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.
68 * g_file_get_contents ("foo.txt", &contents, NULL, &err);
69 * g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL));
72 * /* Report error to user, and free error */
73 * g_assert (contents == NULL);
74 * fprintf (stderr, "Unable to read file: %s\n", err->message);
79 * /* Use file contents */
80 * g_assert (contents != NULL);
83 * Note that <literal>err != NULL</literal> in this example is a
84 * reliable indicator of whether g_file_get_contents() failed.
85 * Additionally, g_file_get_contents() returns a boolean which
86 * indicates whether it was successful.
88 * Because g_file_get_contents() returns %FALSE on failure, if you
89 * are only interested in whether it failed and don't need to display
90 * an error message, you can pass %NULL for the @error argument:
92 * if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) /* ignore errors */
93 * /* no error occurred */ ;
95 * /* error */ ;
98 * The #GError object contains three fields: @domain indicates the module
99 * the error-reporting function is located in, @code indicates the specific
100 * error that occurred, and @message is a user-readable error message with
101 * as many details as possible. Several functions are provided to deal
102 * with an error received from a called function: g_error_matches()
103 * returns %TRUE if the error matches a given domain and code,
104 * g_propagate_error() copies an error into an error location (so the
105 * calling function will receive it), and g_clear_error() clears an
106 * error location by freeing the error and resetting the location to
107 * %NULL. To display an error to the user, simply display the @message,
108 * perhaps along with additional context known only to the calling
109 * function (the file being opened, or whatever - though in the
110 * g_file_get_contents() case, the @message already contains a filename).
112 * When implementing a function that can report errors, the basic
113 * tool is g_set_error(). Typically, if a fatal error occurs you
114 * want to g_set_error(), then return immediately. g_set_error()
115 * does nothing if the error location passed to it is %NULL.
119 * foo_open_file (GError **error)
123 * fd = open ("file.txt", O_RDONLY);
127 * g_set_error (error,
128 * FOO_ERROR, /* error domain */
129 * FOO_ERROR_BLAH, /* error code */
130 * "Failed to open file: %s", /* error message format string */
131 * g_strerror (errno));
139 * Things are somewhat more complicated if you yourself call another
140 * function that can report a #GError. If the sub-function indicates
141 * fatal errors in some way other than reporting a #GError, such as
142 * by returning %TRUE on success, you can simply do the following:
145 * my_function_that_can_fail (GError **err)
147 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
149 * if (!sub_function_that_can_fail (err))
151 * /* assert that error was set by the sub-function */
152 * g_assert (err == NULL || *err != NULL);
156 * /* otherwise continue, no error occurred */
157 * g_assert (err == NULL || *err == NULL);
161 * If the sub-function does not indicate errors other than by
162 * reporting a #GError, you need to create a temporary #GError
163 * since the passed-in one may be %NULL. g_propagate_error() is
164 * intended for use in this case.
167 * my_function_that_can_fail (GError **err)
171 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
174 * sub_function_that_can_fail (&tmp_error);
176 * if (tmp_error != NULL)
178 * /* store tmp_error in err, if err != NULL,
179 * * otherwise call g_error_free() on tmp_error
181 * g_propagate_error (err, tmp_error);
185 * /* otherwise continue, no error occurred */
189 * Error pileups are always a bug. For example, this code is incorrect:
192 * my_function_that_can_fail (GError **err)
196 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
199 * sub_function_that_can_fail (&tmp_error);
200 * other_function_that_can_fail (&tmp_error);
202 * if (tmp_error != NULL)
204 * g_propagate_error (err, tmp_error);
209 * @tmp_error should be checked immediately after sub_function_that_can_fail(),
210 * and either cleared or propagated upward. The rule is: after each error,
211 * you must either handle the error, or return it to the calling function.
213 * Note that passing %NULL for the error location is the equivalent
214 * of handling an error by always doing nothing about it. So the
215 * following code is fine, assuming errors in sub_function_that_can_fail()
216 * are not fatal to my_function_that_can_fail():
219 * my_function_that_can_fail (GError **err)
223 * g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
225 * sub_function_that_can_fail (NULL); /* ignore errors */
228 * other_function_that_can_fail (&tmp_error);
230 * if (tmp_error != NULL)
232 * g_propagate_error (err, tmp_error);
238 * Note that passing %NULL for the error location ignores errors; it's
240 * <literal>try { sub_function_that_can_fail (); } catch (...) {}</literal>
241 * in C++. It does not mean to leave errors unhandled; it means to
242 * handle them by doing nothing.
