaceable>
	  <indexterm><primary><option>-I</option></primary>
	    <secondary>RTS option</secondary>
	  </indexterm>
	  <indexterm><primary>idle GC</primary>
	  </indexterm>
	  </term>
	<listitem>
	  <para>(default: 0.3) In the threaded and SMP versions of the RTS (see
	    <option>-threaded</option>, <xref linkend="options-linker" />), a
	    major GC is automatically performed if the runtime has been idle
	    (no Haskell computation has been running) for a period of time.
	    The amount of idle time which must pass before a GC is performed is
	    set by the <option>-I</option><replaceable>seconds</replaceable>
	    option.  Specifying <option>-I0</option> disables the idle GC.</para>

	  <para>For an interactive application, it is probably a good idea to
	    use the idle GC, because this will allow finalizers to run and
	    deadlocked threads to be detected in the idle time when no Haskell
	    computation is happening.  Also, it will mean that a GC is less
	    likely to happen when the application is busy, and so
	    responsiveness may be improved.   However, if the amount of live data in
	    the heap is particularly large, then the idle GC can cause a
	    significant delay, and too small an interval could adversely affect
	    interactive responsiveness.</para>

	  <para>This is an experimental feature, please let us know if it
	    causes problems and/or could benefit from further tuning.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
         <option>-k</option><replaceable>size</replaceable>
         <indexterm><primary><option>-k</option></primary><secondary>RTS option</secondary></indexterm>
         <indexterm><primary>stack, minimum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 1k&rsqb; Set the initial stack size for
          new threads.  Thread stacks (including the main thread's
          stack) live on the heap, and grow as required.  The default
          value is good for concurrent applications with lots of small
          threads; if your program doesn't fit this model then
          increasing this option may help performance.</para>

	  <para>The main thread is normally started with a slightly
          larger heap to cut down on unnecessary stack growth while
          the program is starting up.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-K</option><replaceable>size</replaceable>
          <indexterm><primary><option>-K</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>stack, maximum size</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: 8M&rsqb; Set the maximum stack size for
          an individual thread to <replaceable>size</replaceable>
          bytes.  This option is there purely to stop the program
          eating up all the available memory in the machine if it gets
          into an infinite loop.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-m</option><replaceable>n</replaceable>
          <indexterm><primary><option>-m</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap, minimum free</primary></indexterm>
        </term>
	<listitem>
	  <para>Minimum &percnt; <replaceable>n</replaceable> of heap
          which must be available for allocation.  The default is
          3&percnt;.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-M</option><replaceable>size</replaceable>
          <indexterm><primary><option>-M</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>heap size, maximum</primary></indexterm>
        </term>
	<listitem>
	  <para>&lsqb;Default: unlimited&rsqb; Set the maximum heap size to
          <replaceable>size</replaceable> bytes.  The heap normally
          grows and shrinks according to the memory requirements of
          the program.  The only reason for having this option is to
          stop the heap growing without bound and filling up all the
          available swap space, which at the least will result in the
          program being summarily killed by the operating
          system.</para>

	  <para>The maximum heap size also affects other garbage
	  collection parameters: when the amount of live data in the
	  heap exceeds a certain fraction of the maximum heap size,
	  compacting collection will be automatically enabled for the
	  oldest generation, and the <option>-F</option> parameter
	  will be reduced in order to avoid exceeding the maximum heap
	  size.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-s</option><replaceable>file</replaceable>
          <indexterm><primary><option>-s</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<term>
          <option>-S</option><replaceable>file</replaceable>
          <indexterm><primary><option>-S</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Write modest (<option>-s</option>) or verbose
          (<option>-S</option>) garbage-collector statistics into file
          <replaceable>file</replaceable>. The default
          <replaceable>file</replaceable> is
          <filename><replaceable>program</replaceable>.stat</filename>. The
          <replaceable>file</replaceable> <constant>stderr</constant>
          is treated specially, with the output really being sent to
          <constant>stderr</constant>.</para>

	  <para>This option is useful for watching how the storage
          manager adjusts the heap size based on the current amount of
          live data.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-t<replaceable>file</replaceable></option>
          <indexterm><primary><option>-t</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Write a one-line GC stats summary after running the
	  program.  This output is in the same format as that produced
	  by the <option>-Rghc-timing</option> option.</para>

