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1<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> 2<html> 3<head> 4<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 5<title>Error Reporting</title> 6<meta name="generator" content="DocBook XSL Stylesheets V1.74.0"> 7<link rel="home" href="index.html" title="GLib Reference Manual"> 8<link rel="up" href="glib-core.html" title="GLib Core Application Support"> 9<link rel="prev" href="glib-IO-Channels.html" title="IO Channels"> 10<link rel="next" href="glib-Warnings-and-Assertions.html" title="Message Output and Debugging Functions"> 11<meta name="generator" content="GTK-Doc V1.10 (XML mode)"> 12<link rel="stylesheet" href="style.css" type="text/css"> 13<link rel="chapter" href="glib.html" title="GLib Overview"> 14<link rel="chapter" href="glib-fundamentals.html" title="GLib Fundamentals"> 15<link rel="chapter" href="glib-core.html" title="GLib Core Application Support"> 16<link rel="chapter" href="glib-utilities.html" title="GLib Utilities"> 17<link rel="chapter" href="glib-data-types.html" title="GLib Data Types"> 18<link rel="chapter" href="tools.html" title="GLib Tools"> 19<link rel="index" href="ix01.html" title="Index"> 20<link rel="index" href="ix02.html" title="Index of deprecated symbols"> 21<link rel="index" href="ix03.html" title="Index of new symbols in 2.2"> 22<link rel="index" href="ix04.html" title="Index of new symbols in 2.4"> 23<link rel="index" href="ix05.html" title="Index of new symbols in 2.6"> 24<link rel="index" href="ix06.html" title="Index of new symbols in 2.8"> 25<link rel="index" href="ix07.html" title="Index of new symbols in 2.10"> 26<link rel="index" href="ix08.html" title="Index of new symbols in 2.12"> 27<link rel="index" href="ix09.html" title="Index of new symbols in 2.14"> 28<link rel="index" href="ix10.html" title="Index of new symbols in 2.16"> 29</head> 30<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 31<table class="navigation" id="top" width="100%" summary="Navigation header" cellpadding="2" cellspacing="2"> 32<tr valign="middle"> 33<td><a accesskey="p" href="glib-IO-Channels.html"><img src="left.png" width="24" height="24" border="0" alt="Prev"></a></td> 34<td><a accesskey="u" href="glib-core.html"><img src="up.png" width="24" height="24" border="0" alt="Up"></a></td> 35<td><a accesskey="h" href="index.html"><img src="home.png" width="24" height="24" border="0" alt="Home"></a></td> 36<th width="100%" align="center">GLib Reference Manual</th> 37<td><a accesskey="n" href="glib-Warnings-and-Assertions.html"><img src="right.png" width="24" height="24" border="0" alt="Next"></a></td> 38</tr> 39<tr><td colspan="5" class="shortcuts"><nobr><a href="#glib-Error-Reporting.synopsis" class="shortcut">Top</a> 40 | 41 <a href="#glib-Error-Reporting.description" class="shortcut">Description</a></nobr></td></tr> 42</table> 43<div class="refentry" lang="en"> 44<a name="glib-Error-Reporting"></a><div class="titlepage"></div> 45<div class="refnamediv"><table width="100%"><tr> 46<td valign="top"> 47<h2><span class="refentrytitle"><a name="glib-Error-Reporting.top_of_page"></a>Error Reporting</span></h2> 48<p>Error Reporting — a system for reporting errors</p> 49</td> 50<td valign="top" align="right"></td> 51</tr></table></div> 52<div class="refsynopsisdiv"> 53<a name="glib-Error-Reporting.synopsis"></a><h2>Synopsis</h2> 54<pre class="synopsis"> 55 56#include <glib.h> 57 58 <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>; 59<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* <a class="link" href="glib-Error-Reporting.html#g-error-new" title="g_error_new ()">g_error_new</a> (<a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 60 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 61 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 62 ...); 63<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* <a class="link" href="glib-Error-Reporting.html#g-error-new-literal" title="g_error_new_literal ()">g_error_new_literal</a> (<a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 64 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 65 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *message); 66void <a class="link" href="glib-Error-Reporting.html#g-error-free" title="g_error_free ()">g_error_free</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error); 67<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* <a class="link" href="glib-Error-Reporting.html#g-error-copy" title="g_error_copy ()">g_error_copy</a> (const <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error); 68<a class="link" href="glib-Basic-Types.html#gboolean" title="gboolean">gboolean</a> <a class="link" href="glib-Error-Reporting.html#g-error-matches" title="g_error_matches ()">g_error_matches</a> (const <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error, 69 <a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 70 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code); 71void <a class="link" href="glib-Error-Reporting.html#g-set-error" title="g_set_error ()">g_set_error</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err, 