glib/gerror.c

   1 /* GLIB - Library of useful routines for C programming
   2  * Copyright (C) 1995-1997  Peter Mattis, Spencer Kimball and Josh MacDonald
   3  *
   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.
   8  *
   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.
  13  *
  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/>.
  16  */
  17
  18 /*
  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/.
  23  */
  24
  25 /**
  26  * SECTION:error_reporting
  27  * @Title: Error Reporting
  28  * @Short_description: a system for reporting errors
  29  *
  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.
  37  *
  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.)
  45  *
  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.
  53  *
  54  * Functions that can fail take a return location for a #GError as their
  55  * last argument. On error, a new #GError instance will be allocated and
  56  * returned to the caller via this argument. For example:
  57  * |[<!-- language="C" -->
  58  * gboolean g_file_get_contents (const gchar  *filename,
  59  *                               gchar       **contents,
  60  *                               gsize        *length,
  61  *                               GError      **error);
  62  * ]|
  63  * If you pass a non-%NULL value for the `error` argument, it should
  64  * point to a location where an error can be placed. For example:
  65  * |[<!-- language="C" -->
  66  * gchar *contents;
  67  * GError *err = NULL;
  68  *
  69  * g_file_get_contents ("foo.txt", &contents, NULL, &err);
  70  * g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL));
  71  * if (err != NULL)
  72  *   {
  73  *     // Report error to user, and free error
  74  *     g_assert (contents == NULL);
  75  *     fprintf (stderr, "Unable to read file: %s\n", err->message);
  76  *     g_error_free (err);
  77  *   }
  78  * else
  79  *   {
  80  *     // Use file contents
  81  *     g_assert (contents != NULL);
  82  *   }
  83  * ]|
  84  * Note that `err != NULL` in this example is a reliable indicator
  85  * of whether g_file_get_contents() failed. Additionally,
  86  * g_file_get_contents() returns a boolean which
  87  * indicates whether it was successful.
  88  *
  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:
  92  * |[<!-- language="C" -->
  93  * if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) // ignore errors
  94  *   // no error occurred
  95  *   ;
  96  * else
  97  *   // error
  98  *   ;
  99  * ]|
 100  *
 101  * The #GError object contains three fields: @domain indicates the module
 102  * the error-reporting function is located in, @code indicates the specific
 103  * error that occurred, and @message is a user-readable error message with
 104  * as many details as possible. Several functions are provided to deal
 105  * with an error received from a called function: g_error_matches()
 106  * returns %TRUE if the error matches a given domain and code,
 107  * g_propagate_error() copies an error into an error location (so the
 108  * calling function will receive it), and g_clear_error() clears an
 109  * error location by freeing the error and resetting the location to
 110  * %NULL. To display an error to the user, simply display the @message,
 111  * perhaps along with additional context known only to the calling
 112  * function (the file being opened, or whatever - though in the
 113  * g_file_get_contents() case, the @message already contains a filename).
 114  *
 115  * When implementing a function that can report errors, the basic
 116  * tool is g_set_error(). Typically, if a fatal error occurs you
 117  * want to g_set_error(), then return immediately. g_set_error()
 118  * does nothing if the error location passed to it is %NULL.