244 * Error domains and codes are conventionally named as follows:
247 * The error domain is called
248 * <literal><NAMESPACE>_<MODULE>_ERROR</literal>,
249 * for example %G_SPAWN_ERROR or %G_THREAD_ERROR:
251 * #define G_SPAWN_ERROR g_spawn_error_quark ()
254 * g_spawn_error_quark (void)
256 * return g_quark_from_static_string ("g-spawn-error-quark");
261 * The quark function for the error domain is called
262 * <literal><namespace>_<module>_error_quark</literal>,
263 * for example g_spawn_error_quark() or g_thread_error_quark().
266 * The error codes are in an enumeration called
267 * <literal><Namespace><Module>Error</literal>;
268 * for example,#GThreadError or #GSpawnError.
271 * Members of the error code enumeration are called
272 * <literal><NAMESPACE>_<MODULE>_ERROR_<CODE></literal>,
273 * for example %G_SPAWN_ERROR_FORK or %G_THREAD_ERROR_AGAIN.
276 * If there's a "generic" or "unknown" error code for unrecoverable
277 * errors it doesn't make sense to distinguish with specific codes,
278 * it should be called <literal><NAMESPACE>_<MODULE>_ERROR_FAILED</literal>,
279 * for example %G_SPAWN_ERROR_FAILED.
283 * Summary of rules for use of #GError:
286 * Do not report programming errors via #GError.
289 * The last argument of a function that returns an error should
290 * be a location where a #GError can be placed (i.e. "#GError** error").
291 * If #GError is used with varargs, the #GError** should be the last
292 * argument before the "...".
295 * The caller may pass %NULL for the #GError** if they are not interested
296 * in details of the exact error that occurred.
299 * If %NULL is passed for the #GError** argument, then errors should
300 * not be returned to the caller, but your function should still
301 * abort and return if an error occurs. That is, control flow should
302 * not be affected by whether the caller wants to get a #GError.
305 * If a #GError is reported, then your function by definition had a
306 * fatal failure and did not complete whatever it was supposed to do.
307 * If the failure was not fatal, then you handled it and you should not
308 * report it. If it was fatal, then you must report it and discontinue
309 * whatever you were doing immediately.
312 * If a #GError is reported, out parameters are not guaranteed to
313 * be set to any defined value.
316 * A #GError* must be initialized to %NULL before passing its address
317 * to a function that can report errors.
320 * "Piling up" errors is always a bug. That is, if you assign a
321 * new #GError to a #GError* that is non-%NULL, thus overwriting
322 * the previous error, it indicates that you should have aborted
323 * the operation instead of continuing. If you were able to continue,
324 * you should have cleared the previous error with g_clear_error().
325 * g_set_error() will complain if you pile up errors.
328 * By convention, if you return a boolean value indicating success
329 * then %TRUE means success and %FALSE means failure. If %FALSE is
330 * returned, the error must be set to a non-%NULL
334 * A %NULL return value is also frequently used to mean that an error
335 * occurred. You should make clear in your documentation whether %NULL
336 * is a valid return value in non-error cases; if %NULL is a valid value,
337 * then users must check whether an error was returned to see if the
338 * function succeeded.
341 * When implementing a function that can report errors, you may want
342 * to add a check at the top of your function that the error return
343 * location is either %NULL or contains a %NULL error (e.g.
344 * <literal>g_return_if_fail (error == NULL || *error == NULL);</literal>).
354 #include "gstrfuncs.h"
355 #include "gtestutils.h"
358 * g_error_new_valist:
359 * @domain: error domain
361 * @format: printf()-style format for error message
362 * @args: #va_list of parameters for the message format
364 * Creates a new #GError with the given @domain and @code,
365 * and a message formatted with @format.
367 * Returns: a new #GError
372 g_error_new_valist (GQuark domain,
379 /* Historically, GError allowed this (although it was never meant to work),
380 * and it has significant use in the wild, which g_return_val_if_fail
381 * would break. It should maybe g_return_val_if_fail in GLib 4.
382 * (GNOME#660371, GNOME#560482)
384 g_warn_if_fail (domain != 0);
385 g_warn_if_fail (format != NULL);
387 error = g_slice_new (GError);
389 error->domain = domain;
391 error->message = g_strdup_vprintf (format, args);
398 * @domain: error domain
400 * @format: printf()-style format for error message
401 * @...: parameters for message format
403 * Creates a new #GError with the given @domain and @code,
404 * and a message formatted with @format.
