/* GLIB - Library of useful routines for C programming
* Copyright (C) 1995-1997 Peter Mattis, Spencer Kimball and Josh MacDonald
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, see .
*/
/*
* Modified by the GLib Team and others 1997-2000. See the AUTHORS
* file for a list of people on the GLib Team. See the ChangeLog
* files for a list of changes. These files are distributed with
* GLib at ftp://ftp.gtk.org/pub/gtk/.
*/
/**
* SECTION:error_reporting
* @Title: Error Reporting
* @Short_description: a system for reporting errors
*
* GLib provides a standard method of reporting errors from a called
* function to the calling code. (This is the same problem solved by
* exceptions in other languages.) It's important to understand that
* this method is both a data type (the #GError struct) and a [set of
* rules][gerror-rules]. If you use #GError incorrectly, then your code will not
* properly interoperate with other code that uses #GError, and users
* of your API will probably get confused. In most cases, [using #GError is
* preferred over numeric error codes][gerror-comparison], but there are
* situations where numeric error codes are useful for performance.
*
* First and foremost: #GError should only be used to report recoverable
* runtime errors, never to report programming errors. If the programmer
* has screwed up, then you should use g_warning(), g_return_if_fail(),
* g_assert(), g_error(), or some similar facility. (Incidentally,
* remember that the g_error() function should only be used for
* programming errors, it should not be used to print any error
* reportable via #GError.)
*
* Examples of recoverable runtime errors are "file not found" or
* "failed to parse input." Examples of programming errors are "NULL
* passed to strcmp()" or "attempted to free the same pointer twice."
* These two kinds of errors are fundamentally different: runtime errors
* should be handled or reported to the user, programming errors should
* be eliminated by fixing the bug in the program. This is why most
* functions in GLib and GTK+ do not use the #GError facility.
*
* Functions that can fail take a return location for a #GError as their
* last argument. On error, a new #GError instance will be allocated and
* returned to the caller via this argument. For example:
* |[
* gboolean g_file_get_contents (const gchar *filename,
* gchar **contents,
* gsize *length,
* GError **error);
* ]|
* If you pass a non-%NULL value for the `error` argument, it should
* point to a location where an error can be placed. For example:
* |[
* gchar *contents;
* GError *err = NULL;
*
* g_file_get_contents ("foo.txt", &contents, NULL, &err);
* g_assert ((contents == NULL && err != NULL) || (contents != NULL && err == NULL));
* if (err != NULL)
* {
* // Report error to user, and free error
* g_assert (contents == NULL);
* fprintf (stderr, "Unable to read file: %s\n", err->message);
* g_error_free (err);
* }
* else
* {
* // Use file contents
* g_assert (contents != NULL);
* }
* ]|
* Note that `err != NULL` in this example is a reliable indicator
* of whether g_file_get_contents() failed. Additionally,
* g_file_get_contents() returns a boolean which
* indicates whether it was successful.
*
* Because g_file_get_contents() returns %FALSE on failure, if you
* are only interested in whether it failed and don't need to display
* an error message, you can pass %NULL for the @error argument:
* |[
* if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) // ignore errors
* // no error occurred
* ;
* else
* // error
* ;
* ]|
*
* The #GError object contains three fields: @domain indicates the module
* the error-reporting function is located in, @code indicates the specific
* error that occurred, and @message is a user-readable error message with
* as many details as possible. Several functions are provided to deal
* with an error received from a called function: g_error_matches()
* returns %TRUE if the error matches a given domain and code,
* g_propagate_error() copies an error into an error location (so the
* calling function will receive it), and g_clear_error() clears an
* error location by freeing the error and resetting the location to
* %NULL. To display an error to the user, simply display the @message,
* perhaps along with additional context known only to the calling
* function (the file being opened, or whatever - though in the
* g_file_get_contents() case, the @message already contains a filename).
*
* When implementing a function that can report errors, the basic
* tool is g_set_error(). Typically, if a fatal error occurs you
* want to g_set_error(), then return immediately. g_set_error()
* does nothing if the error location passed to it is %NULL.
