Message Output and Debugging Functions: GLib Reference Manual

## Message Output and Debugging Functions Message Output and Debugging Functions — functions to output messages and help debug applications |

## Functions

void | (*GLogFunc) ()
void | g_log ()
void | g_logv ()
#define | g_message()
#define | g_warning()
#define | g_critical()
#define | g_error()
#define | g_info()
#define | g_debug()
guint | g_log_set_handler ()
guint | g_log_set_handler_full ()
void | g_log_remove_handler ()
GLogLevelFlags | g_log_set_always_fatal ()
GLogLevelFlags | g_log_set_fatal_mask ()
void | g_log_default_handler ()
GLogFunc | g_log_set_default_handler ()
void | g_log_structured ()
void | g_log_variant ()
void | g_log_structured_array ()
#define | G_DEBUG_HERE
GLogWriterOutput | (*GLogWriterFunc) ()
void | g_log_set_writer_func ()
gboolean | g_log_writer_supports_color ()
gboolean | g_log_writer_is_journald ()
gchar * | g_log_writer_format_fields ()
GLogWriterOutput | g_log_writer_journald ()
GLogWriterOutput | g_log_writer_standard_streams ()
GLogWriterOutput | g_log_writer_default ()

## Types and Values

#define | G_LOG_DOMAIN
#define | G_LOG_FATAL_MASK
#define | G_LOG_LEVEL_USER_SHIFT
enum | GLogLevelFlags
struct | GLogField
enum | GLogWriterOutput

## Includes

    #include <glib.h>

## Description

These functions provide support for outputting messages.

The g_return family of macros (`g_return_if_fail()`, `g_return_val_if_fail()`, `g_return_if_reached()`, `g_return_val_if_reached()`) should only be used for programming errors, a typical use case is checking for invalid parameters at the beginning of a public function. They should not be used if you just mean "if (error) return", they should only be used if you mean "if (bug in program) return". The program behavior is generally considered undefined after one of these checks fails. They are not intended for normal control flow, only to give a perhaps-helpful warning before giving up.

Structured logging output is supported using `g_log_structured()`. This differs from the traditional `g_log()` API in that log messages are handled as a collection of key–value pairs representing individual pieces of information, rather than as a single string containing all the information in an arbitrary format.

The convenience macros `g_info()`, `g_message()`, `g_debug()`, `g_warning()` and `g_error()` will use the traditional `g_log()` API unless you define the symbol `G_LOG_USE_STRUCTURED` before including `glib.h`. But note that even messages logged through the traditional `g_log()` API are ultimatively passed to `g_log_structured()`, so that all log messages end up in same destination. If `G_LOG_USE_STRUCTURED` is defined, `g_test_expect_message()` will become ineffective for the wrapper macros `g_warning()` and friends (see Testing for Messages).

The support for structured logging was motivated by the following needs (some of which were supported previously; others weren’t):

- Support for multiple logging levels.

- Structured log support with the ability to add `MESSAGE_ID`s (see `g_log_structured()`).

- Moving the responsibility for filtering log messages from the program to the log viewer — instead of libraries and programs installing log handlers (with `g_log_set_handler()`) which filter messages before output, all log messages are outputted, and the log viewer program (such as `journalctl`) must filter them. This is based on the idea that bugs are sometimes hard to reproduce, so it is better to log everything possible and then use tools to analyse the logs than it is to not be able to reproduce a bug to get additional log data. Code which uses logging in performance-critical sections should compile out the `g_log_structured()` calls in release builds, and compile them in in debugging builds.

- A single writer function which handles all log messages in a process, from all libraries and program code; rather than multiple log handlers with poorly defined interactions between them. This allows a program to easily change its logging policy by changing the writer function, for example to log to an additional location or to change what logging output fallbacks are used. The log writer functions provided by GLib are exposed publicly so they can be used from programs’ log writers. This allows log writer policy and implementation to be kept separate.

