/usr/lib/swi-prolog/doc/Manual/limits.html

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<h2 id="sec:limits"><a id="sec:2.19"><span class="sec-nr">2.19</span> <span class="sec-title">System 
limits</span></a></h2>

<a id="sec:limits"></a>

<p><h3 id="sec:memlimit"><a id="sec:2.19.1"><span class="sec-nr">2.19.1</span> <span class="sec-title">Limits 
on memory areas</span></a></h3>

<a id="sec:memlimit"></a>

<p>SWI-Prolog has a number of memory areas which are only enlarged to a 
certain limit. The internal data representation limits the local, global 
and trail stack to 128&nbsp;MB on 32-bit processors, or more generally 
to <var>2 ** bits-per-pointer - 5</var> bytes. Considering that almost 
all modern hardware can deal with this amount of memory with ease, the 
default limits are set to their maximum on 32-bit hardware. The 
representation limits can easily exceed physical memory on 64-bit 
hardware. The default limits on 64-bit hardware are double that of 
32-bit hardware, which allows for storing the same amount of (Prolog) 
data.

<p>The limits can be changed from the command line as well as at runtime 
using <a id="idx:setprologstack2:242"></a><a class="pred" href="memory.html#set_prolog_stack/2">set_prolog_stack/2</a>. 
The table below shows these areas. The first column gives the option 
name to modify the size of the area. The option character is immediately 
followed by a number and optionally by a
<code>k</code> or <code>m</code>. With <code>k</code> or no unit 
indicator, the value is interpreted in Kbytes (1024&nbsp;bytes); with <code>m</code>, 
the value is interpreted in Mbytes (<var>1024 &times; 1024</var> bytes).

<p>The PrologScript facility described in <a class="sec" href="compilation.html">section 
2.10.2.1</a> provides a mechanism for specifying options with the load 
file. On Windows the default stack sizes are controlled using the 
Windows <a id="idx:registry:243">registry</a> on the key <code>HKEY_CURRENT_USER\Software\SWI\Prolog</code> 
using the names <code>localSize</code>, <code>globalSize</code> and <code>trailSize</code>. 
The value is a <code>DWORD</code> expressing the default stack size in 
Kbytes. A GUI for modifying these values is provided using the XPCE 
package. To use this, start the XPCE manual tools using <a id="idx:manpce0:244"></a><span class="pred-ext">manpce/0</span>, 
after which you find <i>Preferences</i> in the <i>File</i> menu.

<p>Considering portability, applications that need to modify the default 
limits are advised to do so using <a id="idx:setprologstack2:245"></a><a class="pred" href="memory.html#set_prolog_stack/2">set_prolog_stack/2</a>.

<p><table class="latex frame-box center">
<tr><td align=center>Option</td><td align=center>Default</td><td>Area 
name</td><td>Description </td></tr>
<tr class="hline"><td align=center><strong>-L</strong> </td><td align=center>128M</td><td><b>local 
stack</b></td><td>The local stack is used to store the execution 
environments of procedure invocations. The space for an environment is 
reclaimed when it fails, exits without leaving choice points, the 
alternatives are cut off with the !/0 predicate or no choice points have 
been created since the invocation and the last subclause is started 
(last call optimisation). </td></tr>
<tr><td align=center><strong>-G</strong> </td><td align=center>128M</td><td><b>global 
stack</b></td><td>The global stack is used to store terms created during 
Prolog's execution. Terms on this stack will be reclaimed by 
backtracking to a point before the term was created or by garbage 
collection (provided the term is no longer referenced). </td></tr>
<tr><td align=center><strong>-T</strong> </td><td align=center>128M</td><td><b>trail 
stack</b></td><td>The trail stack is used to store assignments during 
execution. Entries on this stack remain alive until backtracking before 
the point of creation or the garbage collector determines they are no 
longer needed. </td></tr>
<tr><td align=center><strong>-A</strong> </td><td align=center>1M</td><td><b>argument 
stack</b></td><td>The argument stack is used to store one of the Virtual 
Machine's registers. The amount of space needed on this stack is 
determined entirely by the depth in which terms are nested in the 
clauses that constitute the program. Overflow is unlikely. </td></tr>
</table>
<div class="caption"><b>Table 2 : </b>Memory areas</div>
<a id="tab:areas"></a>

<p><h4 id="sec:heap"><a id="sec:2.19.1.1"><span class="sec-nr">2.19.1.1</span> <span class="sec-title">The 
heap</span></a></h4>

<a id="sec:heap"></a>

<p><a id="idx:stackmemorymanagement:246"></a><a id="idx:memorylayout:247"></a>With 
the heap, we refer to the memory area used by malloc() and friends. 
SWI-Prolog uses the area to store atoms, functors, predicates and their 
clauses, records and other dynamic data. No limits are imposed on the 
addresses returned by malloc() and friends.