* Here's an example:
* |[
* gint
* foo_open_file (GError **error)
* {
* gint fd;
* int saved_errno;
*
* fd = open ("file.txt", O_RDONLY);
* saved_errno = errno;
*
* if (fd < 0)
* {
* g_set_error (error,
* FOO_ERROR, // error domain
* FOO_ERROR_BLAH, // error code
* "Failed to open file: %s", // error message format string
* g_strerror (saved_errno));
* return -1;
* }
* else
* return fd;
* }
* ]|
*
* Things are somewhat more complicated if you yourself call another
* function that can report a #GError. If the sub-function indicates
* fatal errors in some way other than reporting a #GError, such as
* by returning %TRUE on success, you can simply do the following:
* |[
* gboolean
* my_function_that_can_fail (GError **err)
* {
* g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
*
* if (!sub_function_that_can_fail (err))
* {
* // assert that error was set by the sub-function
* g_assert (err == NULL || *err != NULL);
* return FALSE;
* }
*
* // otherwise continue, no error occurred
* g_assert (err == NULL || *err == NULL);
* }
* ]|
*
* If the sub-function does not indicate errors other than by
* reporting a #GError (or if its return value does not reliably indicate
* errors) you need to create a temporary #GError
* since the passed-in one may be %NULL. g_propagate_error() is
* intended for use in this case.
* |[
* gboolean
* my_function_that_can_fail (GError **err)
* {
* GError *tmp_error;
*
* g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
*
* tmp_error = NULL;
* sub_function_that_can_fail (&tmp_error);
*
* if (tmp_error != NULL)
* {
* // store tmp_error in err, if err != NULL,
* // otherwise call g_error_free() on tmp_error
* g_propagate_error (err, tmp_error);
* return FALSE;
* }
*
* // otherwise continue, no error occurred
* }
* ]|
*
* Error pileups are always a bug. For example, this code is incorrect:
* |[
* gboolean
* my_function_that_can_fail (GError **err)
* {
* GError *tmp_error;
*
* g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
*
* tmp_error = NULL;
* sub_function_that_can_fail (&tmp_error);
* other_function_that_can_fail (&tmp_error);
*
* if (tmp_error != NULL)
* {
* g_propagate_error (err, tmp_error);
* return FALSE;
* }
* }
* ]|
* @tmp_error should be checked immediately after sub_function_that_can_fail(),
* and either cleared or propagated upward. The rule is: after each error,
* you must either handle the error, or return it to the calling function.
*
* Note that passing %NULL for the error location is the equivalent
* of handling an error by always doing nothing about it. So the
* following code is fine, assuming errors in sub_function_that_can_fail()
* are not fatal to my_function_that_can_fail():
* |[
* gboolean
* my_function_that_can_fail (GError **err)
* {
* GError *tmp_error;
*
* g_return_val_if_fail (err == NULL || *err == NULL, FALSE);
*
* sub_function_that_can_fail (NULL); // ignore errors
*
* tmp_error = NULL;
* other_function_that_can_fail (&tmp_error);
*
* if (tmp_error != NULL)
* {
* g_propagate_error (err, tmp_error);
* return FALSE;
* }
* }
* ]|
*
* Note that passing %NULL for the error location ignores errors;
* it's equivalent to
* `try { sub_function_that_can_fail (); } catch (...) {}`
* in C++. It does not mean to leave errors unhandled; it means
* to handle them by doing nothing.
*
* Error domains and codes are conventionally named as follows:
*
* - The error domain is called __ERROR,
* for example %G_SPAWN_ERROR or %G_THREAD_ERROR:
* |[
* #define G_SPAWN_ERROR g_spawn_error_quark ()
*
* GQuark
* g_spawn_error_quark (void)
* {
* return g_quark_from_static_string ("g-spawn-error-quark");
* }
* ]|
*
* - The quark function for the error domain is called
* __error_quark,
* for example g_spawn_error_quark() or g_thread_error_quark().
*
* - The error codes are in an enumeration called
* Error;
* for example, #GThreadError or #GSpawnError.
*
* - Members of the error code enumeration are called
* __ERROR_,
* for example %G_SPAWN_ERROR_FORK or %G_THREAD_ERROR_AGAIN.