 119  * Here's an example:
 120  * |[<!-- language="C" -->
 121  * gint
 122  * foo_open_file (GError **error)
 123  * {
 124  *   gint fd;
 125  *
 126  *   fd = open ("file.txt", O_RDONLY);
 127  *
 128  *   if (fd < 0)
 129  *     {
 130  *       g_set_error (error,
 131  *                    FOO_ERROR,                 // error domain
 132  *                    FOO_ERROR_BLAH,            // error code
 133  *                    "Failed to open file: %s", // error message format string
 134  *                    g_strerror (errno));
 135  *       return -1;
 136  *     }
 137  *   else
 138  *     return fd;
 139  * }
 140  * ]|
 141  *
 142  * Things are somewhat more complicated if you yourself call another
 143  * function that can report a #GError. If the sub-function indicates
 144  * fatal errors in some way other than reporting a #GError, such as
 145  * by returning %TRUE on success, you can simply do the following:
 146  * |[<!-- language="C" -->
 147  * gboolean
 148  * my_function_that_can_fail (GError **err)
 149  * {
 150  *   g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
 151  *
 152  *   if (!sub_function_that_can_fail (err))
 153  *     {
 154  *       // assert that error was set by the sub-function
 155  *       g_assert (err == NULL || *err != NULL);
 156  *       return FALSE;
 157  *     }
 158  *
 159  *   // otherwise continue, no error occurred
 160  *   g_assert (err == NULL || *err == NULL);
 161  * }
 162  * ]|
 163  *
 164  * If the sub-function does not indicate errors other than by
 165  * reporting a #GError (or if its return value does not reliably indicate
 166  * errors) you need to create a temporary #GError
 167  * since the passed-in one may be %NULL. g_propagate_error() is
 168  * intended for use in this case.
 169  * |[<!-- language="C" -->
 170  * gboolean
 171  * my_function_that_can_fail (GError **err)
 172  * {
 173  *   GError *tmp_error;
 174  *
 175  *   g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
 176  *
 177  *   tmp_error = NULL;
 178  *   sub_function_that_can_fail (&tmp_error);
 179  *
 180  *   if (tmp_error != NULL)
 181  *     {
 182  *       // store tmp_error in err, if err != NULL,
 183  *       // otherwise call g_error_free() on tmp_error
 184  *       g_propagate_error (err, tmp_error);
 185  *       return FALSE;
 186  *     }
 187  *
 188  *   // otherwise continue, no error occurred
 189  * }
 190  * ]|
 191  *
 192  * Error pileups are always a bug. For example, this code is incorrect:
 193  * |[<!-- language="C" -->
 194  * gboolean
 195  * my_function_that_can_fail (GError **err)
 196  * {
 197  *   GError *tmp_error;
 198  *
 199  *   g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
 200  *
 201  *   tmp_error = NULL;
 202  *   sub_function_that_can_fail (&tmp_error);
 203  *   other_function_that_can_fail (&tmp_error);
 204  *
 205  *   if (tmp_error != NULL)
 206  *     {
 207  *       g_propagate_error (err, tmp_error);
 208  *       return FALSE;
 209  *     }
 210  * }
 211  * ]|
 212  * @tmp_error should be checked immediately after sub_function_that_can_fail(),
 213  * and either cleared or propagated upward. The rule is: after each error,
 214  * you must either handle the error, or return it to the calling function.
 215  *
 216  * Note that passing %NULL for the error location is the equivalent
 217  * of handling an error by always doing nothing about it. So the
 218  * following code is fine, assuming errors in sub_function_that_can_fail()
 219  * are not fatal to my_function_that_can_fail():
 220  * |[<!-- language="C" -->
 221  * gboolean
 222  * my_function_that_can_fail (GError **err)
 223  * {
 224  *   GError *tmp_error;
 225  *
 226  *   g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
 227  *
 228  *   sub_function_that_can_fail (NULL); // ignore errors
 229  *
 230  *   tmp_error = NULL;
 231  *   other_function_that_can_fail (&tmp_error);
 232  *
 233  *   if (tmp_error != NULL)
 234  *     {
 235  *       g_propagate_error (err, tmp_error);
 236  *       return FALSE;
 237  *     }
 238  * }
 239  * ]|
 240  *
 241  * Note that passing %NULL for the error location ignores errors;
 242  * it's equivalent to
 243  * `try { sub_function_that_can_fail (); } catch (...) {}`
 244  * in C++. It does not mean to leave errors unhandled; it means
 245  * to handle them by doing nothing.