- If a library wants to add standard information to all of its log messages (such as library state) or to redact private data (such as passwords or network credentials), it should use a wrapper function around its `g_log_structured()` calls or implement that in the single log writer function.

- If a program wants to pass context data from a `g_log_structured()` call to its log writer function so that, for example, it can use the correct server connection to submit logs to, that user data can be passed as a zero-length GLogField to `g_log_structured_array()`.

- Color output needed to be supported on the terminal, to make reading through logs easier.

#### Using Structured Logging

To use structured logging (rather than the old-style logging), either use the `g_log_structured()` and `g_log_structured_array()` functions; or define `G_LOG_USE_STRUCTURED` before including any GLib header, and use the `g_message()`, `g_debug()`, `g_error()` (etc.) macros.

You do not need to define `G_LOG_USE_STRUCTURED` to use `g_log_structured()`, but it is a good idea to avoid confusion.

#### Log Domains

Log domains may be used to broadly split up the origins of log messages. Typically, there are one or a few log domains per application or library. `G_LOG_DOMAIN` should be used to define the default log domain for the current compilation unit — it is typically defined at the top of a source file, or in the preprocessor flags for a group of source files.

Log domains must be unique, and it is recommended that they are the application or library name, optionally followed by a hyphen and a sub-domain name. For example, `bloatpad` or `bloatpad-io`.

#### Debug Message Output

The default log functions (`g_log_default_handler()` for the old-style API and `g_log_writer_default()` for the structured API) both drop debug and informational messages by default, unless the log domains of those messages are listed in the `G_MESSAGES_DEBUG` environment variable (or it is set to `all`).

It is recommended that custom log writer functions re-use the `G_MESSAGES_DEBUG` environment variable, rather than inventing a custom one, so that developers can re-use the same debugging techniques and tools across projects.

#### Testing for Messages

With the old `g_log()` API, `g_test_expect_message()` and `g_test_assert_expected_messages()` could be used in simple cases to check whether some code under test had emitted a given log message. These functions have been deprecated with the structured logging API, for several reasons:

- They relied on an internal queue which was too inflexible for many use cases, where messages might be emitted in several orders, some messages might not be emitted deterministically, or messages might be emitted by unrelated log domains.

- They do not support structured log fields.

- Examining the log output of code is a bad approach to testing it, and while it might be necessary for legacy code which uses `g_log()`, it should be avoided for new code using `g_log_structured()`.

They will continue to work as before if `g_log()` is in use (and `G_LOG_USE_STRUCTURED` is not defined). They will do nothing if used with the structured logging API.

Examining the log output of code is discouraged: libraries should not emit to `stderr` during defined behaviour, and hence this should not be tested. If the log emissions of a library during undefined behaviour need to be tested, they should be limited to asserting that the library aborts and prints a suitable error message before aborting. This should be done with `g_test_trap_assert_stderr()`.

If it is really necessary to test the structured log messages emitted by a particular piece of code – and the code cannot be restructured to be more suitable to more conventional unit testing – you should write a custom log writer function (see `g_log_set_writer_func()`) which appends all log messages to a queue. When you want to check the log messages, examine and clear the queue, ignoring irrelevant log messages (for example, from log domains other than the one under test).

## Functions

### GLogFunc ()

    void
    (*GLogFunc) (const gchar *log_domain,
                 GLogLevelFlags log_level,
                 const gchar *message,
                 gpointer user_data);

Specifies the prototype of log handler functions.

The default log handler, `g_log_default_handler()`, automatically appends a new-line character to `message` when printing it. It is advised that any custom log handler functions behave similarly, so that logging calls in user code do not need modifying to add a new-line character to the message if the log handler is changed.

This is not used if structured logging is enabled; see Using Structured Logging.

#### Parameters

log_domain | the log domain of the message |
log_level | the log level of the message (including the fatal and recursion flags) |
message | the message to process |
user_data | user data, set in `g_log_set_handler()` |

### g_log ()

    void
    g_log (const gchar *log_domain,
           GLogLevelFlags log_level,
           const gchar *format,
           ...);

Logs an error or debugging message.