*
* - If there's a "generic" or "unknown" error code for unrecoverable
* errors it doesn't make sense to distinguish with specific codes,
* it should be called __ERROR_FAILED,
* for example %G_SPAWN_ERROR_FAILED. In the case of error code
* enumerations that may be extended in future releases, you should
* generally not handle this error code explicitly, but should
* instead treat any unrecognized error code as equivalent to
* FAILED.
*
* ## Comparison of #GError and traditional error handling # {#gerror-comparison}
*
* #GError has several advantages over traditional numeric error codes:
* importantly, tools like
* [gobject-introspection](https://developer.gnome.org/gi/stable/) understand
* #GErrors and convert them to exceptions in bindings; the message includes
* more information than just a code; and use of a domain helps prevent
* misinterpretation of error codes.
*
* #GError has disadvantages though: it requires a memory allocation, and
* formatting the error message string has a performance overhead. This makes it
* unsuitable for use in retry loops where errors are a common case, rather than
* being unusual. For example, using %G_IO_ERROR_WOULD_BLOCK means hitting these
* overheads in the normal control flow. String formatting overhead can be
* eliminated by using g_set_error_literal() in some cases.
*
* These performance issues can be compounded if a function wraps the #GErrors
* returned by the functions it calls: this multiplies the number of allocations
* and string formatting operations. This can be partially mitigated by using
* g_prefix_error().
*
* ## Rules for use of #GError # {#gerror-rules}
*
* Summary of rules for use of #GError:
*
* - Do not report programming errors via #GError.
*
* - The last argument of a function that returns an error should
* be a location where a #GError can be placed (i.e. "#GError** error").
* If #GError is used with varargs, the #GError** should be the last
* argument before the "...".
*
* - The caller may pass %NULL for the #GError** if they are not interested
* in details of the exact error that occurred.
*
* - If %NULL is passed for the #GError** argument, then errors should
* not be returned to the caller, but your function should still
* abort and return if an error occurs. That is, control flow should
* not be affected by whether the caller wants to get a #GError.
*
* - If a #GError is reported, then your function by definition had a
* fatal failure and did not complete whatever it was supposed to do.
* If the failure was not fatal, then you handled it and you should not
* report it. If it was fatal, then you must report it and discontinue
* whatever you were doing immediately.
*
* - If a #GError is reported, out parameters are not guaranteed to
* be set to any defined value.
*
* - A #GError* must be initialized to %NULL before passing its address
* to a function that can report errors.
*
* - "Piling up" errors is always a bug. That is, if you assign a
* new #GError to a #GError* that is non-%NULL, thus overwriting
* the previous error, it indicates that you should have aborted
* the operation instead of continuing. If you were able to continue,
* you should have cleared the previous error with g_clear_error().
* g_set_error() will complain if you pile up errors.
*
* - By convention, if you return a boolean value indicating success
* then %TRUE means success and %FALSE means failure. Avoid creating
* functions which have a boolean return value and a GError parameter,
* but where the boolean does something other than signal whether the
* GError is set. Among other problems, it requires C callers to allocate
* a temporary error. Instead, provide a "gboolean *" out parameter.
* There are functions in GLib itself such as g_key_file_has_key() that
* are deprecated because of this. If %FALSE is returned, the error must
* be set to a non-%NULL value. One exception to this is that in situations
* that are already considered to be undefined behaviour (such as when a
* g_return_val_if_fail() check fails), the error need not be set.
* Instead of checking separately whether the error is set, callers
* should ensure that they do not provoke undefined behaviour, then
* assume that the error will be set on failure.
*
* - A %NULL return value is also frequently used to mean that an error
* occurred. You should make clear in your documentation whether %NULL
* is a valid return value in non-error cases; if %NULL is a valid value,
* then users must check whether an error was returned to see if the
* function succeeded.
*
* - When implementing a function that can report errors, you may want
* to add a check at the top of your function that the error return
* location is either %NULL or contains a %NULL error (e.g.
* `g_return_if_fail (error == NULL || *error == NULL);`).
