[glib] Move GError docs inline and ditch template
- From: Matthias Clasen <matthiasc src gnome org>
- To: commits-list gnome org
- Cc:
- Subject: [glib] Move GError docs inline and ditch template
- Date: Tue, 19 Jul 2011 04:04:30 +0000 (UTC)
commit c9379bcfe1423c43b09ea7e979242a3621dab2b2
Author: Matthias Clasen <mclasen redhat com>
Date: Mon Jul 18 23:58:32 2011 -0400
Move GError docs inline and ditch template
docs/reference/glib/tmpl/.gitignore | 1 +
docs/reference/glib/tmpl/error_reporting.sgml | 531 -------------------------
glib/gerror.c | 324 +++++++++++++++
glib/gerror.h | 9 +
4 files changed, 334 insertions(+), 531 deletions(-)
---
diff --git a/docs/reference/glib/tmpl/.gitignore b/docs/reference/glib/tmpl/.gitignore
index 63e2b75..e2c7359 100644
--- a/docs/reference/glib/tmpl/.gitignore
+++ b/docs/reference/glib/tmpl/.gitignore
@@ -13,6 +13,7 @@ conversions.sgml
datasets.sgml
datalist.sgml
date-time.sgml
+error_reporting.sgml
ghostutils.sgml
gregex.sgml
gurifuncs.sgml
diff --git a/glib/gerror.c b/glib/gerror.c
index 1f39739..2f5f8a9 100644
--- a/glib/gerror.c
+++ b/glib/gerror.c
@@ -24,6 +24,330 @@
* 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 <emphasis>data type</emphasis> (the #GError
+ * object) and a <emphasis>set of rules.</emphasis> 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.
+ *
+ * First and foremost: <emphasis>#GError should only be used to report
+ * recoverable runtime errors, never to report programming
+ * errors.</emphasis> 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 <emphasis>only</emphasis> 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. 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 <literal>error</literal>
+ * 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 <literal>err != NULL</literal> in this example is a
+ * <emphasis>reliable</emphasis> 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 <literal>error</literal>
+ * argument:
+ * |[
+ * if (g_file_get_contents ("foo.txt", &contents, NULL, NULL)) /* ignore errors */
+ * /* no error occurred */ ;
+ * else
+ * /* error */ ;
+ * ]|
+ *
+ * The #GError object contains three fields: <literal>domain</literal>
+ * indicates the module the error-reporting function is located in,
+ * <literal>code</literal> indicates the specific error that occurred,
+ * and <literal>message</literal> 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
+ * <literal>error->message</literal>, 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,
+ * <literal>error->message</literal> 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;
+ *
+ * fd = open ("file.txt", O_RDONLY);
+ *
+ * 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 (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, 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;
+ * }
+ * }
+ * ]|
+ * <literal>tmp_error</literal> should be checked immediately after
+ * sub_function_that_can_fail(), and either cleared or propagated
+ * upward. The rule is: <emphasis>after each error, you must either
+ * handle the error, or return it to the calling function</emphasis>.
+ * 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
+ * <emphasis>ignores</emphasis> errors; it's equivalent to
+ * <literal>try { sub_function_that_can_fail (); } catch (...) {}</literal>
+ * in C++. It does <emphasis>not</emphasis> mean to leave errors
+ * unhandled; it means to handle them by doing nothing.
+ *
+ * Error domains and codes are conventionally named as follows:
+ * <itemizedlist>
+ * <listitem><para>
+ * The error domain is called
+ * <literal><NAMESPACE>_<MODULE>_ERROR</literal>,
+ * 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");
+ * }
+ * ]|
+ * </para></listitem>
+ * <listitem><para>
+ * The quark function for the error domain is called
+ * <literal><namespace>_<module>_error_quark</literal>,
+ * for example g_spawn_error_quark() or %g_thread_error_quark().
+ * </para></listitem>
+ * <listitem><para>
+ * The error codes are in an enumeration called
+ * <literal><Namespace><Module>Error</literal>;
+ * for example,#GThreadError or #GSpawnError.
+ * </para></listitem>
+ * <listitem><para>
+ * Members of the error code enumeration are called
+ * <literal><NAMESPACE>_<MODULE>_ERROR_<CODE></literal>,
+ * for example %G_SPAWN_ERROR_FORK or %G_THREAD_ERROR_AGAIN.
+ * </para></listitem>
+ * <listitem><para>
+ * 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 <literal><NAMESPACE>_<MODULE>_ERROR_FAILED</literal>,
+ * for example %G_SPAWN_ERROR_FAILED or %G_THREAD_ERROR_FAILED.
+ * </para></listitem>
+ * </itemizedlist>
+ *
+ * Summary of rules for use of #GError:
+ * <itemizedlist>
+ * <listitem><para>
+ * Do not report programming errors via #GError.
+ * </para></listitem>
+ * <listitem><para>
+ * 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 "...".
+ * </para></listitem>
+ * <listitem><para>
+ * The caller may pass %NULL for the #GError** if they are not interested
+ * in details of the exact error that occurred.
+ * </para></listitem>
+ * <listitem><para>
+ * 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.
+ * </para></listitem>
+ * <listitem><para>
+ * If a #GError is reported, then your function by definition
+ * <emphasis>had a fatal failure and did not complete whatever
+ * it was supposed to do</emphasis>. 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.
+ * </para></listitem>
+ * <listitem><para>
+ * A #GError* must be initialized to %NULL before passing its address
+ * to a function that can report errors.
+ * </para></listitem>
+ * <listitem><para>
+ * "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.
+ * </para></listitem>
+ * <listitem><para>
+ * By convention, if you return a boolean value indicating success
+ * then %TRUE means success and %FALSE means failure. If %FALSE is
+ * returned, the error <emphasis>must</emphasis> be set to a non-%NULL
+ * value.
+ * </para></listitem>
+ * <listitem><para>
+ * 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.
+ * </para></listitem>
+ * <listitem><para>
+ * 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.
+ * <literal>g_return_if_fail (error == NULL || *error == NULL);</literal>).
+ * </para></listitem>
+ * </itemizedlist>
+ */
+
#include "config.h"
#include "gerror.h"
diff --git a/glib/gerror.h b/glib/gerror.h
index b303487..871f310 100644
--- a/glib/gerror.h
+++ b/glib/gerror.h
@@ -31,6 +31,15 @@
G_BEGIN_DECLS
+/**
+ * GError:
+ * @domain: error domain, e.g. #G_FILE_ERROR
+ * @code: error code, e.g. %G_FILE_ERROR_NOENT
+ * @message: human-readable informative error message
+ *
+ * The <structname>GError</structname> structure contains
+ * information about an error that has occurred.
+ */
typedef struct _GError GError;
struct _GError
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