View source for SetString (System function)

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The <var>SetString</var> shared [[Notation conventions for methods#Callable functions|callable]] function sets a [[System class#System and subsystem globals and strings|system-wide string]].

==Syntax==
{{Template:System:SetString syntax}}

===Syntax terms===
<table class="syntaxTable">
<tr><th>%number</th><td>A numeric result that indicates whether the string update succeeded.  <var class="term">%number</var> is set to 1 upon success. If <var class="term">oldValue</var> is specified and does not match the current setting of the string, the update is not performed and a 0 is returned.</td></tr>

<tr><th><var class="nobr">%(System)</var></th>
<td>The class name in parentheses denotes a [[Notation conventions for methods#Shared methods|shared]] method. <var>SetString</var> can also be invoked via a <var>System</var> object variable, which may be <var>null</var>.</td></tr>

<tr><th>name</th>
<td>A string that identifies the global string to be set.</td></tr>

<tr><th>value</th>
<td>A string that identifies the new value for the global string.</td></tr>

<tr><th>oldValue</th>
<td>A string that identifies a value to be compared against the current value of the <var>System</var> string before doing the update. This is an optional argument.</td></tr>
</table>

==Usage notes==
<ul>
<li>All errors result in request cancellation.
<li>It is not an error to set a string that is not set.
<li>Unlike <var>[[SetGlobal_(System_subroutine)#Usage_notes|SetGlobal]]</var>, <var>SetString</var> does not turn off multiprocessing, and it is designed for fairly high levels of updating.
</ul>

==Examples==
<ol>
<li>The following statement sets the system string called <code>DIAMONDS</code>:
<p class="code">%(system):setString('DIAMONDS', 'CLEAR')</p>

<li>The <var class="term">oldValue</var> argument makes it possible to use <var>System</var> strings as structures that can be safely updated by multiple users. For example, one might want to maintain a system-wide counter. This can be accomplished as follows:
<p class="code">%value    is float
%newValue is float

%value = %(system):string('Counter')
%newValue = %value + 1
%(system):setString('Counter', %newValue)
</p>
Unfortunately, another thread might update the value of the counter between the time the above code retrieves the value (with <var>[[String_(System_function)|String]]</var>) and the time it updates it (with <var>SetString</var>).  In such a case, an increment of the counter might be missed. This is more likely, of course, in an <var class="product">MP/204</var> environment, but you should not count on it being impossible in a non-<var class="product">MP/204</var> environment.  To safely update a counter, you can do the following:
<p class="code">%value    is float
%newValue is float
%success  is float

repeat while not %success
   %value = %(system):string('Counter')
   %newValue = %value + 1
   %success = %(system):setString('Counter', %newValue,  %value)
end repeat
</p>
If no user has updated <code>Counter</code> between the <var>[[String_(System_function)|String]]</var> and the <var>SetString</var> methods, the update would succeed. If some other user has updated it, the update would not be performed, <var>SetString</var> would return a 0, and the repeat loop would be repeated.
<p>
While this looks nasty &mdash; it seems the code could loop forever &mdash; it's not that bad. Since the only thing that causes the <var>SetString</var> to fail is another user updating the same string at the same time; even if a hundred users were in this loop, at least one of them would succeed on each iteration. So unless the system is doing almost nothing else, all the updates would be performed in fairly short order.</p>

<li>This technique can be used with considerably more complex data-structures. For example, one can actually store a serialized <var>[[XmlDoc_API#The_XmlDoc_class|XmlDoc]]</var> in a <var>system</var> string. A request can retrieve the current structure, manipulate it, and then replace the updated structure:
<p class="code">%value    is longString
%newValue is longString
%myDoc    is object xmlDoc
%success is float

repeat while not %success
   %value = %(system):string('Complicated')
   %myDoc = new
   %myDoc:loadXml(%value)
     ... manipulate myDoc
   %newValue = %myDoc:serial(,'EBCDIC')
   %success = %(system):setString('Complicated', -
                                  %newValue, %value)
end repeat
</p>
Note that the above code assumes that the <code>Complicated</code> system string is already set. If the application cannot guarantee that <code>Complicated</code> has been set, there should be a test for the null string, bypassing the <var>[[LoadXml_(XmlDoc/XmlNode_function)|loadXml]]</var> operation and instead populating the <var>XmlDoc</var> with some initial nodes.
<p>
The document is stored in the system string in EBCDIC. This is not necessary &mdash; the document could be stored in UTF-8 encoding &mdash; it is just easier to debug this code if it is saved in EBCDIC.</p>
</ol>

==See also==
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