1 /* GLIB - Library of useful routines for C programming
2 * Copyright (C) 1995-1997 Peter Mattis, Spencer Kimball and Josh MacDonald
4 * This library is free software; you can redistribute it and/or
5 * modify it under the terms of the GNU Lesser General Public
6 * License as published by the Free Software Foundation; either
7 * version 2 of the License, or (at your option) any later version.
9 * This library is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
12 * Lesser General Public License for more details.
14 * You should have received a copy of the GNU Lesser General Public
15 * License along with this library; if not, see <http://www.gnu.org/licenses/>.
19 * Modified by the GLib Team and others 1997-2000. See the AUTHORS
20 * file for a list of people on the GLib Team. See the ChangeLog
21 * files for a list of changes. These files are distributed with
22 * GLib at ftp://ftp.gtk.org/pub/gtk/.
26 * SECTION:error_reporting
27 * @Title: Error Reporting
28 * @Short_description: a system for reporting errors
30 * GLib provides a standard method of reporting errors from a called
31 * function to the calling code. (This is the same problem solved by
32 * exceptions in other languages.) It's important to understand that
33 * this method is both a data type (the #GError struct) and a set of
34 * rules. If you use #GError incorrectly, then your code will not
35 * properly interoperate with other code that uses #GError, and users
36 * of your API will probably get confused.
38 * First and foremost: #GError should only be used to report recoverable
39 * runtime errors, never to report programming errors. If the programmer
40 * has screwed up, then you should use g_warning(), g_return_if_fail(),
41 * g_assert(), g_error(), or some similar facility. (Incidentally,
42 * remember that the g_error() function should only be used for
43 * programming errors, it should not be used to print any error
44 * reportable via #GError.)
46 * Examples of recoverable runtime errors are "file not found" or
47 * "failed to parse input." Examples of programming errors are "NULL
48 * passed to strcmp()" or "attempted to free the same pointer twice."
49 * These two kinds of errors are fundamentally different: runtime errors
50 * should be handled or reported to the user, programming errors should
51 * be eliminated by fixing the bug in the program. This is why most
52 * functions in GLib and GTK+ do not use the #GError facility.
54 * Functions that can fail take a return location for a #GError as their
55 * last argument. For example:
57 * gboolean g_file_get_contents (const gchar *filename,
62 * If you pass a non-%NULL value for the <literal>error</literal>
63 * argument, it should point to a location where an error can be placed.
69 * g_file_get_contents ("foo.txt", &contents, NULL, &err);
70 * g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL));
73 * /* Report error to user, and free error */
74 * g_assert (contents == NULL);
75 * fprintf (stderr, "Unable to read file: %s\n", err->message);
80 * /* Use file contents */
81 * g_assert (contents != NULL);
84 * Note that <literal>err != NULL</literal> in this example is a
85 * reliable indicator of whether g_file_get_contents() failed.
86 * Additionally, g_file_get_contents() returns a boolean which
87 * indicates whether it was successful.
89 * Because g_file_get_contents() returns %FALSE on failure, if you
90 * are only interested in whether it failed and don't need to display
91 * an error message, you can pass %NULL for the @error argument:
93 * if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) /* ignore errors */
94 * /* no error occurred */ ;
96 * /* error */ ;
99 * The #GError object contains three fields: @domain indicates the module
100 * the error-reporting function is located in, @code indicates the specific
101 * error that occurred, and @message is a user-readable error message with
102 * as many details as possible. Several functions are provided to deal
103 * with an error received from a called function: g_error_matches()
104 * returns %TRUE if the error matches a given domain and code,
105 * g_propagate_error() copies an error into an error location (so the
106 * calling function will receive it), and g_clear_error() clears an
107 * error location by freeing the error and resetting the location to
108 * %NULL. To display an error to the user, simply display the @message,
109 * perhaps along with additional context known only to the calling
110 * function (the file being opened, or whatever - though in the
111 * g_file_get_contents() case, the @message already contains a filename).