<p><h3 id="sec:morelimits"><a id="sec:2.19.2"><span class="sec-nr">2.19.2</span> <span class="sec-title">Other 
Limits</span></a></h3>

<a id="sec:morelimits"></a>

<dl class="latex">
<dt><b>Clauses</b></dt>
<dd>
The only limit on clauses is their arity (the number of arguments to the 
head), which is limited to 1024. Raising this limit is easy and 
relatively cheap; removing it is harder.</dd>
<dt><b>Atoms and Strings</b></dt>
<dd>
SWI-Prolog has no limits on the sizes of atoms and strings. <a id="idx:read1:248"></a><a class="pred" href="termrw.html#read/1">read/1</a> 
and its derivatives, however, normally limit the number of newlines in 
an atom or string to 6 to improve error detection and recovery. This can 
be switched off with <a id="idx:stylecheck1:249"></a><a class="pred" href="debugger.html#style_check/1">style_check/1</a>.

<p>The number of atoms is limited to 16777216 (16M) on 32-bit machines. 
On 64-bit machines this is virtually unlimited. See also <a class="sec" href="foreigninclude.html">section 
9.4.2.1</a>.</dd>
<dt><b>Memory areas</b></dt>
<dd>
On 32-bit hardware, SWI-Prolog data is packed in a 32-bit word, which 
contains both type and value information. The size of the various memory 
areas is limited to 128&nbsp;MB for each of the areas, except for the 
program heap, which is not limited. On 64-bit hardware there are no 
meaningful limits.</dd>
<dt><b>Nesting of terms</b></dt>
<dd>
Most built-in predicates that process Prolog terms create an explicitly 
managed stack and perform optimization for processing the last argument 
of a term. This implies they can process deeply nested terms at constant 
and low usage of the C stack, and the system raises a resource error if 
no more stack can be allocated. Currently only <a id="idx:read1:250"></a><a class="pred" href="termrw.html#read/1">read/1</a> 
and <a id="idx:write1:251"></a><a class="pred" href="termrw.html#write/1">write/1</a> 
(and all variations thereof) still use the C stack and may cause the 
system to crash in an uncontrolled way (i.e., not mapped to a Prolog 
exception that can be caught).</dd>
<dt><b>Integers</b></dt>
<dd>
On most systems SWI-Prolog is compiled with support for unbounded 
integers by means of the GNU GMP library. In practice this means that 
integers are bound by the global stack size. Too large integers cause a
<code>resource_error</code>. On systems that lack GMP, integers are 
64-bit on 32- as well as 64-bit machines.

<p>Integers up to the value of the <a class="flag" href="flags.html#flag:max_tagged_integer">max_tagged_integer</a> 
Prolog flag are represented more efficiently on the stack. For integers 
that appear in clauses, the value (below <a class="flag" href="flags.html#flag:max_tagged_integer">max_tagged_integer</a> 
or not) has little impact on the size of the clause.</dd>
<dt><b>Floating point numbers</b></dt>
<dd>
Floating point numbers are represented as C-native double precision 
floats, 64-bit IEEE on most machines.
</dd>
</dl>

<p><h3 id="sec:resnames"><a id="sec:2.19.3"><span class="sec-nr">2.19.3</span> <span class="sec-title">Reserved 
Names</span></a></h3>

<a id="sec:resnames"></a>

<p>The boot compiler (see <strong>-b</strong> option) does not support 
the module system. As large parts of the system are written in Prolog 
itself we need some way to avoid name clashes with the user's 
predicates, database keys, etc. Like Edinburgh C-Prolog <cite><a class="cite" href="Bibliography.html#CPROLOG:manual">Pereira, 
1986</a></cite> all predicates, database keys, etc., that should be 
hidden from the user start with a dollar (<code>$</code>) sign.

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swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / limits.html