	  <para>As with <option>-s</option>, the default
          <replaceable>file</replaceable> is
          <filename><replaceable>program</replaceable>.stat</filename>. The
          <replaceable>file</replaceable> <constant>stderr</constant>
          is treated specially, with the output really being sent to
          <constant>stderr</constant>.</para>
	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2>
    <title>RTS options for profiling and parallelism</title>

    <para>The RTS options related to profiling are described in <xref
    linkend="rts-options-heap-prof"/>, those for concurrency in
      <xref linkend="using-concurrent" />, and those for parallelism in
      <xref linkend="parallel-options"/>.</para>
  </sect2>

  <sect2 id="rts-options-debugging">
    <title>RTS options for hackers, debuggers, and over-interested
    souls</title>

    <indexterm><primary>RTS options, hacking/debugging</primary></indexterm>

    <para>These RTS options might be used (a)&nbsp;to avoid a GHC bug,
    (b)&nbsp;to see &ldquo;what's really happening&rdquo;, or
    (c)&nbsp;because you feel like it.  Not recommended for everyday
    use!</para>

    <variablelist>

      <varlistentry>
	<term>
          <option>-B</option>
          <indexterm><primary><option>-B</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Sound the bell at the start of each (major) garbage
	  collection.</para>

	  <para>Oddly enough, people really do use this option!  Our
          pal in Durham (England), Paul Callaghan, writes: &ldquo;Some
          people here use it for a variety of
          purposes&mdash;honestly!&mdash;e.g., confirmation that the
          code/machine is doing something, infinite loop detection,
          gauging cost of recently added code. Certain people can even
          tell what stage &lsqb;the program&rsqb; is in by the beep
          pattern. But the major use is for annoying others in the
          same office&hellip;&rdquo;</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-D</option><replaceable>num</replaceable>
          <indexterm><primary>-D</primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>An RTS debugging flag; varying quantities of output
          depending on which bits are set in
          <replaceable>num</replaceable>.  Only works if the RTS was
          compiled with the <option>DEBUG</option> option.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-r</option><replaceable>file</replaceable>
          <indexterm><primary><option>-r</option></primary><secondary>RTS option</secondary></indexterm>
          <indexterm><primary>ticky ticky profiling</primary></indexterm>
          <indexterm><primary>profiling</primary><secondary>ticky ticky</secondary></indexterm>
        </term>
	<listitem>
	  <para>Produce &ldquo;ticky-ticky&rdquo; statistics at the
          end of the program run.  The <replaceable>file</replaceable>
          business works just like on the <option>-S</option> RTS
          option (above).</para>

	  <para>&ldquo;Ticky-ticky&rdquo; statistics are counts of
          various program actions (updates, enters, etc.)  The program
          must have been compiled using
          <option>-ticky</option><indexterm><primary><option>-ticky</option></primary></indexterm>
          (a.k.a. &ldquo;ticky-ticky profiling&rdquo;), and, for it to
          be really useful, linked with suitable system libraries.
          Not a trivial undertaking: consult the installation guide on
          how to set things up for easy &ldquo;ticky-ticky&rdquo;
          profiling.  For more information, see <xref
          linkend="ticky-ticky"/>.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-xc</option>
          <indexterm><primary><option>-xc</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>(Only available when the program is compiled for
	  profiling.)  When an exception is raised in the program,
	  this option causes the current cost-centre-stack to be
	  dumped to <literal>stderr</literal>.</para>

	  <para>This can be particularly useful for debugging: if your
	  program is complaining about a <literal>head []</literal>
	  error and you haven't got a clue which bit of code is
	  causing it, compiling with <literal>-prof
	  -auto-all</literal> and running with <literal>+RTS -xc
	  -RTS</literal> will tell you exactly the call stack at the
	  point the error was raised.</para>

	  <para>The output contains one line for each exception raised
	  in the program (the program might raise and catch several
	  exceptions during its execution), where each line is of the
	  form:</para>

