Commit 29b65294 authored by Havoc Pennington's avatar Havoc Pennington Committed by Havoc Pennington

docs

2000-09-06  Havoc Pennington  <hp@redhat.com>

	* gerror.c: docs

	* docs/reference/glib/tmpl/error_reporting.sgml: docs
parent 98dbc6a7
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
2000-09-06 Havoc Pennington <hp@redhat.com>
* gerror.c: docs
* docs/reference/glib/tmpl/error_reporting.sgml: docs
Wed Sep 6 10:28:34 2000 Owen Taylor <otaylor@redhat.com>
* guniprop.c gunicode.h gutf8.c: Some inline docs fixes.
......
......@@ -3,10 +3,364 @@ Error Reporting
<!-- ##### SECTION Short_Description ##### -->
System for reporting errors
<!-- ##### SECTION Long_Description ##### -->
<para>
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.
</para>
<para>
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.)
</para>
<para>
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.
</para>
<para>
Functions that can fail take a return location for a #GError as their last argument.
For example:
<programlisting>
gchar* g_file_get_contents (const gchar *filename, GError **error);
</programlisting>
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:
<programlisting>
gchar *contents;
GError *err = NULL;
contents = g_file_get_contents ("foo.txt", &amp;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 foo.txt: %s\n", err->message);
g_error_free (err);
}
else
{
/* Use file contents */
g_assert (contents != NULL);
}
</programlisting>
Note that <literal>err != NULL</literal> in this example is a
<emphasis>reliable</emphasis> indicator of whether g_file_get_contents()
failed. Also, g_file_get_contents() uses the convention that a NULL return value
means an error occurred (but not all functions use this convention).
</para>
<para>
Because g_file_get_contents() returns NULL 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:
<programlisting>
contents = g_file_get_contents ("foo.txt", NULL); /* ignore errors */
if (contents != NULL)
/* no error occurred */ ;
else
/* error */ ;
</programlisting>
</para>
<para>
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-&gt;message</literal>, perhaps along with
additional context known only to the calling function (the file being opened, or
whatever).
</para>
<para>
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:
<programlisting>
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;
}
</programlisting>
</para>
<para>
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:
<programlisting>
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);
}
</programlisting>
</para>
<para>
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.
<programlisting>
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 (&amp;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 */
}
</programlisting>
</para>
<para>
Error pileups are always a bug. For example, this code is incorrect:
<programlisting>
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 (&amp;tmp_error);
other_function_that_can_fail (&amp;tmp_error);
if (tmp_error != NULL)
{
g_propagate_error (err, tmp_error);
return FALSE;
}
}
</programlisting>
<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():
<programlisting>
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 (&amp;tmp_error);
if (tmp_error != NULL)
{
g_propagate_error (err, tmp_error);
return FALSE;
}
}
</programlisting>
</para>
<para>
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.
</para>
<para>
Error domains and codes are conventionally named as follows:
<itemizedlist>
<listitem>
<para>
The error domain is called
<literal>&lt;NAMESPACE&gt;_&lt;MODULE&gt;_ERROR</literal>, for example
%G_EXEC_ERROR or %G_THREAD_ERROR.
</para>
</listitem>
<listitem>
<para>
The error codes are in an enumeration called
<literal>&lt;Namespace&gt;_&lt;Module&gt;_Error</literal>; for example,
#GThreadError or #GExecError.
</para>
</listitem>
<listitem>
<para>
Members of the error code enumeration are called <literal>&lt;NAMESPACE&gt;_&lt;MODULE&gt;_ERROR_&lt;CODE&gt;</literal>, for example %G_EXEC_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>&lt;NAMESPACE&gt;_&lt;MODULE&gt;_ERROR_FAILED</literal>, for
example %G_EXEC_ERROR_FAILED or %G_THREAD_ERROR_FAILED.
</para>
</listitem>
</itemizedlist>
</para>
<para>
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>
</para>
<!-- ##### SECTION See_Also ##### -->
......@@ -101,5 +455,11 @@ Error Reporting
</para>
@err:
<!--
Local variables:
mode: sgml
sgml-parent-document: ("../glib-docs.sgml" "book" "refsect2" "")
End:
-->
......@@ -43,6 +43,18 @@ g_error_new_valist(GQuark domain,
return error;
}
/**
* g_error_new:
* @domain: error domain