 246  *
 247  * Error domains and codes are conventionally named as follows:
 248  *
 249  * - The error domain is called <NAMESPACE>_<MODULE>_ERROR,
 250  *   for example %G_SPAWN_ERROR or %G_THREAD_ERROR:
 251  *   |[<!-- language="C" -->
 252  *   #define G_SPAWN_ERROR g_spawn_error_quark ()
 253  *
 254  *   GQuark
 255  *   g_spawn_error_quark (void)
 256  *   {
 257  *       return g_quark_from_static_string ("g-spawn-error-quark");
 258  *   }
 259  *   ]|
 260  *
 261  * - The quark function for the error domain is called
 262  *   <namespace>_<module>_error_quark,
 263  *   for example g_spawn_error_quark() or g_thread_error_quark().
 264  *
 265  * - The error codes are in an enumeration called
 266  *   <Namespace><Module>Error;
 267  *   for example, #GThreadError or #GSpawnError.
 268  *
 269  * - Members of the error code enumeration are called
 270  *   <NAMESPACE>_<MODULE>_ERROR_<CODE>,
 271  *   for example %G_SPAWN_ERROR_FORK or %G_THREAD_ERROR_AGAIN.
 272  *
 273  * - If there's a "generic" or "unknown" error code for unrecoverable
 274  *   errors it doesn't make sense to distinguish with specific codes,
 275  *   it should be called <NAMESPACE>_<MODULE>_ERROR_FAILED,
 276  *   for example %G_SPAWN_ERROR_FAILED. In the case of error code
 277  *   enumerations that may be extended in future releases, you should
 278  *   generally not handle this error code explicitly, but should
 279  *   instead treat any unrecognized error code as equivalent to
 280  *   FAILED.
 281  *
 282  * Summary of rules for use of #GError:
 283  *
 284  * - Do not report programming errors via #GError.
 285  *
 286  * - The last argument of a function that returns an error should
 287  *   be a location where a #GError can be placed (i.e. "#GError** error").
 288  *   If #GError is used with varargs, the #GError** should be the last
 289  *   argument before the "...".
 290  *
 291  * - The caller may pass %NULL for the #GError** if they are not interested
 292  *   in details of the exact error that occurred.
 293  *
 294  * - If %NULL is passed for the #GError** argument, then errors should
 295  *   not be returned to the caller, but your function should still
 296  *   abort and return if an error occurs. That is, control flow should
 297  *   not be affected by whether the caller wants to get a #GError.
 298  *
 299  * - If a #GError is reported, then your function by definition had a
 300  *   fatal failure and did not complete whatever it was supposed to do.
 301  *   If the failure was not fatal, then you handled it and you should not
 302  *   report it. If it was fatal, then you must report it and discontinue
 303  *   whatever you were doing immediately.
 304  *
 305  * - If a #GError is reported, out parameters are not guaranteed to
 306  *   be set to any defined value.
 307  *
 308  * - A #GError* must be initialized to %NULL before passing its address
 309  *   to a function that can report errors.
 310  *
 311  * - "Piling up" errors is always a bug. That is, if you assign a
 312  *   new #GError to a #GError* that is non-%NULL, thus overwriting
 313  *   the previous error, it indicates that you should have aborted
 314  *   the operation instead of continuing. If you were able to continue,
 315  *   you should have cleared the previous error with g_clear_error().
 316  *   g_set_error() will complain if you pile up errors.
 317  *
 318  * - By convention, if you return a boolean value indicating success
 319  *   then %TRUE means success and %FALSE means failure.
 320  *   <footnote><para>Avoid creating functions which have a boolean
 321  *   return value and a GError parameter, but where the boolean does
 322  *   something other than signal whether the GError is set.  Among other
 323  *   problems, it requires C callers to allocate a temporary error.  Instead,
 324  *   provide a "gboolean *" out parameter. There are functions in GLib
 325  *   itself such as g_key_file_has_key() that are deprecated because of this.
 326  *   </para></footnote>
 327  *   If %FALSE is
 328  *   returned, the error must be set to a non-%NULL value.
 329  *   <footnote><para>One exception to this is that in situations that are
 330  *   already considered to be undefined behaviour (such as when a
 331  *   g_return_val_if_fail() check fails), the error need not be set.
