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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html> <head> <title> Frequently Asked Questions </title> <meta name="GENERATOR" content= "Modular DocBook HTML Stylesheet Version 1.45"> <link rel="HOME" title="GTK+ / Gnome Application Development" href="ggad.html"> <link rel="UP" title="Appendices" href="appendices.html"> <link rel="PREVIOUS" title="Table of Header Files" href= "headers.html"> <link rel="NEXT" title="Online Resources" href="online.html"> </head> <body bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink= "#840084" alink="#0000FF"> <div class="NAVHEADER"> <table width="100%" border="0" bgcolor="#ffffff" cellpadding= "1" cellspacing="0"> <tr> <th colspan="4" align="center"> <font color="#000000" size="2">GTK+ / Gnome Application Development</font> </th> </tr> <tr> <td width="25%" bgcolor="#ffffff" align="left"> <a href="headers.html"><font color="#0000ff" size="2"> <b><<< Previous</b></font></a> </td> <td width="25%" colspan="2" bgcolor="#ffffff" align= "center"> <font color="#0000ff" size="2"><b><a href="ggad.html"> <font color="#0000ff" size="2"><b> Home</b></font></a></b></font> </td> <td width="25%" bgcolor="#ffffff" align="right"> <a href="online.html"><font color="#0000ff" size="2"> <b>Next >>></b></font></a> </td> </tr> </table> </div> <div class="CHAPTER"> <h1> <a name="FAQS">Frequently Asked Questions</a> </h1> <p> This chapter contains some commonly-asked questions, and answers, with references to the rest of the book. See the table of contents for a summary of the questions. </p> <div class="SECT1"> <h1 class="SECT1"> <a name="Z827">Questions, with Answers</a> </h1> <div class="SECT2"> <h2 class="SECT2"> <a name="Z828">How do I make my application beep?</a> </h2> <p> Call the <tt class="FUNCTION">gdk_beep()</tt> function. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z829">When do I need to destroy my widgets?</a> </h2> <p> See <a href="z57.html#WIDGETLIFECYCLE">the section called <i>Widget Life Cycle</i> in the chapter called <i>GTK+ Basics</i></a> for the simple answer, and <a href="sec-finalization.html">the section called <i> Object Finalization</i> in the chapter called <i>The GTK+ Object and Type System</i></a> for more details. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z830">When I turn on memory profiling in glib, my application becomes unstable. What gives?</a> </h2> <p> Normally <tt class="FUNCTION">g_malloc()</tt> and <tt class="FUNCTION">g_free()</tt> are just wrappers around <tt class="FUNCTION">malloc()</tt> and <tt class= "FUNCTION">free()</tt>, with a couple of extra features described in <a href="cha-glib.html#GLIB-MEMORY">the section called <i>Memory</i> in the chapter called <i> glib: Portability and Utility</i></a>. However, when you turn on memory profiling, they are no longer interchangeable with <tt class="FUNCTION">malloc()</tt> and <tt class="FUNCTION">free()</tt>. So anytime you incorrectly mix the two pairs of functions, your program will crash. </p> <p> If you're using the GNU C library, which comes with nearly all Linux distributions, it has a special feature which can help you debug this. Set the <tt class="APPLICATION">MALLOC_CHECK_</tt> environment variable to <tt class="APPLICATION">2</tt> before running your program, then run the program in <tt class="APPLICATION">gdb</tt>. As soon as <tt class= "FUNCTION">free()</tt> gets a pointer not created by <tt class="FUNCTION">malloc()</tt>, <tt class= "FUNCTION">abort()</tt> will be called. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z831">To create a custom display, I want to place widgets in arbitrary locations, or move them around rapidly, or draw to them directly. How?</a> </h2> <p> You are probably fighting a losing battle. Widgets really aren't what you want, most likely. Consider using a <tt class="CLASSNAME">GtkDrawingArea</tt> or the <tt class="CLASSNAME">GnomeCanvas</tt> to create your custom display. </p> <p> If you really need interactive widgets, such as a <tt class="CLASSNAME">GtkEntry</tt> or <tt class= "CLASSNAME">GtkButton</tt>, you can try to use <tt class="CLASSNAME">GtkLayout</tt> or <tt class= "CLASSNAME">GtkFixed</tt>. </p> <p> If you have very specialized needs, you probably need to write your own widget. <a href="cha-widget.html">the chapter called <i>Writing a <tt class="CLASSNAME"> GtkWidget</tt></i></a> tells you how to do so. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z832">Why does my memory debugging tool show memory leaks in glib?