If the log level has been set as fatal, the `abort()` function is called to terminate the program.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled this will output via the structured log writer function (see `g_log_set_writer_func()`).

#### Parameters

log_domain | the log domain, usually G_LOG_DOMAIN, or `NULL` for the default. | [nullable]
log_level | the log level, either from GLogLevelFlags or a user-defined level |
format | the message format. See the `printf()` documentation |
... | the parameters to insert into the format string |

### g_logv ()

    void
    g_logv (const gchar *log_domain,
            GLogLevelFlags log_level,
            const gchar *format,
            va_list args);

Logs an error or debugging message.

If the log level has been set as fatal, the `abort()` function is called to terminate the program.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled this will output via the structured log writer function (see `g_log_set_writer_func()`).

#### Parameters

log_domain | the log domain, or `NULL` for the default "" application domain. | [nullable]
log_level | the log level |
format | the message format. See the `printf()` documentation |
args | the parameters to insert into the format string |

### g_message()

    #define             g_message(...)

A convenience function/macro to log a normal message.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

### g_warning()

    #define             g_warning(...)

A convenience function/macro to log a warning message.

This is not intended for end user error reporting. Use of GError is preferred for that instead, as it allows calling functions to perform actions conditional on the type of error.

You can make warnings fatal at runtime by setting the `G_DEBUG` environment variable (see Running GLib Applications).

If `g_log_default_handler()` is used as the log handler function, a newline character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

### g_critical()

    #define             g_critical(...)

Logs a "critical warning" (G_LOG_LEVEL_CRITICAL). It's more or less application-defined what constitutes a critical vs. a regular warning. You could call `g_log_set_always_fatal()` to make critical warnings exit the program, then use `g_critical()` for fatal errors, for example.

You can also make critical warnings fatal at runtime by setting the `G_DEBUG` environment variable (see Running GLib Applications).

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

### g_error()

    #define             g_error(...)

A convenience function/macro to log an error message.

This is not intended for end user error reporting. Use of GError is preferred for that instead, as it allows calling functions to perform actions conditional on the type of error.

Error messages are always fatal, resulting in a call to `abort()` to terminate the application. This function will result in a core dump; don't use it for errors you expect. Using this function indicates a bug in your program, i.e. an assertion failure.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

### g_info()

    #define             g_info(...)

A convenience function/macro to log an informational message. Seldom used.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

Such messages are suppressed by the `g_log_default_handler()` and `g_log_writer_default()` unless the `G_MESSAGES_DEBUG` environment variable is set appropriately.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

Since: 2.40

### g_debug()

    #define             g_debug(...)

A convenience function/macro to log a debug message.

If `g_log_default_handler()` is used as the log handler function, a new-line character will automatically be appended to @..., and need not be entered manually.

Such messages are suppressed by the `g_log_default_handler()` and `g_log_writer_default()` unless the `G_MESSAGES_DEBUG` environment variable is set appropriately.

If structured logging is enabled, this will use `g_log_structured()`; otherwise it will use `g_log()`. See Using Structured Logging.

#### Parameters

... | format string, followed by parameters to insert into the format string (as with `printf()`) |

Since: 2.6

### g_log_set_handler ()

    guint
    g_log_set_handler (const gchar *log_domain,
                       GLogLevelFlags log_levels,
                       GLogFunc log_func,
                       gpointer user_data);

Sets the log handler for a domain and a set of log levels. To handle fatal and recursive messages the `log_levels` parameter must be combined with the G_LOG_FLAG_FATAL and G_LOG_FLAG_RECURSION bit flags.

Note that since the G_LOG_LEVEL_ERROR log level is always fatal, if you want to set a handler for this log level you must combine it with G_LOG_FLAG_FATAL.

This has no effect if structured logging is enabled; see Using Structured Logging.