72 <a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 73 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 74 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 75 ...); 76void <a class="link" href="glib-Error-Reporting.html#g-propagate-error" title="g_propagate_error ()">g_propagate_error</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **dest, 77 <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *src); 78void <a class="link" href="glib-Error-Reporting.html#g-clear-error" title="g_clear_error ()">g_clear_error</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err); 79void <a class="link" href="glib-Error-Reporting.html#g-prefix-error" title="g_prefix_error ()">g_prefix_error</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err, 80 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 81 ...); 82void <a class="link" href="glib-Error-Reporting.html#g-propagate-prefixed-error" title="g_propagate_prefixed_error ()">g_propagate_prefixed_error</a> (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **dest, 83 <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *src, 84 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 85 ...); 86</pre> 87</div> 88<div class="refsect1" lang="en"> 89<a name="glib-Error-Reporting.description"></a><h2>Description</h2> 90<p> 91GLib provides a standard method of reporting errors from a called function to 92the calling code. (This is the same problem solved by exceptions in other 93languages.) It's important to understand that this method is both a 94<span class="emphasis"><em>data type</em></span> (the <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> object) and a <span class="emphasis"><em>set of 95rules.</em></span> If you use <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> incorrectly, then your code will not 96properly interoperate with other code that uses <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>, and users of your API 97will probably get confused. 98</p> 99<p> 100First and foremost: <span class="emphasis"><em><a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> should only be used to report 101recoverable runtime errors, never to report programming errors.</em></span> If 102the programmer has screwed up, then you should use <a class="link" href="glib-Message-Logging.html#g-warning" title="g_warning()"><code class="function">g_warning()</code></a>, 103<a class="link" href="glib-Warnings-and-Assertions.html#g-return-if-fail" title="g_return_if_fail()"><code class="function">g_return_if_fail()</code></a>, <a class="link" href="glib-Testing.html#g-assert" title="g_assert()"><code class="function">g_assert()</code></a>, <a class="link" href="glib-Message-Logging.html#g-error" title="g_error()"><code class="function">g_error()</code></a>, or some similar facility. 104(Incidentally, remember that the <a class="link" href="glib-Message-Logging.html#g-error" title="g_error()"><code class="function">g_error()</code></a> function should 105<span class="emphasis"><em>only</em></span> be used for programming errors, it should not be used 106to print any error reportable via <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>.) 107</p> 108<p> 109Examples of recoverable runtime errors are "file not found" or "failed to parse 110input." Examples of programming errors are "NULL passed to <code class="function">strcmp()</code>" or 111"attempted to free the same pointer twice." These two kinds of errors are 112fundamentally different: runtime errors should be handled or reported to the 113user, programming errors should be eliminated by fixing the bug in the program. 114This is why most functions in GLib and GTK+ do not use the <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> facility. 115</p> 116<p> 117Functions that can fail take a return location for a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> as their last argument. 118For example: 119</p> 120<div class="informalexample"><pre class="programlisting"> 121gboolean g_file_get_contents (const gchar *filename, 122 gchar **contents, 123 gsize *length, 124 GError **error); 125</pre></div> 126<p> 127If you pass a non-<a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> value for the <code class="literal">error</code> argument, it should 128point to a location where an error can be placed. For example: 129</p> 130<div class="informalexample"><pre class="programlisting"> 131gchar *contents; 132GError *err = NULL; 133g_file_get_contents ("foo.txt", &contents, NULL, &err); 134g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL)); 135if (err != NULL) 136 { 137 /* Report error to user, and free error */ 138 g_assert (contents == NULL); 139 fprintf (stderr, "Unable to read file: %s\n", err->message); 140 g_error_free (err); 141 } 142else 143 { 144 /* Use file contents */ 145 g_assert (contents != NULL); 146 } 147</pre></div> 148<p> 149Note that <code class="literal">err != NULL</code> in this example is a 150<span class="emphasis"><em>reliable</em></span> indicator of whether 151<a class="link" href="glib-File-Utilities.html#g-file-get-contents" title="g_file_get_contents ()"><code class="function">g_file_get_contents()</code></a> failed. Additionally, <a class="link" href="glib-File-Utilities.html#g-file-get-contents" title="g_file_get_contents ()"><code class="function">g_file_get_contents()</code></a> returns 152a boolean which indicates whether it was successful. 