<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> <title>Missing Components (GNU Octave (version 5.1.0))</title> <meta name="description" content="Missing Components (GNU Octave (version 5.1.0))"> <meta name="keywords" content="Missing Components (GNU Octave (version 5.1.0))"> <meta name="resource-type" content="document"> <meta name="distribution" content="global"> <meta name="Generator" content="makeinfo"> <link href="index.html#Top" rel="start" title="Top"> <link href="Concept-Index.html#Concept-Index" rel="index" title="Concept Index"> <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> <link href="Creating-Packages.html#Creating-Packages" rel="up" title="Creating Packages"> <link href="External-Code-Interface.html#External-Code-Interface" rel="next" title="External Code Interface"> <link href="PKG_005fADD-and-PKG_005fDEL-Directives.html#PKG_005fADD-and-PKG_005fDEL-Directives" rel="prev" title="PKG_ADD and PKG_DEL Directives"> <style type="text/css"> <!-- a.summary-letter {text-decoration: none} blockquote.indentedblock {margin-right: 0em} blockquote.smallindentedblock {margin-right: 0em; font-size: smaller} blockquote.smallquotation {font-size: smaller} div.display {margin-left: 3.2em} div.example {margin-left: 3.2em} div.lisp {margin-left: 3.2em} div.smalldisplay {margin-left: 3.2em} div.smallexample {margin-left: 3.2em} div.smalllisp {margin-left: 3.2em} kbd {font-style: oblique} pre.display {font-family: inherit} pre.format {font-family: inherit} pre.menu-comment {font-family: serif} pre.menu-preformatted {font-family: serif} pre.smalldisplay {font-family: inherit; font-size: smaller} pre.smallexample {font-size: smaller} pre.smallformat {font-family: inherit; font-size: smaller} pre.smalllisp {font-size: smaller} span.nolinebreak {white-space: nowrap} span.roman {font-family: initial; font-weight: normal} span.sansserif {font-family: sans-serif; font-weight: normal} ul.no-bullet {list-style: none} --> </style> <link rel="stylesheet" type="text/css" href="octave.css"> </head> <body lang="en"> <a name="Missing-Components"></a> <div class="header"> <p> Previous: <a href="PKG_005fADD-and-PKG_005fDEL-Directives.html#PKG_005fADD-and-PKG_005fDEL-Directives" accesskey="p" rel="prev">PKG_ADD and PKG_DEL Directives</a>, Up: <a href="Creating-Packages.html#Creating-Packages" accesskey="u" rel="up">Creating Packages</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p> </div> <hr> <a name="Missing-Components-1"></a> <h4 class="subsection">37.4.4 Missing Components</h4> <p>If a package relies on a component, such as another Octave package, that may not be present it may be useful to install a function which informs users what to do when a particular component is missing. The function must be written by the package maintainer and registered with Octave using <code>missing_component_hook</code>. </p> <a name="XREFmissing_005fcomponent_005fhook"></a><dl> <dt><a name="index-missing_005fcomponent_005fhook"></a><em><var>val</var> =</em> <strong>missing_component_hook</strong> <em>()</em></dt> <dt><a name="index-missing_005fcomponent_005fhook-1"></a><em><var>old_val</var> =</em> <strong>missing_component_hook</strong> <em>(<var>new_val</var>)</em></dt> <dt><a name="index-missing_005fcomponent_005fhook-2"></a><em></em> <strong>missing_component_hook</strong> <em>(<var>new_val</var>, "local")</em></dt> <dd><p>Query or set the internal variable that specifies the function to call when a component of Octave is missing. </p> <p>This can be useful for packagers that may split the Octave installation into multiple sub-packages, for example, to provide a hint to users for how to install the missing components. </p> <p>When called from inside a function with the <code>"local"</code> option, the variable is changed locally for the function and any subroutines it calls. The original variable value is restored when exiting the function. </p> <p>The hook function is expected to be of the form </p> <div class="example"> <pre class="example"><var>fcn</var> (<var>component</var>) </pre></div> <p>Octave will call <var>fcn</var> with the name of the function that requires the component and a string describing the missing component. The hook function should return an error message to be displayed. </p> <p><strong>See also:</strong> <a href="Parser.html#XREFmissing_005ffunction_005fhook">missing_function_hook</a>. </p></dd></dl> </body> </html>