 332  *   Instead of checking separately whether the error is set, callers
 333  *   should ensure that they do not provoke undefined behaviour, then
 334  *   assume that the error will be set on failure.</para></footnote>
 335  *
 336  * - A %NULL return value is also frequently used to mean that an error
 337  *   occurred. You should make clear in your documentation whether %NULL
 338  *   is a valid return value in non-error cases; if %NULL is a valid value,
 339  *   then users must check whether an error was returned to see if the
 340  *   function succeeded.
 341  *
 342  * - When implementing a function that can report errors, you may want
 343  *   to add a check at the top of your function that the error return
 344  *   location is either %NULL or contains a %NULL error (e.g.
 345  *   `g_return_if_fail (error == NULL || *error == NULL);`).
 346  */
 347
 348 #include "config.h"
 349
 350 #include "gerror.h"
 351
 352 #include "gslice.h"
 353 #include "gstrfuncs.h"
 354 #include "gtestutils.h"
 355
 356 /**
 357  * g_error_new_valist:
 358  * @domain: error domain
 359  * @code: error code
 360  * @format: printf()-style format for error message
 361  * @args: #va_list of parameters for the message format
 362  *
 363  * Creates a new #GError with the given @domain and @code,
 364  * and a message formatted with @format.
 365  *
 366  * Returns: a new #GError
 367  *
 368  * Since: 2.22
 369  */
 370 GError*
 371 g_error_new_valist (GQuark       domain,
 372                     gint         code,
 373                     const gchar *format,
 374                     va_list      args)
 375 {
 376   GError *error;
 377
 378   /* Historically, GError allowed this (although it was never meant to work),
 379    * and it has significant use in the wild, which g_return_val_if_fail
 380    * would break. It should maybe g_return_val_if_fail in GLib 4.
 381    * (GNOME#660371, GNOME#560482)
 382    */
 383   g_warn_if_fail (domain != 0);
 384   g_warn_if_fail (format != NULL);
 385
 386   error = g_slice_new (GError);
 387
 388   error->domain = domain;
 389   error->code = code;
 390   error->message = g_strdup_vprintf (format, args);
 391
 392   return error;
 393 }
 394
 395 /**
 396  * g_error_new:
 397  * @domain: error domain
 398  * @code: error code
 399  * @format: printf()-style format for error message
 400  * @...: parameters for message format
 401  *
 402  * Creates a new #GError with the given @domain and @code,
 403  * and a message formatted with @format.
 404  *
 405  * Returns: a new #GError
 406  */
 407 GError*
 408 g_error_new (GQuark       domain,
 409              gint         code,
 410              const gchar *format,
 411              ...)
 412 {
 413   GError* error;
 414   va_list args;
 415
 416   g_return_val_if_fail (format != NULL, NULL);
 417   g_return_val_if_fail (domain != 0, NULL);
 418
 419   va_start (args, format);
 420   error = g_error_new_valist (domain, code, format, args);
 421   va_end (args);
 422
 423   return error;
 424 }
 425
 426 /**
 427  * g_error_new_literal:
 428  * @domain: error domain
 429  * @code: error code
 430  * @message: error message
 431  *
 432  * Creates a new #GError; unlike g_error_new(), @message is
 433  * not a printf()-style format string. Use this function if
 434  * @message contains text you don't have control over,
 435  * that could include printf() escape sequences.
 436  *
 437  * Returns: a new #GError
 438  **/
 439 GError*
 440 g_error_new_literal (GQuark         domain,
 441                      gint           code,
 442                      const gchar   *message)
 443 {
 444   GError* err;
 445
 446   g_return_val_if_fail (message != NULL, NULL);
 447   g_return_val_if_fail (domain != 0, NULL);
 448
 449   err = g_slice_new (GError);
 450
 451   err->domain = domain;
 452   err->code = code;
 453   err->message = g_strdup (message);
 454
 455   return err;
 456 }
 457
 458 /**
 459  * g_error_free:
 460  * @error: a #GError
 461  *
 462  * Frees a #GError and associated resources.