153</p> 154<p> 155Because <a class="link" href="glib-File-Utilities.html#g-file-get-contents" title="g_file_get_contents ()"><code class="function">g_file_get_contents()</code></a> returns <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a> on failure, if you are only 156interested in whether it failed and don't need to display an error message, you 157can pass <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> for the <code class="literal">error</code> argument: 158</p> 159<div class="informalexample"><pre class="programlisting"> 160if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) /* ignore errors */ 161 /* no error occurred */ ; 162else 163 /* error */ ; 164</pre></div> 165<p> 166</p> 167<p> 168The <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> object contains three fields: <code class="literal">domain</code> indicates 169the module the error-reporting function is located in, <code class="literal">code</code> 170indicates the specific error that occurred, and <code class="literal">message</code> is a 171user-readable error message with as many details as possible. Several functions 172are provided to deal with an error received from a called function: 173<a class="link" href="glib-Error-Reporting.html#g-error-matches" title="g_error_matches ()"><code class="function">g_error_matches()</code></a> returns <a class="link" href="glib-Standard-Macros.html#TRUE:CAPS" title="TRUE"><code class="literal">TRUE</code></a> if the error matches a given domain and code, 174<a class="link" href="glib-Error-Reporting.html#g-propagate-error" title="g_propagate_error ()"><code class="function">g_propagate_error()</code></a> copies an error into an error location (so the calling 175function will receive it), and <a class="link" href="glib-Error-Reporting.html#g-clear-error" title="g_clear_error ()"><code class="function">g_clear_error()</code></a> clears an error location by 176freeing the error and resetting the location to <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>. To display an error to the 177user, simply display <code class="literal">error->message</code>, perhaps along with 178additional context known only to the calling function (the file being opened, or 179whatever -- though in the <a class="link" href="glib-File-Utilities.html#g-file-get-contents" title="g_file_get_contents ()"><code class="function">g_file_get_contents()</code></a> case, 180<code class="literal">error->message</code> already contains a filename). 181</p> 182<p> 183When implementing a function that can report errors, the basic tool is 184<a class="link" href="glib-Error-Reporting.html#g-set-error" title="g_set_error ()"><code class="function">g_set_error()</code></a>. Typically, if a fatal error occurs you want to <a class="link" href="glib-Error-Reporting.html#g-set-error" title="g_set_error ()"><code class="function">g_set_error()</code></a>, 185then return immediately. <a class="link" href="glib-Error-Reporting.html#g-set-error" title="g_set_error ()"><code class="function">g_set_error()</code></a> does nothing if the error location passed 186to it is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>. Here's an example: 187</p> 188<div class="informalexample"><pre class="programlisting"> 189gint 190foo_open_file (GError **error) 191{ 192 gint fd; 193 194 fd = open ("file.txt", O_RDONLY); 195 196 if (fd < 0) 197 { 198 g_set_error (error, 199 FOO_ERROR, /* error domain */ 200 FOO_ERROR_BLAH, /* error code */ 201 "Failed to open file: %s", /* error message format string */ 202 g_strerror (errno)); 203 return -1; 204 } 205 else 206 return fd; 207} 208</pre></div> 209<p> 210</p> 211<p> 212Things are somewhat more complicated if you yourself call another function that 213can report a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>. If the sub-function indicates fatal errors in some way 214other than reporting a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>, such as by returning <a class="link" href="glib-Standard-Macros.html#TRUE:CAPS" title="TRUE"><code class="literal">TRUE</code></a> on success, you can 215simply do the following: 216</p> 217<div class="informalexample"><pre class="programlisting"> 218gboolean 219my_function_that_can_fail (GError **err) 220{ 221 g_return_val_if_fail (err == NULL || *err == NULL, FALSE); 222 223 if (!sub_function_that_can_fail (err)) 224 { 225 /* assert that error was set by the sub-function */ 226 g_assert (err == NULL || *err != NULL); 227 return FALSE; 228 } 229 230 /* otherwise continue, no error occurred */ 231 g_assert (err == NULL || *err == NULL); 232} 233</pre></div> 234<p> 235</p> 236<p> 237If the sub-function does not indicate errors other than by reporting a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>, 238you need to create a temporary <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> since the passed-in one may be <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>. 