Here is an example for adding a log handler for all warning messages in the default domain:

    1
    2
    g_log_set_handler (NULL, G_LOG_LEVEL_WARNING | G_LOG_FLAG_FATAL
                       | G_LOG_FLAG_RECURSION, my_log_handler, NULL);

This example adds a log handler for all critical messages from GTK+:

    1
    2
    g_log_set_handler ("Gtk", G_LOG_LEVEL_CRITICAL | G_LOG_FLAG_FATAL
                       | G_LOG_FLAG_RECURSION, my_log_handler, NULL);

This example adds a log handler for all messages from GLib:

    1
    2
    g_log_set_handler ("GLib", G_LOG_LEVEL_MASK | G_LOG_FLAG_FATAL
                       | G_LOG_FLAG_RECURSION, my_log_handler, NULL);

#### Parameters

log_domain | the log domain, or `NULL` for the default "" application domain. | [nullable]
log_levels | the log levels to apply the log handler for. To handle fatal and recursive messages as well, combine the log levels with the G_LOG_FLAG_FATAL and G_LOG_FLAG_RECURSION bit flags. |
log_func | the log handler function |
user_data | data passed to the log handler |

#### Returns

the id of the new handler

### g_log_set_handler_full ()

    guint
    g_log_set_handler_full (const gchar *log_domain,
                            GLogLevelFlags log_levels,
                            GLogFunc log_func,
                            gpointer user_data,
                            GDestroyNotify destroy);

Like `g_log_sets_handler()`, but takes a destroy notify for the `user_data` .

This has no effect if structured logging is enabled; see Using Structured Logging.

[rename-to g_log_set_handler]

#### Parameters

log_domain | the log domain, or `NULL` for the default "" application domain. | [nullable]
log_levels | the log levels to apply the log handler for. To handle fatal and recursive messages as well, combine the log levels with the G_LOG_FLAG_FATAL and G_LOG_FLAG_RECURSION bit flags. |
log_func | the log handler function |
user_data | data passed to the log handler |
destroy | destroy notify for `user_data` , or `NULL` |

#### Returns

the id of the new handler

Since: 2.46

### g_log_remove_handler ()

    void
    g_log_remove_handler (const gchar *log_domain,
                          guint handler_id);

Removes the log handler.

This has no effect if structured logging is enabled; see Using Structured Logging.

#### Parameters

log_domain | the log domain |
handler_id | the id of the handler, which was returned in `g_log_set_handler()` |

### g_log_set_always_fatal ()

    GLogLevelFlags
    g_log_set_always_fatal (GLogLevelFlags fatal_mask);

Sets the message levels which are always fatal, in any log domain. When a message with any of these levels is logged the program terminates. You can only set the levels defined by GLib to be fatal. `G_LOG_LEVEL_ERROR` is always fatal.

You can also make some message levels fatal at runtime by setting the `G_DEBUG` environment variable (see Running GLib Applications).

Libraries should not call this function, as it affects all messages logged by a process, including those from other libraries.

Structured log messages (using `g_log_structured()` and `g_log_structured_array()`) are fatal only if the default log writer is used; otherwise it is up to the writer function to determine which log messages are fatal. See Using Structured Logging.

#### Parameters

fatal_mask | the mask containing bits set for each level of error which is to be fatal |

#### Returns

the old fatal mask

### g_log_set_fatal_mask ()

    GLogLevelFlags
    g_log_set_fatal_mask (const gchar *log_domain,
                          GLogLevelFlags fatal_mask);

Sets the log levels which are fatal in the given domain. `G_LOG_LEVEL_ERROR` is always fatal.

This has no effect on structured log messages (using `g_log_structured()` or `g_log_structured_array()`). To change the fatal behaviour for specific log messages, programs must install a custom log writer function using `g_log_set_writer_func()`. See Using Structured Logging.