* @code: error code
* @format: printf()-style format for error message
* @Varargs: parameters for message format
*
* Creates a new #GError with the given @domain and @code,
* and a message formatted with @format.
*
* Return value: a new #GError
**/
GError*
g_error_new (GQuark domain,
gint code,
......@@ -62,6 +74,19 @@ g_error_new (GQuark domain,
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.
*
* Return value: a new #GError
**/
GError*
g_error_new_literal (GQuark domain,
gint code,
......@@ -81,6 +106,13 @@ g_error_new_literal (GQuark domain,
return err;
}
/**
* g_error_free:
* @error: a #GError
*
* Frees a #GError and associated resources.
*
**/
void
g_error_free (GError *error)
{
......@@ -91,6 +123,14 @@ g_error_free (GError *error)
g_free (error);
}
/**
* g_error_copy:
* @error: a #GError
*
* Makes a copy of @error.
*
* Return value: a new #GError
**/
GError*
g_error_copy (const GError *error)
{
......@@ -107,6 +147,17 @@ g_error_copy (const GError *error)
return copy;
}
/**
* g_error_matches:
* @error: a #GError
* @domain: an error domain
* @code: an error code
*
* Returns TRUE if @error matches @domain and @code, FALSE
* otherwise.
*
* Return value: whether @error has @domain and @code
**/
gboolean
g_error_matches (const GError *error,
GQuark domain,
......@@ -120,6 +171,17 @@ g_error_matches (const GError *error,
#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."
/**
* g_set_error:
* @err: a return location for a #GError, or NULL
* @domain: error domain
* @code: error code
* @format: printf()-style format
* @Varargs: 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,
......@@ -140,6 +202,14 @@ g_set_error (GError **err,
va_end (args);
}
/**
* g_propagate_error:
* @dest: error return location
* @src: error to move into the return location
*
* Does nothing if @dest is NULL; otherwise,
* moves @src into *@dest. *@dest must be NULL.
**/
void
g_propagate_error (GError **dest,
GError *src)
......@@ -155,6 +225,13 @@ g_propagate_error (GError **dest,
*dest = src;
}
/**
* g_clear_error:
* @err: a #GError return location
*
* If @err is NULL, does nothing. If @err is non-NULL,
* calls g_error_free() on *@err and sets *@err to NULL.
**/
void
g_clear_error (GError **err)
{
......
......@@ -43,6 +43,18 @@ g_error_new_valist(GQuark domain,
return error;
}
/**
* g_error_new:
* @domain: error domain
* @code: error code
* @format: printf()-style format for error message
* @Varargs: parameters for message format
*
* Creates a new #GError with the given @domain and @code,
* and a message formatted with @format.
*
* Return value: a new #GError
**/
GError*
g_error_new (GQuark domain,
gint code,
......@@ -62,6 +74,19 @@ g_error_new (GQuark domain,
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.
*
* Return value: a new #GError
**/
GError*
g_error_new_literal (GQuark domain,
gint code,
......@@ -81,6 +106,13 @@ g_error_new_literal (GQuark domain,
return err;
}
/**
* g_error_free:
* @error: a #GError
*
* Frees a #GError and associated resources.
*
**/
void
g_error_free (GError *error)
{
......@@ -91,6 +123,14 @@ g_error_free (GError *error)
g_free (error);
}
/**
* g_error_copy:
* @error: a #GError
*
* Makes a copy of @error.
*
* Return value: a new #GError
**/
GError*
g_error_copy (const GError *error)
{
......@@ -107,6 +147,17 @@ g_error_copy (const GError *error)
return copy;
}
/**
* g_error_matches:
* @error: a #GError
* @domain: an error domain
* @code: an error code
*
* Returns TRUE if @error matches @domain and @code, FALSE
* otherwise.
*
* Return value: whether @error has @domain and @code
**/
gboolean
g_error_matches (const GError *error,
GQuark domain,
......@@ -120,6 +171,17 @@ g_error_matches (const GError *error,
#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."
/**
* g_set_error:
* @err: a return location for a #GError, or NULL
* @domain: error domain
* @code: error code
* @format: printf()-style format
* @Varargs: 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,
......@@ -140,6 +202,14 @@ g_set_error (GError **err,
va_end (args);
}
/**
* g_propagate_error:
* @dest: error return location
* @src: error to move into the return location
*
* Does nothing if @dest is NULL; otherwise,
* moves @src into *@dest. *@dest must be NULL.
**/
void
g_propagate_error (GError **dest,
GError *src)
......@@ -155,6 +225,13 @@ g_propagate_error (GError **dest,
*dest = src;
}
/**
* g_clear_error:
* @err: a #GError return location
*
* If @err is NULL, does nothing. If @err is non-NULL,
* calls g_error_free() on *@err and sets *@err to NULL.
**/
void
g_clear_error (GError **err)
{
......
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