239<a class="link" href="glib-Error-Reporting.html#g-propagate-error" title="g_propagate_error ()"><code class="function">g_propagate_error()</code></a> is intended for use in this case. 240</p> 241<div class="informalexample"><pre class="programlisting"> 242gboolean 243my_function_that_can_fail (GError **err) 244{ 245 GError *tmp_error; 246 247 g_return_val_if_fail (err == NULL || *err == NULL, FALSE); 248 249 tmp_error = NULL; 250 sub_function_that_can_fail (&tmp_error); 251 252 if (tmp_error != NULL) 253 { 254 /* store tmp_error in err, if err != NULL, 255 * otherwise call g_error_free() on tmp_error 256 */ 257 g_propagate_error (err, tmp_error); 258 return FALSE; 259 } 260 261 /* otherwise continue, no error occurred */ 262} 263</pre></div> 264<p> 265</p> 266<p> 267Error pileups are always a bug. For example, this code is incorrect: 268</p> 269<div class="informalexample"><pre class="programlisting"> 270gboolean 271my_function_that_can_fail (GError **err) 272{ 273 GError *tmp_error; 274 275 g_return_val_if_fail (err == NULL || *err == NULL, FALSE); 276 277 tmp_error = NULL; 278 sub_function_that_can_fail (&tmp_error); 279 other_function_that_can_fail (&tmp_error); 280 281 if (tmp_error != NULL) 282 { 283 g_propagate_error (err, tmp_error); 284 return FALSE; 285 } 286} 287</pre></div> 288<p> 289<code class="literal">tmp_error</code> should be checked immediately after 290<code class="function"><code class="function">sub_function_that_can_fail()</code></code>, and either cleared or propagated upward. The rule 291is: <span class="emphasis"><em>after each error, you must either handle the error, or return it to the 292calling function</em></span>. Note that passing <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> for the error location is the 293equivalent of handling an error by always doing nothing about it. So the 294following code is fine, assuming errors in <code class="function"><code class="function">sub_function_that_can_fail()</code></code> are not 295fatal to <code class="function"><code class="function">my_function_that_can_fail()</code></code>: 296</p> 297<div class="informalexample"><pre class="programlisting"> 298gboolean 299my_function_that_can_fail (GError **err) 300{ 301 GError *tmp_error; 302 303 g_return_val_if_fail (err == NULL || *err == NULL, FALSE); 304 305 sub_function_that_can_fail (NULL); /* ignore errors */ 306 307 tmp_error = NULL; 308 other_function_that_can_fail (&tmp_error); 309 310 if (tmp_error != NULL) 311 { 312 g_propagate_error (err, tmp_error); 313 return FALSE; 314 } 315} 316</pre></div> 317<p> 318</p> 319<p> 320Note that passing <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> for the error location <span class="emphasis"><em>ignores</em></span> 321errors; it's equivalent to <code class="literal">try { <code class="function">sub_function_that_can_fail()</code>; } catch 322(...) {}</code> in C++. It does <span class="emphasis"><em>not</em></span> mean to leave errors 323unhandled; it means to handle them by doing nothing. 324</p> 325<p> 326Error domains and codes are conventionally named as follows: 327</p> 328<div class="itemizedlist"><ul type="disc"> 329<li> 330<p> 331The error domain is called 332<code class="literal"><NAMESPACE>_<MODULE>_ERROR</code>, for example 333<a class="link" href="glib-Spawning-Processes.html#G-SPAWN-ERROR:CAPS" title="G_SPAWN_ERROR"><code class="literal">G_SPAWN_ERROR</code></a> or <a class="link" href="glib-Threads.html#G-THREAD-ERROR:CAPS" title="G_THREAD_ERROR"><code class="literal">G_THREAD_ERROR</code></a>: 334</p> 335<div class="informalexample"><pre class="programlisting"> 336#define G_SPAWN_ERROR g_spawn_error_quark () 337 338GQuark 339g_spawn_error_quark (void) 340{ 341 return g_quark_from_static_string ("g-spawn-error-quark"); 342} 343</pre></div> 344<p> 345</p> 346</li> 347<li><p> 348The error codes are in an enumeration called 349<code class="literal"><Namespace><Module>Error</code>; for example, 350<a class="link" href="glib-Threads.html#GThreadError" title="enum GThreadError"><span class="type">GThreadError</span></a> or <a class="link" href="glib-Spawning-Processes.html#GSpawnError" title="enum GSpawnError"><span class="type">GSpawnError</span></a>. 351</p></li> 352<li><p> 353Members of the error code enumeration are called <code class="literal"><NAMESPACE>_<MODULE>_ERROR_<CODE></code>, for example <a class="link" href="glib-Spawning-Processes.html#G-SPAWN-ERROR-FORK:CAPS"><code class="literal">G_SPAWN_ERROR_FORK</code></a> or <a class="link" href="glib-Threads.html#G-THREAD-ERROR-AGAIN:CAPS"><code class="literal">G_THREAD_ERROR_AGAIN</code></a>. 354</p></li> 355<li><p> 356If there's a "generic" or "unknown" error code for unrecoverable errors it 357doesn't make sense to distinguish with specific codes, it should be called 358<code class="literal"><NAMESPACE>_<MODULE>_ERROR_FAILED</code>, for 359example <a class="link" href="glib-Spawning-Processes.html#G-SPAWN-ERROR-FAILED:CAPS"><code class="literal">G_SPAWN_ERROR_FAILED</code></a> or <code class="literal">G_THREAD_ERROR_FAILED</code>. 