#### Parameters

log_domain | the log domain |
fatal_mask | the new fatal mask |

#### Returns

the old fatal mask for the log domain

### g_log_default_handler ()

    void
    g_log_default_handler (const gchar *log_domain,
                           GLogLevelFlags log_level,
                           const gchar *message,
                           gpointer unused_data);

The default log handler set up by GLib; `g_log_set_default_handler()` allows to install an alternate default log handler. This is used if no log handler has been set for the particular log domain and log level combination. It outputs the message to stderr or stdout and if the log level is fatal it calls `abort()`. It automatically prints a new-line character after the message, so one does not need to be manually included in `message` .

The behavior of this log handler can be influenced by a number of environment variables:

- `G_MESSAGES_PREFIXED`: A :-separated list of log levels for which messages should be prefixed by the program name and PID of the aplication.

- `G_MESSAGES_DEBUG`: A space-separated list of log domains for which debug and informational messages are printed. By default these messages are not printed.

stderr is used for levels `G_LOG_LEVEL_ERROR`, `G_LOG_LEVEL_CRITICAL`, `G_LOG_LEVEL_WARNING` and `G_LOG_LEVEL_MESSAGE`. stdout is used for the rest.

This has no effect if structured logging is enabled; see Using Structured Logging.

#### Parameters

log_domain | the log domain of the message, or `NULL` for the default "" application domain. | [nullable]
log_level | the level of the message |
message | the message. | [nullable]
unused_data | data passed from `g_log()` which is unused. | [nullable]

### g_log_set_default_handler ()

    GLogFunc
    g_log_set_default_handler (GLogFunc log_func,
                               gpointer user_data);

Installs a default log handler which is used if no log handler has been set for the particular log domain and log level combination. By default, GLib uses `g_log_default_handler()` as default log handler.

This has no effect if structured logging is enabled; see Using Structured Logging.

#### Parameters

log_func | the log handler function |
user_data | data passed to the log handler |

#### Returns

the previous default log handler

Since: 2.6

### g_log_structured ()

    void
    g_log_structured (const gchar *log_domain,
                      GLogLevelFlags log_level,
                      ...);

Log a message with structured data. The message will be passed through to the log writer set by the application using `g_log_set_writer_func()`. If the message is fatal (i.e. its log level is `G_LOG_LEVEL_ERROR`), the program will be aborted at the end of this function. If the log writer returns `G_LOG_WRITER_UNHANDLED` (failure), no other fallback writers will be tried. See the documentation for GLogWriterFunc for information on chaining writers.

The structured data is provided as key–value pairs, where keys are UTF-8 strings, and values are arbitrary pointers — typically pointing to UTF-8 strings, but that is not a requirement. To pass binary (non-nul-terminated) structured data, use `g_log_structured_array()`. The keys for structured data should follow the systemd journal fields specification. It is suggested that custom keys are namespaced according to the code which sets them. For example, custom keys from GLib all have a `GLIB_` prefix.

The `log_domain` will be converted into a `GLIB_DOMAIN` field. `log_level` will be converted into a `PRIORITY` field. The format string will have its placeholders substituted for the provided values and be converted into a `MESSAGE` field.

Other fields you may commonly want to pass into this function:

- `MESSAGE_ID`

- `CODE_FILE`

- `CODE_LINE`

- `CODE_FUNC`

- `ERRNO`

Note that `CODE_FILE`, `CODE_LINE` and `CODE_FUNC` are automatically set by the logging macros, `G_DEBUG_HERE()`, `g_message()`, `g_warning()`, `g_critical()`, `g_error()`, etc, if the symbols `G_LOG_USE_STRUCTURED` is defined before including glib.h.

For example:

    1
    2
    3
    4
    5
    g_log_structured (G_LOG_DOMAIN, G_LOG_LEVEL_DEBUG,
                      "MESSAGE_ID", "06d4df59e6c24647bfe69d2c27ef0b4e",
                      "MY_APPLICATION_CUSTOM_FIELD", "some debug string",
                      "MESSAGE", "This is a debug message about pointer %p and integer %u.",
                      some_pointer, some_integer);

Note that each `MESSAGE_ID` must be uniquely and randomly generated. If adding a `MESSAGE_ID`, consider shipping a message catalog with your software.