113 * When implementing a function that can report errors, the basic
114 * tool is g_set_error(). Typically, if a fatal error occurs you
115 * want to g_set_error(), then return immediately. g_set_error()
116 * does nothing if the error location passed to it is %NULL.
120 * foo_open_file (GError **error)
124 * fd = open ("file.txt", O_RDONLY);
128 * g_set_error (error,
129 * FOO_ERROR, /* error domain */
130 * FOO_ERROR_BLAH, /* error code */
131 * "Failed to open file: %s", /* error message format string */
132 * g_strerror (errno));
140 * Things are somewhat more complicated if you yourself call another
141 * function that can report a #GError. If the sub-function indicates
142 * fatal errors in some way other than reporting a #GError, such as
143 * by returning %TRUE on success, you can simply do the following:
146 * my_function_that_can_fail (GError **err)
148 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
150 * if (!sub_function_that_can_fail (err))
152 * /* assert that error was set by the sub-function */
153 * g_assert (err == NULL || *err != NULL);
157 * /* otherwise continue, no error occurred */
158 * g_assert (err == NULL || *err == NULL);
162 * If the sub-function does not indicate errors other than by
163 * reporting a #GError, you need to create a temporary #GError
164 * since the passed-in one may be %NULL. g_propagate_error() is
165 * intended for use in this case.
168 * my_function_that_can_fail (GError **err)
172 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
175 * sub_function_that_can_fail (&tmp_error);
177 * if (tmp_error != NULL)
179 * /* store tmp_error in err, if err != NULL,
180 * * otherwise call g_error_free() on tmp_error
182 * g_propagate_error (err, tmp_error);
186 * /* otherwise continue, no error occurred */
190 * Error pileups are always a bug. For example, this code is incorrect:
193 * my_function_that_can_fail (GError **err)
197 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
200 * sub_function_that_can_fail (&tmp_error);
201 * other_function_that_can_fail (&tmp_error);
203 * if (tmp_error != NULL)
205 * g_propagate_error (err, tmp_error);
210 * @tmp_error should be checked immediately after sub_function_that_can_fail(),
211 * and either cleared or propagated upward. The rule is: after each error,
212 * you must either handle the error, or return it to the calling function.
214 * Note that passing %NULL for the error location is the equivalent
215 * of handling an error by always doing nothing about it. So the
216 * following code is fine, assuming errors in sub_function_that_can_fail()
217 * are not fatal to my_function_that_can_fail():
220 * my_function_that_can_fail (GError **err)
224 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
226 * sub_function_that_can_fail (NULL); /* ignore errors */
229 * other_function_that_can_fail (&tmp_error);
231 * if (tmp_error != NULL)
233 * g_propagate_error (err, tmp_error);
239 * Note that passing %NULL for the error location ignores errors; it's
241 * <literal>try { sub_function_that_can_fail (); } catch (...) {}</literal>
242 * in C++. It does not mean to leave errors unhandled; it means to
243 * handle them by doing nothing.
245 * Error domains and codes are conventionally named as follows:
247 * - The error domain is called <NAMESPACE>_<MODULE>_ERROR,
248 * for example %G_SPAWN_ERROR or %G_THREAD_ERROR:
250 * #define G_SPAWN_ERROR g_spawn_error_quark ()
253 * g_spawn_error_quark (void)
255 * return g_quark_from_static_string ("g-spawn-error-quark");
259 * - The quark function for the error domain is called
260 * <namespace>_<module>_error_quark,
261 * for example g_spawn_error_quark() or g_thread_error_quark().
263 * - The error codes are in an enumeration called
264 * <Namespace><Module>Error;
265 * for example,#GThreadError or #GSpawnError.
267 * - Members of the error code enumeration are called
268 * <NAMESPACE>_<MODULE>_ERROR_<CODE>,
269 * for example %G_SPAWN_ERROR_FORK or %G_THREAD_ERROR_AGAIN.
271 * - If there's a "generic" or "unknown" error code for unrecoverable
272 * errors it doesn't make sense to distinguish with specific codes,
273 * it should be called <NAMESPACE>_<MODULE>_ERROR_FAILED,
274 * for example %G_SPAWN_ERROR_FAILED.