360</p></li> 361</ul></div> 362<p> 363</p> 364<p> 365Summary of rules for use of <span class="type">""</span> 366 </p> 367<div class="itemizedlist"><ul type="disc"> 368<li><p> 369 Do not report programming errors via <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>. 370 </p></li> 371<li><p> 372 The last argument of a function that returns an error should be a 373 location where a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> can be placed (i.e. "<a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>** error"). If 374 <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> is used with varargs, the <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>** should be the last 375 argument before the "...". 376 </p></li> 377<li><p> 378 The caller may pass <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> for the <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>** if they are not interested 379 in details of the exact error that occurred. 380 </p></li> 381<li><p> 382 If <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> is passed for the <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>** argument, then errors should 383 not be returned to the caller, but your function should still 384 abort and return if an error occurs. That is, control flow should 385 not be affected by whether the caller wants to get a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>. 386 </p></li> 387<li><p> 388 If a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> is reported, then your function by definition 389 <span class="emphasis"><em>had a fatal failure and did not complete whatever it was supposed 390 to do</em></span>. If the failure was not fatal, then you handled it 391 and you should not report it. If it was fatal, then you must report it 392 and discontinue whatever you were doing immediately. 393 </p></li> 394<li><p> 395 A <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>* must be initialized to <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> before passing its address to 396 a function that can report errors. 397 </p></li> 398<li><p> 399 "Piling up" errors is always a bug. That is, if you assign a new 400 <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> to a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>* that is non-<a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, thus overwriting the previous 401 error, it indicates that you should have aborted the operation instead 402 of continuing. If you were able to continue, you should have cleared 403 the previous error with <a class="link" href="glib-Error-Reporting.html#g-clear-error" title="g_clear_error ()"><code class="function">g_clear_error()</code></a>. <a class="link" href="glib-Error-Reporting.html#g-set-error" title="g_set_error ()"><code class="function">g_set_error()</code></a> will complain 404 if you pile up errors. 405 </p></li> 406<li><p> 407 By convention, if you return a boolean value indicating success 408 then <a class="link" href="glib-Standard-Macros.html#TRUE:CAPS" title="TRUE"><code class="literal">TRUE</code></a> means success and <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a> means failure. If <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a> is returned, 409 the error <span class="emphasis"><em>must</em></span> be set to a non-<a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> value. 410 </p></li> 411<li><p> 412 A <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> return value is also frequently used to mean that an error 413 occurred. You should make clear in your documentation whether <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> is 414 a valid return value in non-error cases; if <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> is a valid value, 415 then users must check whether an error was returned to see if the 416 function succeeded. 417 </p></li> 418<li><p> 419 When implementing a function that can report errors, you may want to 420 add a check at the top of your function that the error return location 421 is either <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> or contains a <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> error 422 (e.g. <code class="literal">g_return_if_fail (error == NULL || *error == 423 NULL);</code>). 424 </p></li> 425</ul></div> 426<p> 427</p> 428</div> 429<div class="refsect1" lang="en"> 430<a name="glib-Error-Reporting.details"></a><h2>Details</h2> 431<div class="refsect2" lang="en"> 432<a name="GError"></a><h3>GError</h3> 433<pre class="programlisting">typedef struct { 434 GQuark domain; 435 gint code; 436 gchar *message; 437} GError; 438</pre> 439<p> 440The <span class="structname">GError</span> structure contains 441information about an error that has occurred. 