GLib – 2.0: Error Reporting
Description: Reference for GLib-2.0: Error Reporting

#### Error Reporting

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. In most cases, using `GError` is preferred over numeric error codes, 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 char  *filename,
                                  char       **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:

    char *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).

Since error messages may be displayed to the user, they need to be valid UTF-8 (all GTK widgets expect text to be UTF-8). Keep this in mind in particular when formatting error messages with filenames, which are in the ‘filename encoding’, and need to be turned into UTF-8 using `g_filename_to_utf8()`, `g_filename_display_name()` or `g_utf8_make_valid()`.

Note, however, that many error messages are too technical to display to the user in an application, so prefer to use `g_error_matches()` to categorize errors from called functions, and build an appropriate error message for the context within your application. Error messages from a `GError` are more appropriate to be printed in system logs or on the command line. They are typically translated.

## Reporting errors

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:

    int
    foo_open_file (GError **error)
    {
      int fd;
      int saved_errno;

      g_return_val_if_fail (error == NULL || *error == NULL, -1);

      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

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`: “`c #define G_SPAWN_ERROR g_spawn_error_quark ()

G_DEFINE_QUARK (g-spawn-error-quark, g_spawn_error) “`

- 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`.

## Comparison of `GError` and traditional error handling

`GError` has several advantages over traditional numeric error codes: importantly, tools like gobject-introspection understand `GError`s 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 `GError`s 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`

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.

- `GError` structs must not be stack-allocated.

- “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 hard to use 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);`).

## Extended `GError` Domains

Since GLib 2.68 it is possible to extend the `GError` type. This is done with the `G_DEFINE_EXTENDED_ERROR()` macro. To create an extended `GError` type do something like this in the header file:

    typedef enum
    {
      MY_ERROR_BAD_REQUEST,
    } MyError;
    #define MY_ERROR (my_error_quark ())
    GQuark my_error_quark (void);
    int
    my_error_get_parse_error_id (GError *error);
    const char *
    my_error_get_bad_request_details (GError *error);

and in the implementation:

    typedef struct
    {
      int parse_error_id;
      char *bad_request_details;
    } MyErrorPrivate;

    static void
    my_error_private_init (MyErrorPrivate *priv)
    {
      priv->parse_error_id = -1;
      // No need to set priv->bad_request_details to NULL,
      // the struct is initialized with zeros.
    }

    static void
    my_error_private_copy (const MyErrorPrivate *src_priv, MyErrorPrivate *dest_priv)
    {
      dest_priv->parse_error_id = src_priv->parse_error_id;
      dest_priv->bad_request_details = g_strdup (src_priv->bad_request_details);
    }

    static void
    my_error_private_clear (MyErrorPrivate *priv)
    {
      g_free (priv->bad_request_details);
    }

    // This defines the my_error_get_private and my_error_quark functions.
    G_DEFINE_EXTENDED_ERROR (MyError, my_error)

    int
    my_error_get_parse_error_id (GError *error)
    {
      MyErrorPrivate *priv = my_error_get_private (error);
      g_return_val_if_fail (priv != NULL, -1);
      return priv->parse_error_id;
    }

    const char *
    my_error_get_bad_request_details (GError *error)
    {
      MyErrorPrivate *priv = my_error_get_private (error);
      g_return_val_if_fail (priv != NULL, NULL);
      g_return_val_if_fail (error->code != MY_ERROR_BAD_REQUEST, NULL);
      return priv->bad_request_details;
    }

    static void
    my_error_set_bad_request (GError     **error,
                              const char  *reason,
                              int          error_id,
                              const char  *details)
    {
      MyErrorPrivate *priv;
      g_set_error (error, MY_ERROR, MY_ERROR_BAD_REQUEST, "Invalid request: %s", reason);
      if (error != NULL && *error != NULL)
        {
          priv = my_error_get_private (error);
          g_return_val_if_fail (priv != NULL, NULL);
          priv->parse_error_id = error_id;
          priv->bad_request_details = g_strdup (details);
        }
    }

An example of use of the error could be:

    gboolean
    send_request (GBytes *request, GError **error)
    {
      ParseFailedStatus *failure = validate_request (request);
      if (failure != NULL)
        {
          my_error_set_bad_request (error, failure->reason, failure->error_id, failure->details);
          parse_failed_status_free (failure);
          return FALSE;
        }

      return send_one (request, error);
    }

Please note that if you are a library author and your library exposes an existing error domain, then you can’t make this error domain an extended one without breaking ABI. This is because earlier it was possible to create an error with this error domain on the stack and then copy it with `g_error_copy()`. If the new version of your library makes the error domain an extended one, then `g_error_copy()` called by code that allocated the error on the stack will try to copy more data than it used to, which will lead to undefined behavior. You must not stack-allocate errors with an extended error domain, and it is bad practice to stack-allocate any other `GError`s.

Extended error domains in unloadable plugins/modules are not supported.