276 * Summary of rules for use of #GError:
278 * - Do not report programming errors via #GError.
280 * - The last argument of a function that returns an error should
281 * be a location where a #GError can be placed (i.e. "#GError** error").
282 * If #GError is used with varargs, the #GError** should be the last
283 * argument before the "...".
285 * - The caller may pass %NULL for the #GError** if they are not interested
286 * in details of the exact error that occurred.
288 * - If %NULL is passed for the #GError** argument, then errors should
289 * not be returned to the caller, but your function should still
290 * abort and return if an error occurs. That is, control flow should
291 * not be affected by whether the caller wants to get a #GError.
293 * - If a #GError is reported, then your function by definition had a
294 * fatal failure and did not complete whatever it was supposed to do.
295 * If the failure was not fatal, then you handled it and you should not
296 * report it. If it was fatal, then you must report it and discontinue
297 * whatever you were doing immediately.
299 * - If a #GError is reported, out parameters are not guaranteed to
300 * be set to any defined value.
302 * - A #GError* must be initialized to %NULL before passing its address
303 * to a function that can report errors.
305 * - "Piling up" errors is always a bug. That is, if you assign a
306 * new #GError to a #GError* that is non-%NULL, thus overwriting
307 * the previous error, it indicates that you should have aborted
308 * the operation instead of continuing. If you were able to continue,
309 * you should have cleared the previous error with g_clear_error().
310 * g_set_error() will complain if you pile up errors.
312 * - By convention, if you return a boolean value indicating success
313 * then %TRUE means success and %FALSE means failure. If %FALSE is
314 * returned, the error must be set to a non-%NULL value.
316 * - A %NULL return value is also frequently used to mean that an error
317 * occurred. You should make clear in your documentation whether %NULL
318 * is a valid return value in non-error cases; if %NULL is a valid value,
319 * then users must check whether an error was returned to see if the
320 * function succeeded.
322 * - When implementing a function that can report errors, you may want
323 * to add a check at the top of your function that the error return
324 * location is either %NULL or contains a %NULL error (e.g.
325 * <literal>g_return_if_fail (error == NULL || *error == NULL);</literal>).
333 #include "gstrfuncs.h"
334 #include "gtestutils.h"
337 * g_error_new_valist:
338 * @domain: error domain
340 * @format: printf()-style format for error message
341 * @args: #va_list of parameters for the message format
343 * Creates a new #GError with the given @domain and @code,
344 * and a message formatted with @format.
346 * Returns: a new #GError
351 g_error_new_valist (GQuark domain,
358 /* Historically, GError allowed this (although it was never meant to work),
359 * and it has significant use in the wild, which g_return_val_if_fail
360 * would break. It should maybe g_return_val_if_fail in GLib 4.
361 * (GNOME#660371, GNOME#560482)
363 g_warn_if_fail (domain != 0);
364 g_warn_if_fail (format != NULL);
366 error = g_slice_new (GError);
368 error->domain = domain;
370 error->message = g_strdup_vprintf (format, args);
377 * @domain: error domain
379 * @format: printf()-style format for error message
380 * @...: parameters for message format
382 * Creates a new #GError with the given @domain and @code,
383 * and a message formatted with @format.
385 * Return value: a new #GError
388 g_error_new (GQuark domain,
396 g_return_val_if_fail (format != NULL, NULL);
397 g_return_val_if_fail (domain != 0, NULL);
399 va_start (args, format);
400 error = g_error_new_valist (domain, code, format, args);
407 * g_error_new_literal:
408 * @domain: error domain
410 * @message: error message
412 * Creates a new #GError; unlike g_error_new(), @message is
413 * not a printf()-style format string. Use this function if
414 * @message contains text you don't have control over,
415 * that could include printf() escape sequences.
417 * Return value: a new #GError
420 g_error_new_literal (GQuark domain,
422 const gchar *message)
426 g_return_val_if_fail (message != NULL, NULL);
427 g_return_val_if_fail (domain != 0, NULL);
429 err = g_slice_new (GError);
431 err->domain = domain;
433 err->message = g_strdup (message);
442 * Frees a #GError and associated resources.