442</p> 443<div class="variablelist"><table border="0"> 444<col align="left" valign="top"> 445<tbody> 446<tr> 447<td><p><span class="term"><a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> <em class="structfield"><code>domain</code></em>;</span></p></td> 448<td>error domain, e.g. <a class="link" href="glib-File-Utilities.html#G-FILE-ERROR:CAPS" title="G_FILE_ERROR"><span class="type">G_FILE_ERROR</span></a>. 449</td> 450</tr> 451<tr> 452<td><p><span class="term"><a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> <em class="structfield"><code>code</code></em>;</span></p></td> 453<td>error code, e.g. <a class="link" href="glib-File-Utilities.html#G-FILE-ERROR-NOENT:CAPS"><code class="literal">G_FILE_ERROR_NOENT</code></a>. 454</td> 455</tr> 456<tr> 457<td><p><span class="term"><a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *<em class="structfield"><code>message</code></em>;</span></p></td> 458<td>human-readable informative error message. 459 460</td> 461</tr> 462</tbody> 463</table></div> 464</div> 465<hr> 466<div class="refsect2" lang="en"> 467<a name="g-error-new"></a><h3>g_error_new ()</h3> 468<pre class="programlisting"><a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* g_error_new (<a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 469 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 470 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 471 ...);</pre> 472<p> 473Creates a new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> with the given <em class="parameter"><code>domain</code></em> and <em class="parameter"><code>code</code></em>, 474and a message formatted with <em class="parameter"><code>format</code></em>.</p> 475<p> 476 477</p> 478<div class="variablelist"><table border="0"> 479<col align="left" valign="top"> 480<tbody> 481<tr> 482<td><p><span class="term"><em class="parameter"><code>domain</code></em> :</span></p></td> 483<td> error domain 484</td> 485</tr> 486<tr> 487<td><p><span class="term"><em class="parameter"><code>code</code></em> :</span></p></td> 488<td> error code 489</td> 490</tr> 491<tr> 492<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td> 493<td> <code class="function">printf()</code>-style format for error message 494</td> 495</tr> 496<tr> 497<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td> 498<td> parameters for message format 499</td> 500</tr> 501<tr> 502<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td> 503<td> a new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 504</td> 505</tr> 506</tbody> 507</table></div> 508</div> 509<hr> 510<div class="refsect2" lang="en"> 511<a name="g-error-new-literal"></a><h3>g_error_new_literal ()</h3> 512<pre class="programlisting"><a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* g_error_new_literal (<a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 513 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 514 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *message);</pre> 515<p> 516Creates a new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>; unlike <a class="link" href="glib-Error-Reporting.html#g-error-new" title="g_error_new ()"><code class="function">g_error_new()</code></a>, <em class="parameter"><code>message</code></em> is not 517a <code class="function">printf()</code>-style format string. Use this 518function if <em class="parameter"><code>message</code></em> contains text you don't have control over, 519that could include <code class="function">printf()</code> escape sequences.</p> 520<p> 521 522</p> 523<div class="variablelist"><table border="0"> 524<col align="left" valign="top"> 525<tbody> 526<tr> 527<td><p><span class="term"><em class="parameter"><code>domain</code></em> :</span></p></td> 528<td> error domain 529</td> 530</tr> 531<tr> 532<td><p><span class="term"><em class="parameter"><code>code</code></em> :</span></p></td> 533<td> error code 534</td> 535</tr> 536<tr> 537<td><p><span class="term"><em class="parameter"><code>message</code></em> :</span></p></td> 538<td> error message 539</td> 540</tr> 541<tr> 542<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td> 543<td> a new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 544</td> 545</tr> 546</tbody> 547</table></div> 548</div> 549<hr> 550<div class="refsect2" lang="en"> 551<a name="g-error-free"></a><h3>g_error_free ()</h3> 552<pre class="programlisting">void g_error_free (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error);</pre> 553<p> 554Frees a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> and associated resources.</p> 555<p> 556 557</p> 558<div class="variablelist"><table border="0"> 559<col align="left" valign="top"> 560<tbody><tr> 561<td><p><span class="term"><em class="parameter"><code>error</code></em> :</span></p></td> 562<td> a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 563</td> 564</tr></tbody> 565</table></div> 566</div> 567<hr> 568<div class="refsect2" lang="en"> 569<a name="g-error-copy"></a><h3>g_error_copy ()</h3> 570<pre class="programlisting"><a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a>* g_error_copy (const <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error);</pre> 571<p> 572Makes a copy of <em class="parameter"><code>error</code></em>.