To pass a user data pointer to the log writer function which is specific to this logging call, you must use `g_log_structured_array()` and pass the pointer as a field with GLogField.length set to zero, otherwise it will be interpreted as a string.

For example:

    1
    2
    3
    4
    5
    6
    7
    const GLogField fields[] = {
      { "MESSAGE", "This is a debug message.", -1 },
      { "MESSAGE_ID", "fcfb2e1e65c3494386b74878f1abf893", -1 },
      { "MY_APPLICATION_CUSTOM_FIELD", "some debug string", -1 },
      { "MY_APPLICATION_STATE", state_object, 0 },
    };
    g_log_structured_array (G_LOG_LEVEL_DEBUG, fields, G_N_ELEMENTS (fields));

Note also that, even if no other structured fields are specified, there must always be a `MESSAGE` key before the format string. The `MESSAGE`-format pair has to be the last of the key-value pairs, and `MESSAGE` is the only field for which `printf()`-style formatting is supported.

The default writer function for `stdout` and `stderr` will automatically append a new-line character after the message, so you should not add one manually to the format string.

#### Parameters

log_domain | log domain, usually `G_LOG_DOMAIN` |
log_level | log level, either from GLogLevelFlags, or a user-defined level |
... | key-value pairs of structured data to add to the log entry, followed by the key "MESSAGE", followed by a `printf()`-style message format, followed by parameters to insert in the format string |

Since: 2.50

### g_log_variant ()

    void
    g_log_variant (const gchar *log_domain,
                   GLogLevelFlags log_level,
                   GVariant *fields);

Log a message with structured data, accepting the data within a GVariant. This version is especially useful for use in other languages, via introspection.

The only mandatory item in the `fields` dictionary is the "MESSAGE" which must contain the text shown to the user.

The values in the `fields` dictionary are likely to be of type String (G_VARIANT_TYPE_STRING). Array of bytes (G_VARIANT_TYPE_BYTESTRING) is also supported. In this case the message is handled as binary and will be forwarded to the log writer as such. The size of the array should not be higher than `G_MAXSSIZE`. Otherwise it will be truncated to this size. For other types `g_variant_print()` will be used to convert the value into a string.

For more details on its usage and about the parameters, see `g_log_structured()`.

#### Parameters

log_domain | log domain, usually `G_LOG_DOMAIN`. | [nullable]
log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | a dictionary (GVariant of the type `G_VARIANT_TYPE_VARDICT`) containing the key-value pairs of message data. |

Since: 2.50

### g_log_structured_array ()

    void
    g_log_structured_array (GLogLevelFlags log_level,
                            const GLogField *fields,
                            gsize n_fields);

Log a message with structured data. The message will be passed through to the log writer set by the application using `g_log_set_writer_func()`. If the message is fatal (i.e. its log level is `G_LOG_LEVEL_ERROR`), the program will be aborted at the end of this function.

See `g_log_structured()` for more documentation.

This assumes that `log_level` is already present in `fields` (typically as the `PRIORITY` field).

#### Parameters

log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | key–value pairs of structured data to add to the log message. | [array length=n_fields]
n_fields | number of elements in the `fields` array |

Since: 2.50

### G_DEBUG_HERE

    #define             G_DEBUG_HERE()

A convenience form of `g_log_structured()`, recommended to be added to functions when debugging. It prints the current monotonic time and the code location using `G_STRLOC`.

Since: 2.50

### GLogWriterFunc ()

    GLogWriterOutput
    (*GLogWriterFunc) (GLogLevelFlags log_level,
                       const GLogField *fields,
                       gsize n_fields,
                       gpointer user_data);

Writer function for log entries. A log entry is a collection of one or more GLogFields, using the standard field names from journal specification. See `g_log_structured()` for more information.