445 g_error_free (GError *error)
447 g_return_if_fail (error != NULL);
449 g_free (error->message);
451 g_slice_free (GError, error);
458 * Makes a copy of @error.
460 * Return value: a new #GError
463 g_error_copy (const GError *error)
467 g_return_val_if_fail (error != NULL, NULL);
468 /* See g_error_new_valist for why these don't return */
469 g_warn_if_fail (error->domain != 0);
470 g_warn_if_fail (error->message != NULL);
472 copy = g_slice_new (GError);
476 copy->message = g_strdup (error->message);
483 * @error: (allow-none): a #GError or %NULL
484 * @domain: an error domain
485 * @code: an error code
487 * Returns %TRUE if @error matches @domain and @code, %FALSE
488 * otherwise. In particular, when @error is %NULL, %FALSE will
491 * Return value: whether @error has @domain and @code
494 g_error_matches (const GError *error,
499 error->domain == domain &&
503 #define ERROR_OVERWRITTEN_WARNING "GError set over the top of a previous GError or uninitialized memory.\n" \
504 "This indicates a bug in someone's code. You must ensure an error is NULL before it's set.\n" \
505 "The overwriting error message was: %s"
509 * @err: (allow-none): a return location for a #GError, or %NULL
510 * @domain: error domain
512 * @format: printf()-style format
513 * @...: args for @format
515 * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
516 * must be %NULL. A new #GError is created and assigned to *@err.
519 g_set_error (GError **err,
532 va_start (args, format);
533 new = g_error_new_valist (domain, code, format, args);
540 g_warning (ERROR_OVERWRITTEN_WARNING, new->message);
546 * g_set_error_literal:
547 * @err: (allow-none): a return location for a #GError, or %NULL
548 * @domain: error domain
550 * @message: error message
552 * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
553 * must be %NULL. A new #GError is created and assigned to *@err.
554 * Unlike g_set_error(), @message is not a printf()-style format string.
555 * Use this function if @message contains text you don't have control over,
556 * that could include printf() escape sequences.
561 g_set_error_literal (GError **err,
564 const gchar *message)
570 *err = g_error_new_literal (domain, code, message);
572 g_warning (ERROR_OVERWRITTEN_WARNING, message);
577 * @dest: error return location
578 * @src: error to move into the return location
580 * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
581 * The error variable @dest points to must be %NULL.
584 g_propagate_error (GError **dest,
587 g_return_if_fail (src != NULL);
599 g_warning (ERROR_OVERWRITTEN_WARNING, src->message);
609 * @err: a #GError return location
611 * If @err is %NULL, does nothing. If @err is non-%NULL,
612 * calls g_error_free() on *@err and sets *@err to %NULL.
615 g_clear_error (GError **err)
626 g_error_add_prefix (gchar **string,
633 prefix = g_strdup_vprintf (format, ap);
635 *string = g_strconcat (prefix, oldstring, NULL);
642 * @err: (allow-none): a return location for a #GError, or %NULL
643 * @format: printf()-style format string
644 * @...: arguments to @format
646 * Formats a string according to @format and prefix it to an existing
647 * error message. If @err is %NULL (ie: no error variable) then do
650 * If *@err is %NULL (ie: an error variable is present but there is no
651 * error condition) then also do nothing. Whether or not it makes sense
652 * to take advantage of this feature is up to you.
657 g_prefix_error (GError **err,
665 va_start (ap, format);
666 g_error_add_prefix (&(*err)->message, format, ap);
672 * g_propagate_prefixed_error:
673 * @dest: error return location
674 * @src: error to move into the return location
675 * @format: printf()-style format string
676 * @...: arguments to @format
678 * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
679 * *@dest must be %NULL. After the move, add a prefix as with
685 g_propagate_prefixed_error (GError **dest,
690 g_propagate_error (dest, src);
696 va_start (ap, format);
697 g_error_add_prefix (&(*dest)->message, format, ap);