<screen>
&lt; cc<subscript>1</subscript>, ..., cc<subscript>n</subscript> &gt;
</screen>
	  <para>each <literal>cc</literal><subscript>i</subscript> is
	  a cost centre in the program (see <xref
	  linkend="cost-centres"/>), and the sequence represents the
	  &ldquo;call stack&rdquo; at the point the exception was
	  raised.  The leftmost item is the innermost function in the
	  call stack, and the rightmost item is the outermost
	  function.</para>

	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <option>-Z</option>
          <indexterm><primary><option>-Z</option></primary><secondary>RTS option</secondary></indexterm>
        </term>
	<listitem>
	  <para>Turn <emphasis>off</emphasis> &ldquo;update-frame
          squeezing&rdquo; at garbage-collection time.  (There's no
          particularly good reason to turn it off, except to ensure
          the accuracy of certain data collected regarding thunk entry
          counts.)</para>
	</listitem>
      </varlistentry>
    </variablelist>

  </sect2>

  <sect2 id="rts-hooks">
    <title>&ldquo;Hooks&rdquo; to change RTS behaviour</title>

    <indexterm><primary>hooks</primary><secondary>RTS</secondary></indexterm>
    <indexterm><primary>RTS hooks</primary></indexterm>
    <indexterm><primary>RTS behaviour, changing</primary></indexterm>

    <para>GHC lets you exercise rudimentary control over the RTS
    settings for any given program, by compiling in a
    &ldquo;hook&rdquo; that is called by the run-time system.  The RTS
    contains stub definitions for all these hooks, but by writing your
    own version and linking it on the GHC command line, you can
    override the defaults.</para>

    <para>Owing to the vagaries of DLL linking, these hooks don't work
    under Windows when the program is built dynamically.</para>

    <para>The hook <literal>ghc_rts_opts</literal><indexterm><primary><literal>ghc_rts_opts</literal></primary>
      </indexterm>lets you set RTS
    options permanently for a given program.  A common use for this is
    to give your program a default heap and/or stack size that is
    greater than the default.  For example, to set <literal>-H128m
    -K1m</literal>, place the following definition in a C source
    file:</para>

<programlisting>
char *ghc_rts_opts = "-H128m -K1m";
</programlisting>

    <para>Compile the C file, and include the object file on the
    command line when you link your Haskell program.</para>

    <para>These flags are interpreted first, before any RTS flags from
    the <literal>GHCRTS</literal> environment variable and any flags
    on the command line.</para>

    <para>You can also change the messages printed when the runtime
    system &ldquo;blows up,&rdquo; e.g., on stack overflow.  The hooks
    for these are as follows:</para>

    <variablelist>

      <varlistentry>
	<term>
          <function>void OutOfHeapHook (unsigned long, unsigned long)</function>
          <indexterm><primary><function>OutOfHeapHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The heap-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void StackOverflowHook (long int)</function>
          <indexterm><primary><function>StackOverflowHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The stack-overflow message.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>
          <function>void MallocFailHook (long int)</function>
          <indexterm><primary><function>MallocFailHook</function></primary></indexterm>
        </term>
	<listitem>
	  <para>The message printed if <function>malloc</function>
	  fails.</para>
	</listitem>
      </varlistentry>
    </variablelist>

    <para>For examples of the use of these hooks, see GHC's own
    versions in the file
    <filename>ghc/compiler/parser/hschooks.c</filename> in a GHC
    source tree.</para>
  </sect2>

  <sect2>
    <title>Getting information about the RTS</title>

    <indexterm><primary>RTS</primary></indexterm>

    <para>It is possible to ask the RTS to give some information about
    itself. To do this, use the <option>--info</option> flag, e.g.</para>
<screen>
$ ./a.out +RTS --info
 [("GHC RTS", "Yes")
 ,("GHC version", "6.7")
 ,("RTS way", "rts_p")
 ,("Host platform", "x86_64-unknown-linux")
 ,("Build platform", "x86_64-unknown-linux")
 ,("Target platform", "x86_64-unknown-linux")
 ,("Compiler unregisterised", "NO")
 ,("Tables next to code", "YES")
 ]
</screen>
    <para>The information is formatted such that it can be read as a
    of type <literal>[(String, String)]</literal>.</para>
  </sect2>
</sect1>

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