406 * Return value: a new #GError
409 g_error_new (GQuark domain,
417 g_return_val_if_fail (format != NULL, NULL);
418 g_return_val_if_fail (domain != 0, NULL);
420 va_start (args, format);
421 error = g_error_new_valist (domain, code, format, args);
428 * g_error_new_literal:
429 * @domain: error domain
431 * @message: error message
433 * Creates a new #GError; unlike g_error_new(), @message is
434 * not a printf()-style format string. Use this function if
435 * @message contains text you don't have control over,
436 * that could include printf() escape sequences.
438 * Return value: a new #GError
441 g_error_new_literal (GQuark domain,
443 const gchar *message)
447 g_return_val_if_fail (message != NULL, NULL);
448 g_return_val_if_fail (domain != 0, NULL);
450 err = g_slice_new (GError);
452 err->domain = domain;
454 err->message = g_strdup (message);
463 * Frees a #GError and associated resources.
466 g_error_free (GError *error)
468 g_return_if_fail (error != NULL);
470 g_free (error->message);
472 g_slice_free (GError, error);
479 * Makes a copy of @error.
481 * Return value: a new #GError
484 g_error_copy (const GError *error)
488 g_return_val_if_fail (error != NULL, NULL);
489 /* See g_error_new_valist for why these don't return */
490 g_warn_if_fail (error->domain != 0);
491 g_warn_if_fail (error->message != NULL);
493 copy = g_slice_new (GError);
497 copy->message = g_strdup (error->message);
504 * @error: (allow-none): a #GError or %NULL
505 * @domain: an error domain
506 * @code: an error code
508 * Returns %TRUE if @error matches @domain and @code, %FALSE
509 * otherwise. In particular, when @error is %NULL, %FALSE will
512 * Return value: whether @error has @domain and @code
515 g_error_matches (const GError *error,
520 error->domain == domain &&
524 #define ERROR_OVERWRITTEN_WARNING "GError set over the top of a previous GError or uninitialized memory.\n" \
525 "This indicates a bug in someone's code. You must ensure an error is NULL before it's set.\n" \
526 "The overwriting error message was: %s"
530 * @err: (allow-none): a return location for a #GError, or %NULL
531 * @domain: error domain
533 * @format: printf()-style format
534 * @...: args for @format
536 * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
537 * must be %NULL. A new #GError is created and assigned to *@err.
540 g_set_error (GError **err,
553 va_start (args, format);
554 new = g_error_new_valist (domain, code, format, args);
561 g_warning (ERROR_OVERWRITTEN_WARNING, new->message);
567 * g_set_error_literal:
568 * @err: (allow-none): a return location for a #GError, or %NULL
569 * @domain: error domain
571 * @message: error message
573 * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
574 * must be %NULL. A new #GError is created and assigned to *@err.
575 * Unlike g_set_error(), @message is not a printf()-style format string.
576 * Use this function if @message contains text you don't have control over,
577 * that could include printf() escape sequences.
582 g_set_error_literal (GError **err,
585 const gchar *message)
591 *err = g_error_new_literal (domain, code, message);
593 g_warning (ERROR_OVERWRITTEN_WARNING, message);
598 * @dest: error return location
599 * @src: error to move into the return location
601 * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
602 * The error variable @dest points to must be %NULL.
605 g_propagate_error (GError **dest,
608 g_return_if_fail (src != NULL);
620 g_warning (ERROR_OVERWRITTEN_WARNING, src->message);
630 * @err: a #GError return location
632 * If @err is %NULL, does nothing. If @err is non-%NULL,
633 * calls g_error_free() on *@err and sets *@err to %NULL.
636 g_clear_error (GError **err)
647 g_error_add_prefix (gchar **string,
654 prefix = g_strdup_vprintf (format, ap);
656 *string = g_strconcat (prefix, oldstring, NULL);
663 * @err: (allow-none): a return location for a #GError, or %NULL
664 * @format: printf()-style format string
665 * @...: arguments to @format
667 * Formats a string according to @format and prefix it to an existing
668 * error message. If @err is %NULL (ie: no error variable) then do
671 * If *@err is %NULL (ie: an error variable is present but there is no
672 * error condition) then also do nothing. Whether or not it makes sense
673 * to take advantage of this feature is up to you.
678 g_prefix_error (GError **err,
686 va_start (ap, format);
687 g_error_add_prefix (&(*err)->message, format, ap);
693 * g_propagate_prefixed_error:
694 * @dest: error return location
695 * @src: error to move into the return location
696 * @format: printf()-style format string
697 * @...: arguments to @format
699 * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
700 * *@dest must be %NULL. After the move, add a prefix as with
706 g_propagate_prefixed_error (GError **dest,
711 g_propagate_error (dest, src);
717 va_start (ap, format);
718 g_error_add_prefix (&(*dest)->message, format, ap);