Writer functions must ignore fields which they do not recognise, unless they can write arbitrary binary output, as field values may be arbitrary binary.

`log_level` is guaranteed to be included in `fields` as the `PRIORITY` field, but is provided separately for convenience of deciding whether or where to output the log entry.

Writer functions should return `G_LOG_WRITER_HANDLED` if they handled the log message successfully or if they deliberately ignored it. If there was an error handling the message (for example, if the writer function is meant to send messages to a remote logging server and there is a network error), it should return `G_LOG_WRITER_UNHANDLED`. This allows writer functions to be chained and fall back to simpler handlers in case of failure.

#### Parameters

log_level | log level of the message |
fields | fields forming the message. | [array length=n_fields]
n_fields | number of `fields` |
user_data | user data passed to `g_log_set_writer_func()` |

#### Returns

`G_LOG_WRITER_HANDLED` if the log entry was handled successfully; `G_LOG_WRITER_UNHANDLED` otherwise

Since: 2.50

### g_log_set_writer_func ()

    void
    g_log_set_writer_func (GLogWriterFunc func,
                           gpointer user_data,
                           GDestroyNotify user_data_free);

Set a writer function which will be called to format and write out each log message. Each program should set a writer function, or the default writer (`g_log_writer_default()`) will be used.

Libraries **must not** call this function — only programs are allowed to install a writer function, as there must be a single, central point where log messages are formatted and outputted.

There can only be one writer function. It is an error to set more than one.

#### Parameters

func | log writer function, which must not be `NULL` |
user_data | user data to pass to `func` . | [closure func]
user_data_free | function to free `user_data` once it’s finished with, if non-`NULL`. | [destroy func]

Since: 2.50

### g_log_writer_supports_color ()

    gboolean
    g_log_writer_supports_color (gint output_fd);

Check whether the given `output_fd` file descriptor supports ANSI color escape sequences. If so, they can safely be used when formatting log messages.

#### Parameters

output_fd | output file descriptor to check |

#### Returns

`TRUE` if ANSI color escapes are supported, `FALSE` otherwise

Since: 2.50

### g_log_writer_is_journald ()

    gboolean
    g_log_writer_is_journald (gint output_fd);

Check whether the given `output_fd` file descriptor is a connection to the systemd journal, or something else (like a log file or `stdout` or `stderr`).

Invalid file descriptors are accepted and return `FALSE`, which allows for the following construct without needing any additional error handling:

    1
    is_journald = g_log_writer_is_journald (fileno (stderr));

#### Parameters

output_fd | output file descriptor to check |

#### Returns

`TRUE` if `output_fd` points to the journal, `FALSE` otherwise

Since: 2.50

### g_log_writer_format_fields ()

    gchar *
    g_log_writer_format_fields (GLogLevelFlags log_level,
                                const GLogField *fields,
                                gsize n_fields,
                                gboolean use_color);

Format a structured log message as a string suitable for outputting to the terminal (or elsewhere). This will include the values of all fields it knows how to interpret, which includes `MESSAGE` and `GLIB_DOMAIN` (see the documentation for `g_log_structured()`). It does not include values from unknown fields.

The returned string does **not** have a trailing new-line character. It is encoded in the character set of the current locale, which is not necessarily UTF-8.

#### Parameters

log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | key–value pairs of structured data forming the log message. | [array length=n_fields]
n_fields | number of elements in the `fields` array |
use_color | `TRUE` to use ANSI color escape sequences when formatting the message, `FALSE` to not |

#### Returns

string containing the formatted log message, in the character set of the current locale.

[transfer full]

Since: 2.50

### g_log_writer_journald ()

    GLogWriterOutput
    g_log_writer_journald (GLogLevelFlags log_level,
                           const GLogField *fields,
                           gsize n_fields,
                           gpointer user_data);

Format a structured log message and send it to the systemd journal as a set of key–value pairs. All fields are sent to the journal, but if a field has length zero (indicating program-specific data) then only its key will be sent.

