GLib Reference Manual: Error Reporting

## Error Reporting Error Reporting — a system for reporting errors |

## Functions

GError * | g_error_new ()
GError * | g_error_new_literal ()
GError * | g_error_new_valist ()
void | g_error_free ()
GError * | g_error_copy ()
gboolean | g_error_matches ()
void | g_set_error ()
void | g_set_error_literal ()
void | g_propagate_error ()
void | g_clear_error ()
void | g_prefix_error ()
void | g_propagate_prefixed_error ()

## Types and Values

struct | GError

## Includes

    #include <glib.h>

## Description

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. 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: 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. For example:

    1
    2
    3
    4
    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:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    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:

    1
    2
    3
    4
    5
    6
    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:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    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:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    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.

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    19
    20
    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:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    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()`:

    1
    2
    3
    4
    5
    6
    7
    8
    9
    10
    11
    12
    13
    14
    15
    16
    17
    18
    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 <NAMESPACE>_<MODULE>_ERROR, for example `G_SPAWN_ERROR` or `G_THREAD_ERROR`:

      1
      2
      3
      4
      5
      6
      7
      #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 <namespace>_<module>_error_quark, for example `g_spawn_error_quark()` or `g_thread_error_quark()`.

- The error codes are in an enumeration called <Namespace><Module>Error; for example, GThreadError or GSpawnError.

- Members of the error code enumeration are called <NAMESPACE>_<MODULE>_ERROR_<CODE>, 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 <NAMESPACE>_<MODULE>_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.

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. <footnote><para>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. </para></footnote> If `FALSE` is returned, the error must be set to a non-`NULL` value. <footnote><para>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.</para></footnote>

- 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);`).

## Functions

### g_error_new ()

    GError *
    g_error_new (GQuark domain,
                 gint code,
                 const gchar *format,
                 ...);

Creates a new GError with the given `domain` and `code` , and a message formatted with `format` .

#### Parameters

domain | error domain |
code | error code |
format | printf()-style format for error message |
... | parameters for message format |

#### Returns

a new GError

### g_error_new_literal ()

    GError *
    g_error_new_literal (GQuark domain,
                         gint code,
                         const gchar *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.

#### Parameters

domain | error domain |
code | error code |
message | error message |

#### Returns

a new GError

### g_error_new_valist ()

    GError *
    g_error_new_valist (GQuark domain,
                        gint code,
                        const gchar *format,
                        va_list args);

Creates a new GError with the given `domain` and `code` , and a message formatted with `format` .

#### Parameters

domain | error domain |
code | error code |
format | printf()-style format for error message |
args | va_list of parameters for the message format |

#### Returns

a new GError

Since 2.22

### g_error_free ()

    void
    g_error_free (GError *error);

Frees a GError and associated resources.

#### Parameters

error | a GError |

### g_error_copy ()

    GError *
    g_error_copy (const GError *error);

Makes a copy of `error` .

#### Parameters

error | a GError |

#### Returns

a new GError

### g_error_matches ()

    gboolean
    g_error_matches (const GError *error,
                     GQuark domain,
                     gint 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 equilalent 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.

#### Parameters

error | a GError or `NULL`. | [allow-none]
domain | an error domain |
code | an error code |

#### Returns

whether `error` has `domain` and `code`

### g_set_error ()

    void
    g_set_error (GError **err,
                 GQuark domain,
                 gint code,
                 const gchar *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` .

#### Parameters

err | a return location for a GError, or `NULL`. | [allow-none]
domain | error domain |
code | error code |
format | printf()-style format |
... | args for `format` |

### g_set_error_literal ()

    void
    g_set_error_literal (GError **err,
                         GQuark domain,
                         gint code,
                         const gchar *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.

#### Parameters

err | a return location for a GError, or `NULL`. | [allow-none]
domain | error domain |
code | error code |
message | error message |

Since 2.18

### g_propagate_error ()

    void
    g_propagate_error (GError **dest,
                       GError *src);

If `dest` is `NULL`, free `src` ; otherwise, moves `src` into *`dest` . The error variable `dest` points to must be `NULL`.

#### Parameters

dest | error return location |
src | error to move into the return location |

### g_clear_error ()

    void
    g_clear_error (GError **err);

If `err` is `NULL`, does nothing. If `err` is non-`NULL`, calls `g_error_free()` on *`err` and sets *`err` to `NULL`.

#### Parameters

err | a GError return location |

### g_prefix_error ()

    void
    g_prefix_error (GError **err,
                    const gchar *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. Whether or not it makes sense to take advantage of this feature is up to you.

#### Parameters

err | a return location for a GError, or `NULL`. | [allow-none]
format | printf()-style format string |
... | arguments to `format` |

Since 2.16

### g_propagate_prefixed_error ()

    void
    g_propagate_prefixed_error (GError **dest,
                                GError *src,
                                const gchar *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()`.

#### Parameters

dest | error return location |
src | error to move into the return location |
format | printf()-style format string |
... | arguments to `format` |

Since 2.16

## Types and Values

### struct GError

    struct GError {
      GQuark       domain;
      gint         code;
      gchar       *message;
    };

The `GError` structure contains information about an error that has occurred.

#### Members

GQuark `domain`; | error domain, e.g. G_FILE_ERROR |
gint `code`; | error code, e.g. `G_FILE_ERROR_NOENT` |
gchar *`message`; | human-readable informative error message |