</p> 573<p> 574 575</p> 576<div class="variablelist"><table border="0"> 577<col align="left" valign="top"> 578<tbody> 579<tr> 580<td><p><span class="term"><em class="parameter"><code>error</code></em> :</span></p></td> 581<td> a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 582</td> 583</tr> 584<tr> 585<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td> 586<td> a new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 587</td> 588</tr> 589</tbody> 590</table></div> 591</div> 592<hr> 593<div class="refsect2" lang="en"> 594<a name="g-error-matches"></a><h3>g_error_matches ()</h3> 595<pre class="programlisting"><a class="link" href="glib-Basic-Types.html#gboolean" title="gboolean">gboolean</a> g_error_matches (const <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *error, 596 <a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 597 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code);</pre> 598<p> 599Returns <a class="link" href="glib-Standard-Macros.html#TRUE:CAPS" title="TRUE"><code class="literal">TRUE</code></a> if <em class="parameter"><code>error</code></em> matches <em class="parameter"><code>domain</code></em> and <em class="parameter"><code>code</code></em>, <a class="link" href="glib-Standard-Macros.html#FALSE:CAPS" title="FALSE"><code class="literal">FALSE</code></a> 600otherwise.</p> 601<p> 602 603</p> 604<div class="variablelist"><table border="0"> 605<col align="left" valign="top"> 606<tbody> 607<tr> 608<td><p><span class="term"><em class="parameter"><code>error</code></em> :</span></p></td> 609<td> a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> 610</td> 611</tr> 612<tr> 613<td><p><span class="term"><em class="parameter"><code>domain</code></em> :</span></p></td> 614<td> an error domain 615</td> 616</tr> 617<tr> 618<td><p><span class="term"><em class="parameter"><code>code</code></em> :</span></p></td> 619<td> an error code 620</td> 621</tr> 622<tr> 623<td><p><span class="term"><span class="emphasis"><em>Returns</em></span> :</span></p></td> 624<td> whether <em class="parameter"><code>error</code></em> has <em class="parameter"><code>domain</code></em> and <em class="parameter"><code>code</code></em> 625</td> 626</tr> 627</tbody> 628</table></div> 629</div> 630<hr> 631<div class="refsect2" lang="en"> 632<a name="g-set-error"></a><h3>g_set_error ()</h3> 633<pre class="programlisting">void g_set_error (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err, 634 <a class="link" href="glib-Quarks.html#GQuark" title="GQuark">GQuark</a> domain, 635 <a class="link" href="glib-Basic-Types.html#gint" title="gint">gint</a> code, 636 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 637 ...);</pre> 638<p> 639Does nothing if <em class="parameter"><code>err</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>; if <em class="parameter"><code>err</code></em> is non-<a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, then *<em class="parameter"><code>err</code></em> must 640be <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>. A new <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> is created and assigned to *<em class="parameter"><code>err</code></em>.</p> 641<p> 642 643</p> 644<div class="variablelist"><table border="0"> 645<col align="left" valign="top"> 646<tbody> 647<tr> 648<td><p><span class="term"><em class="parameter"><code>err</code></em> :</span></p></td> 649<td> a return location for a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>, or <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> 650</td> 651</tr> 652<tr> 653<td><p><span class="term"><em class="parameter"><code>domain</code></em> :</span></p></td> 654<td> error domain 655</td> 656</tr> 657<tr> 658<td><p><span class="term"><em class="parameter"><code>code</code></em> :</span></p></td> 659<td> error code 660</td> 661</tr> 662<tr> 663<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td> 664<td> <code class="function">printf()</code>-style format 665</td> 666</tr> 667<tr> 668<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td> 669<td> args for <em class="parameter"><code>format</code></em> 670</td> 671</tr> 672</tbody> 673</table></div> 674</div> 675<hr> 676<div class="refsect2" lang="en"> 677<a name="g-propagate-error"></a><h3>g_propagate_error ()</h3> 678<pre class="programlisting">void g_propagate_error (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **dest, 679 <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *src);</pre> 680<p> 681If <em class="parameter"><code>dest</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, free <em class="parameter"><code>src</code></em>; otherwise, moves <em class="parameter"><code>src</code></em> into *<em class="parameter"><code>dest</code></em>. 682The error variable <em class="parameter"><code>dest</code></em> points to must be <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>.