*/
#include "config.h"
#include "gerror.h"
#include "gslice.h"
#include "gstrfuncs.h"
#include "gtestutils.h"
/**
* g_error_new_valist:
* @domain: error domain
* @code: error code
* @format: printf()-style format for error message
* @args: #va_list of parameters for the message format
*
* Creates a new #GError with the given @domain and @code,
* and a message formatted with @format.
*
* Returns: a new #GError
*
* Since: 2.22
*/
GError*
g_error_new_valist (GQuark domain,
gint code,
const gchar *format,
va_list args)
{
GError *error;
/* Historically, GError allowed this (although it was never meant to work),
* and it has significant use in the wild, which g_return_val_if_fail
* would break. It should maybe g_return_val_if_fail in GLib 4.
* (GNOME#660371, GNOME#560482)
*/
g_warn_if_fail (domain != 0);
g_warn_if_fail (format != NULL);
error = g_slice_new (GError);
error->domain = domain;
error->code = code;
error->message = g_strdup_vprintf (format, args);
return error;
}
/**
* g_error_new:
* @domain: error domain
* @code: error code
* @format: printf()-style format for error message
* @...: parameters for message format
*
* Creates a new #GError with the given @domain and @code,
* and a message formatted with @format.
*
* Returns: a new #GError
*/
GError*
g_error_new (GQuark domain,
gint code,
const gchar *format,
...)
{
GError* error;
va_list args;
g_return_val_if_fail (format != NULL, NULL);
g_return_val_if_fail (domain != 0, NULL);
va_start (args, format);
error = g_error_new_valist (domain, code, format, args);
va_end (args);
return error;
}
/**
* g_error_new_literal:
* @domain: error domain
* @code: error code
* @message: error message
*
* Creates a new #GError; unlike g_error_new(), @message is
* not a printf()-style format string. Use this function if
* @message contains text you don't have control over,
* that could include printf() escape sequences.
*
* Returns: a new #GError
**/
GError*
g_error_new_literal (GQuark domain,
gint code,
const gchar *message)
{
GError* err;
g_return_val_if_fail (message != NULL, NULL);
g_return_val_if_fail (domain != 0, NULL);
err = g_slice_new (GError);
err->domain = domain;
err->code = code;
err->message = g_strdup (message);
return err;
}
/**
* g_error_free:
* @error: a #GError
*
* Frees a #GError and associated resources.
*/
void
g_error_free (GError *error)
{
g_return_if_fail (error != NULL);
g_free (error->message);
g_slice_free (GError, error);
}
/**
* g_error_copy:
* @error: a #GError
*
* Makes a copy of @error.
*
* Returns: a new #GError
*/
GError*
g_error_copy (const GError *error)
{
GError *copy;
g_return_val_if_fail (error != NULL, NULL);
/* See g_error_new_valist for why these don't return */
g_warn_if_fail (error->domain != 0);
g_warn_if_fail (error->message != NULL);
copy = g_slice_new (GError);
*copy = *error;
copy->message = g_strdup (error->message);
return copy;
}
/**
* g_error_matches:
* @error: (nullable): a #GError
* @domain: an error domain
* @code: an error code
*
* Returns %TRUE if @error matches @domain and @code, %FALSE
* otherwise. In particular, when @error is %NULL, %FALSE will
* be returned.
*
* If @domain contains a `FAILED` (or otherwise generic) error code,
* you should generally not check for it explicitly, but should
* instead treat any not-explicitly-recognized error code as being
* equivalent to the `FAILED` code. This way, if the domain is
* extended in the future to provide a more specific error code for
* a certain case, your code will still work.
*
* Returns: whether @error has @domain and @code
*/
gboolean
g_error_matches (const GError *error,
GQuark domain,
gint code)
{
return error &&
error->domain == domain &&
error->code == code;
}
#define ERROR_OVERWRITTEN_WARNING "GError set over the top of a previous GError or uninitialized memory.\n" \
"This indicates a bug in someone's code. You must ensure an error is NULL before it's set.\n" \
"The overwriting error message was: %s"
/**
* g_set_error:
* @err: (out callee-allocates) (optional): a return location for a #GError
* @domain: error domain
* @code: error code
* @format: printf()-style format
* @...: args for @format
*
* Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
* must be %NULL. A new #GError is created and assigned to *@err.