 463  */
 464 void
 465 g_error_free (GError *error)
 466 {
 467   g_return_if_fail (error != NULL);
 468
 469   g_free (error->message);
 470
 471   g_slice_free (GError, error);
 472 }
 473
 474 /**
 475  * g_error_copy:
 476  * @error: a #GError
 477  *
 478  * Makes a copy of @error.
 479  *
 480  * Returns: a new #GError
 481  */
 482 GError*
 483 g_error_copy (const GError *error)
 484 {
 485   GError *copy;
 486
 487   g_return_val_if_fail (error != NULL, NULL);
 488   /* See g_error_new_valist for why these don't return */
 489   g_warn_if_fail (error->domain != 0);
 490   g_warn_if_fail (error->message != NULL);
 491
 492   copy = g_slice_new (GError);
 493
 494   *copy = *error;
 495
 496   copy->message = g_strdup (error->message);
 497
 498   return copy;
 499 }
 500
 501 /**
 502  * g_error_matches:
 503  * @error: (allow-none): a #GError or %NULL
 504  * @domain: an error domain
 505  * @code: an error code
 506  *
 507  * Returns %TRUE if @error matches @domain and @code, %FALSE
 508  * otherwise. In particular, when @error is %NULL, %FALSE will
 509  * be returned.
 510  *
 511  * If @domain contains a `FAILED` (or otherwise generic) error code,
 512  * you should generally not check for it explicitly, but should
 513  * instead treat any not-explicitly-recognized error code as being
 514  * equilalent to the `FAILED` code. This way, if the domain is
 515  * extended in the future to provide a more specific error code for
 516  * a certain case, your code will still work.
 517  *
 518  * Returns: whether @error has @domain and @code
 519  */
 520 gboolean
 521 g_error_matches (const GError *error,
 522                  GQuark        domain,
 523                  gint          code)
 524 {
 525   return error &&
 526     error->domain == domain &&
 527     error->code == code;
 528 }
 529
 530 #define ERROR_OVERWRITTEN_WARNING "GError set over the top of a previous GError or uninitialized memory.\n" \
 531                "This indicates a bug in someone's code. You must ensure an error is NULL before it's set.\n" \
 532                "The overwriting error message was: %s"
 533
 534 /**
 535  * g_set_error:
 536  * @err: (allow-none): a return location for a #GError, or %NULL
 537  * @domain: error domain
 538  * @code: error code
 539  * @format: printf()-style format
 540  * @...: args for @format
 541  *
 542  * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
 543  * must be %NULL. A new #GError is created and assigned to *@err.
 544  */
 545 void
 546 g_set_error (GError      **err,
 547              GQuark        domain,
 548              gint          code,
 549              const gchar  *format,
 550              ...)
 551 {
 552   GError *new;
 553
 554   va_list args;
 555
 556   if (err == NULL)
 557     return;
 558
 559   va_start (args, format);
 560   new = g_error_new_valist (domain, code, format, args);
 561   va_end (args);
 562
 563   if (*err == NULL)
 564     *err = new;
 565   else
 566     {
 567       g_warning (ERROR_OVERWRITTEN_WARNING, new->message);
 568       g_error_free (new);
 569     }
 570 }
 571
 572 /**
 573  * g_set_error_literal:
 574  * @err: (allow-none): a return location for a #GError, or %NULL
 575  * @domain: error domain
 576  * @code: error code
 577  * @message: error message
 578  *
 579  * Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
 580  * must be %NULL. A new #GError is created and assigned to *@err.
 581  * Unlike g_set_error(), @message is not a printf()-style format string.
 582  * Use this function if @message contains text you don't have control over,
 583  * that could include printf() escape sequences.