</a> </h2> <p> glib does not call <tt class="FUNCTION">malloc()</tt> every time it needs a new node in a data structure. If it did, building linked lists (for example) would be substantially slower. Instead, glib caches pools of equal-sized "memory chunks" for use in these data structures. Since the chunks are still available for recycling when your program exits, they are never <tt class="FUNCTION">free()</tt>d. (Of course, the operating system will reclaim the memory, but tools such as <tt class="APPLICATION">ccmalloc</tt> and <tt class="APPLICATION">Purify</tt> will report it as a memory leak.) </p> <p> To get around this, you can plug a new <span class= "STRUCTNAME">GAllocator</span> into most of the data structures. A <span class="STRUCTNAME"> GAllocator</span> is a pool of memory as described above. Just create an allocator manually, so you have a pointer to it; you can then free the allocator when you are finished. <a href="faqs.html#FL-GLISTALLOCATOR"> Figure 1</a> summarizes the relevant functions for <span class="STRUCTNAME">GList</span>. A quick glance through <tt class="FILENAME">glib.h</tt> will reveal the corresponding functions for other data structures. </p> <p> The <tt class="APPLICATION">name</tt> argument to <tt class="FUNCTION">g_allocator_new()</tt> is used in debugging messages; the <tt class="APPLICATION"> n_preallocs</tt> argument is passed through to <tt class="FUNCTION">g_mem_chunk_new()</tt>. </p> <div class="FIGURE"> <a name="FL-GLISTALLOCATOR"></a> <div class="FUNCSYNOPSIS"> <a name="FL-GLISTALLOCATOR.SYNOPSIS"></a> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="FUNCSYNOPSISINFO">#include <glib.h></pre> </td> </tr> </table> <p> <code><code class="FUNCDEF">void <tt class= "FUNCTION"> g_list_push_allocator</tt></code>(GAllocator* <tt class="PARAMETER"><i>allocator</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class= "FUNCTION"> g_list_pop_allocator</tt></code>(void);</code> </p> <p> <code><code class="FUNCDEF">GAllocator* <tt class= "FUNCTION">g_allocator_new</tt></code>(gchar* <tt class="PARAMETER"><i>name</i></tt>, guint <tt class="PARAMETER"><i>n_preallocs</i></tt>);</code> </p> <p> <code><code class="FUNCDEF">void <tt class= "FUNCTION">g_allocator_free</tt></code>(GAllocator* <tt class="PARAMETER"><i> allocator</i></tt>);</code> </p> </div> <p> <b>Figure 1. Functions for replacing the <span class= "STRUCTNAME">GList</span> memory allocator</b> </p> </div> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z833">I get a bunch of "assertion failed" warnings from GTK+. What causes these?</a> </h2> <p> These come from the <tt class="FUNCTION"> g_return_if_fail()</tt> checks at the beginning of many GTK+ functions. (They will only appear if your copy of GTK+ was compiled with debugging turned on---and hopefully it was if you are writing an application.) You will need to look at the exact assertion that failed to see what causes the warning. A common one: if you accidentally access a destroyed widget or object, you will have a pointer to memory garbage. Among other things, this means the type tag will be invalid; so GTK+'s runtime type checks will fail. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z834">Why are some things in Gnome rather than GTK+?</a> </h2> <p> Historical accident, mostly. Sometimes there is a reason; for example, GTK+ does not include <tt class= "APPLICATION">gdk_imlib</tt>, so does not include any widgets that rely on it. In very general terms, GTK+ imposes less "policy" than Gnome; some Gnome widgets are deliberately inflexible to keep people from creating an inconsistent user interface. GTK+ does not take this approach. Finally, some of the Gnome widgets were considered too "experimental" to go in GTK+ at the time. However, the core Gnome widgets discussed in this book are not in this category. </p> </div> <div class="SECT2"> <h2 class="SECT2"> <a name="Z835">How can I center a window on the screen?</a> </h2> <p> If the window is a <tt class="CLASSNAME"> GnomeDialog</tt>, this is user-configurable and you should not do it. In most other cases it would be a bit strange; but there are exceptions, such as splash screens. The function you want is <tt class="FUNCTION"> gtk_window_set_position()</tt>; you can leave the window's position up to the window manager (the default), ask to have it centered, or ask to have it appear wherever the mouse pointer is. There is an enumeration which corresponds to these settings: <span class="STRUCTNAME">GTK_WIN_POS_NONE</span>, <span class="STRUCTNAME">GTK_WIN_POS_CENTER</span>, <span class="STRUCTNAME">GTK_WIN_POS_MOUSE</span>. For example: </p> <table border="0" bgcolor="#E0E0E0" width="100%"> <tr> <td><pre class="PROGRAMLISTING"> gtk_window_set_position(GTK_WINDOW(window), GTK_WIN_POS_CENTER); </pre> </td> </tr> </table> <p> You should do this <i class="EMPHASIS">before</i> calling <tt class="FUNCTION">gtk_widget_show()</tt>, because the function affects where the window appears when it is first placed on-screen. </p> </div> <div class="SECT2"> <h2 class="SECT2">?? 快捷鍵說明
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