*/
void
g_set_error (GError **err,
GQuark domain,
gint code,
const gchar *format,
...)
{
GError *new;
va_list args;
if (err == NULL)
return;
va_start (args, format);
new = g_error_new_valist (domain, code, format, args);
va_end (args);
if (*err == NULL)
*err = new;
else
{
g_warning (ERROR_OVERWRITTEN_WARNING, new->message);
g_error_free (new);
}
}
/**
* g_set_error_literal:
* @err: (out callee-allocates) (optional): a return location for a #GError
* @domain: error domain
* @code: error code
* @message: error message
*
* Does nothing if @err is %NULL; if @err is non-%NULL, then *@err
* must be %NULL. A new #GError is created and assigned to *@err.
* Unlike g_set_error(), @message is not a printf()-style format string.
* Use this function if @message contains text you don't have control over,
* that could include printf() escape sequences.
*
* Since: 2.18
*/
void
g_set_error_literal (GError **err,
GQuark domain,
gint code,
const gchar *message)
{
if (err == NULL)
return;
if (*err == NULL)
*err = g_error_new_literal (domain, code, message);
else
g_warning (ERROR_OVERWRITTEN_WARNING, message);
}
/**
* g_propagate_error:
* @dest: (out callee-allocates) (optional) (nullable): error return location
* @src: (transfer full): error to move into the return location
*
* If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
* The error variable @dest points to must be %NULL.
*
* @src must be non-%NULL.
*
* Note that @src is no longer valid after this call. If you want
* to keep using the same GError*, you need to set it to %NULL
* after calling this function on it.
*/
void
g_propagate_error (GError **dest,
GError *src)
{
g_return_if_fail (src != NULL);
if (dest == NULL)
{
if (src)
g_error_free (src);
return;
}
else
{
if (*dest != NULL)
{
g_warning (ERROR_OVERWRITTEN_WARNING, src->message);
g_error_free (src);
}
else
*dest = src;
}
}
/**
* g_clear_error:
* @err: a #GError return location
*
* If @err or *@err is %NULL, does nothing. Otherwise,
* calls g_error_free() on *@err and sets *@err to %NULL.
*/
void
g_clear_error (GError **err)
{
if (err && *err)
{
g_error_free (*err);
*err = NULL;
}
}
G_GNUC_PRINTF(2, 0)
static void
g_error_add_prefix (gchar **string,
const gchar *format,
va_list ap)
{
gchar *oldstring;
gchar *prefix;
prefix = g_strdup_vprintf (format, ap);
oldstring = *string;
*string = g_strconcat (prefix, oldstring, NULL);
g_free (oldstring);
g_free (prefix);
}
/**
* g_prefix_error:
* @err: (inout) (optional) (nullable): a return location for a #GError
* @format: printf()-style format string
* @...: arguments to @format
*
* Formats a string according to @format and prefix it to an existing
* error message. If @err is %NULL (ie: no error variable) then do
* nothing.
*
* If *@err is %NULL (ie: an error variable is present but there is no
* error condition) then also do nothing.
*
* Since: 2.16
*/
void
g_prefix_error (GError **err,
const gchar *format,
...)
{
if (err && *err)
{
va_list ap;
va_start (ap, format);
g_error_add_prefix (&(*err)->message, format, ap);
va_end (ap);
}
}
/**
* g_propagate_prefixed_error:
* @dest: error return location
* @src: error to move into the return location
* @format: printf()-style format string
* @...: arguments to @format
*
* If @dest is %NULL, free @src; otherwise, moves @src into *@dest.
* *@dest must be %NULL. After the move, add a prefix as with
* g_prefix_error().
*
* Since: 2.16
**/
void
g_propagate_prefixed_error (GError **dest,
GError *src,
const gchar *format,
...)
{
g_propagate_error (dest, src);
if (dest && *dest)
{
va_list ap;
va_start (ap, format);
g_error_add_prefix (&(*dest)->message, format, ap);
va_end (ap);
}
}