 584  *
 585  * Since: 2.18
 586  */
 587 void
 588 g_set_error_literal (GError      **err,
 589                      GQuark        domain,
 590                      gint          code,
 591                      const gchar  *message)
 592 {
 593   if (err == NULL)
 594     return;
 595
 596   if (*err == NULL)
 597     *err = g_error_new_literal (domain, code, message);
 598   else
 599     g_warning (ERROR_OVERWRITTEN_WARNING, message);
 600 }
 601
 602 /**
 603  * g_propagate_error:
 604  * @dest: error return location
 605  * @src: error to move into the return location
 606  *
 607  * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
 608  * The error variable @dest points to must be %NULL.
 609  *
 610  * Note that @src is no longer valid after this call. If you want
 611  * to keep using the same GError*, you need to set it to %NULL
 612  * after calling this function on it.
 613  */
 614 void
 615 g_propagate_error (GError **dest,
 616                    GError  *src)
 617 {
 618   g_return_if_fail (src != NULL);
 619
 620   if (dest == NULL)
 621     {
 622       if (src)
 623         g_error_free (src);
 624       return;
 625     }
 626   else
 627     {
 628       if (*dest != NULL)
 629         {
 630           g_warning (ERROR_OVERWRITTEN_WARNING, src->message);
 631           g_error_free (src);
 632         }
 633       else
 634         *dest = src;
 635     }
 636 }
 637
 638 /**
 639  * g_clear_error:
 640  * @err: a #GError return location
 641  *
 642  * If @err is %NULL, does nothing. If @err is non-%NULL,
 643  * calls g_error_free() on *@err and sets *@err to %NULL.
 644  */
 645 void
 646 g_clear_error (GError **err)
 647 {
 648   if (err && *err)
 649     {
 650       g_error_free (*err);
 651       *err = NULL;
 652     }
 653 }
 654
 655 G_GNUC_PRINTF(2, 0)
 656 static void
 657 g_error_add_prefix (gchar       **string,
 658                     const gchar  *format,
 659                     va_list       ap)
 660 {
 661   gchar *oldstring;
 662   gchar *prefix;
 663
 664   prefix = g_strdup_vprintf (format, ap);
 665   oldstring = *string;
 666   *string = g_strconcat (prefix, oldstring, NULL);
 667   g_free (oldstring);
 668   g_free (prefix);
 669 }
 670
 671 /**
 672  * g_prefix_error:
 673  * @err: (allow-none): a return location for a #GError, or %NULL
 674  * @format: printf()-style format string
 675  * @...: arguments to @format
 676  *
 677  * Formats a string according to @format and prefix it to an existing
 678  * error message. If @err is %NULL (ie: no error variable) then do
 679  * nothing.
 680  *
 681  * If *@err is %NULL (ie: an error variable is present but there is no
 682  * error condition) then also do nothing. Whether or not it makes sense
 683  * to take advantage of this feature is up to you.
 684  *
 685  * Since: 2.16
 686  */
 687 void
 688 g_prefix_error (GError      **err,
 689                 const gchar  *format,
 690                 ...)
 691 {
 692   if (err && *err)
 693     {
 694       va_list ap;
 695
 696       va_start (ap, format);
 697       g_error_add_prefix (&(*err)->message, format, ap);
 698       va_end (ap);
 699     }
 700 }
 701
 702 /**
 703  * g_propagate_prefixed_error:
 704  * @dest: error return location
 705  * @src: error to move into the return location
 706  * @format: printf()-style format string
 707  * @...: arguments to @format
 708  *
 709  * If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
 710  * *@dest must be %NULL. After the move, add a prefix as with
 711  * g_prefix_error().
 712  *
 713  * Since: 2.16
 714  **/
 715 void
 716 g_propagate_prefixed_error (GError      **dest,
 717                             GError       *src,
 718                             const gchar  *format,
 719                             ...)
 720 {
 721   g_propagate_error (dest, src);
 722
 723   if (dest && *dest)
 724     {
 725       va_list ap;
 726
 727       va_start (ap, format);
 728       g_error_add_prefix (&(*dest)->message, format, ap);
 729       va_end (ap);
 730     }
 731 }