</p> 683<p> 684 685</p> 686<div class="variablelist"><table border="0"> 687<col align="left" valign="top"> 688<tbody> 689<tr> 690<td><p><span class="term"><em class="parameter"><code>dest</code></em> :</span></p></td> 691<td> error return location 692</td> 693</tr> 694<tr> 695<td><p><span class="term"><em class="parameter"><code>src</code></em> :</span></p></td> 696<td> error to move into the return location 697</td> 698</tr> 699</tbody> 700</table></div> 701</div> 702<hr> 703<div class="refsect2" lang="en"> 704<a name="g-clear-error"></a><h3>g_clear_error ()</h3> 705<pre class="programlisting">void g_clear_error (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err);</pre> 706<p> 707If <em class="parameter"><code>err</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, does nothing. If <em class="parameter"><code>err</code></em> is non-<a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, 708calls <a class="link" href="glib-Error-Reporting.html#g-error-free" title="g_error_free ()"><code class="function">g_error_free()</code></a> on *<em class="parameter"><code>err</code></em> and sets *<em class="parameter"><code>err</code></em> to <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>.</p> 709<p> 710 711</p> 712<div class="variablelist"><table border="0"> 713<col align="left" valign="top"> 714<tbody><tr> 715<td><p><span class="term"><em class="parameter"><code>err</code></em> :</span></p></td> 716<td> a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a> return location 717</td> 718</tr></tbody> 719</table></div> 720</div> 721<hr> 722<div class="refsect2" lang="en"> 723<a name="g-prefix-error"></a><h3>g_prefix_error ()</h3> 724<pre class="programlisting">void g_prefix_error (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **err, 725 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 726 ...);</pre> 727<p> 728Formats a string according to <em class="parameter"><code>format</code></em> and 729prefix it to an existing error message. If 730<em class="parameter"><code>err</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> (ie: no error variable) then do 731nothing. 732</p> 733<p> 734If *<em class="parameter"><code>err</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> (ie: an error variable is 735present but there is no error condition) then 736also do nothing. Whether or not it makes 737sense to take advantage of this feature is up 738to you.</p> 739<p> 740 741</p> 742<div class="variablelist"><table border="0"> 743<col align="left" valign="top"> 744<tbody> 745<tr> 746<td><p><span class="term"><em class="parameter"><code>err</code></em> :</span></p></td> 747<td> a return location for a <a class="link" href="glib-Error-Reporting.html#GError" title="GError"><span class="type">GError</span></a>, or <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a> 748</td> 749</tr> 750<tr> 751<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td> 752<td> <code class="function">printf()</code>-style format string 753</td> 754</tr> 755<tr> 756<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td> 757<td> arguments to <em class="parameter"><code>format</code></em> 758</td> 759</tr> 760</tbody> 761</table></div> 762<p class="since">Since 2.16 763</p> 764</div> 765<hr> 766<div class="refsect2" lang="en"> 767<a name="g-propagate-prefixed-error"></a><h3>g_propagate_prefixed_error ()</h3> 768<pre class="programlisting">void g_propagate_prefixed_error (<a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> **dest, 769 <a class="link" href="glib-Error-Reporting.html#GError" title="GError">GError</a> *src, 770 const <a class="link" href="glib-Basic-Types.html#gchar" title="gchar">gchar</a> *format, 771 ...);</pre> 772<p> 773If <em class="parameter"><code>dest</code></em> is <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>, free <em class="parameter"><code>src</code></em>; otherwise, 774moves <em class="parameter"><code>src</code></em> into *<em class="parameter"><code>dest</code></em>. *<em class="parameter"><code>dest</code></em> must be <a class="link" href="glib-Standard-Macros.html#NULL:CAPS" title="NULL"><code class="literal">NULL</code></a>. 775After the move, add a prefix as with 776<a class="link" href="glib-Error-Reporting.html#g-prefix-error" title="g_prefix_error ()"><code class="function">g_prefix_error()</code></a>.</p> 777<p> 778 779</p> 780<div class="variablelist"><table border="0"> 781<col align="left" valign="top"> 782<tbody> 783<tr> 784<td><p><span class="term"><em class="parameter"><code>dest</code></em> :</span></p></td> 785<td> error return location 786</td> 787</tr> 788<tr> 789<td><p><span class="term"><em class="parameter"><code>src</code></em> :</span></p></td> 790<td> error to move into the return location 791</td> 792</tr> 793<tr> 794<td><p><span class="term"><em class="parameter"><code>format</code></em> :</span></p></td> 795<td> <code class="function">printf()</code>-style format string 796</td> 797</tr> 798<tr> 799<td><p><span class="term"><em class="parameter"><code>...</code></em> :</span></p></td> 800<td> arguments to <em class="parameter"><code>format</code></em> 801</td> 802</tr> 803</tbody> 804</table></div> 805<p class="since">Since 2.16 806</p> 807</div> 808</div> 809</div> 810<div class="footer"> 811<hr> 812 Generated by GTK-Doc V1.10</div> 813</body> 814</html>