This is suitable for use as a GLogWriterFunc.

If GLib has been compiled without systemd support, this function is still defined, but will always return `G_LOG_WRITER_UNHANDLED`.

#### Parameters

log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | key–value pairs of structured data forming the log message. | [array length=n_fields]
n_fields | number of elements in the `fields` array |
user_data | user data passed to `g_log_set_writer_func()` |

#### Returns

`G_LOG_WRITER_HANDLED` on success, `G_LOG_WRITER_UNHANDLED` otherwise

Since: 2.50

### g_log_writer_standard_streams ()

    GLogWriterOutput
    g_log_writer_standard_streams (GLogLevelFlags log_level,
                                   const GLogField *fields,
                                   gsize n_fields,
                                   gpointer user_data);

Format a structured log message and print it to either `stdout` or `stderr`, depending on its log level. `G_LOG_LEVEL_INFO` and `G_LOG_LEVEL_DEBUG` messages are sent to `stdout`; all other log levels are sent to `stderr`. Only fields which are understood by this function are included in the formatted string which is printed.

If the output stream supports ANSI color escape sequences, they will be used in the output.

A trailing new-line character is added to the log message when it is printed.

This is suitable for use as a GLogWriterFunc.

#### Parameters

log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | key–value pairs of structured data forming the log message. | [array length=n_fields]
n_fields | number of elements in the `fields` array |
user_data | user data passed to `g_log_set_writer_func()` |

#### Returns

`G_LOG_WRITER_HANDLED` on success, `G_LOG_WRITER_UNHANDLED` otherwise

Since: 2.50

### g_log_writer_default ()

    GLogWriterOutput
    g_log_writer_default (GLogLevelFlags log_level,
                          const GLogField *fields,
                          gsize n_fields,
                          gpointer user_data);

Format a structured log message and output it to the default log destination for the platform. On Linux, this is typically the systemd journal, falling back to `stdout` or `stderr` if running from the terminal or if output is being redirected to a file.

Support for other platform-specific logging mechanisms may be added in future. Distributors of GLib may modify this function to impose their own (documented) platform-specific log writing policies.

This is suitable for use as a GLogWriterFunc, and is the default writer used if no other is set using `g_log_set_writer_func()`.

As with `g_log_default_handler()`, this function drops debug and informational messages unless their log domain (or `all`) is listed in the space-separated `G_MESSAGES_DEBUG` environment variable.

#### Parameters

log_level | log level, either from GLogLevelFlags, or a user-defined level |
fields | key–value pairs of structured data forming the log message. | [array length=n_fields]
n_fields | number of elements in the `fields` array |
user_data | user data passed to `g_log_set_writer_func()` |

#### Returns

`G_LOG_WRITER_HANDLED` on success, `G_LOG_WRITER_UNHANDLED` otherwise

Since: 2.50

## Types and Values

### G_LOG_DOMAIN

    #define G_LOG_DOMAIN    ((gchar*) 0)

Defines the log domain.

Libraries should define this so that any messages which they log can be differentiated from messages from other libraries and application code. But be careful not to define it in any public header files.

For example, GTK+ uses this in its Makefile.am:

    1
    AM_CPPFLAGS = -DG_LOG_DOMAIN=\"Gtk\"

Applications can choose to leave it as the default `NULL` (or "") domain. However, defining the domain offers the same advantages as above.

### G_LOG_FATAL_MASK

    #define G_LOG_FATAL_MASK        (G_LOG_FLAG_RECURSION | G_LOG_LEVEL_ERROR)

GLib log levels that are considered fatal by default.

This is not used if structured logging is enabled; see Using Structured Logging.

### G_LOG_LEVEL_USER_SHIFT

    #define G_LOG_LEVEL_USER_SHIFT  (8)

Log levels below 1<<G_LOG_LEVEL_USER_SHIFT are used by GLib. Higher bits can be used for user-defined log levels.

### enum GLogLevelFlags

Flags specifying the level of log messages.

It is possible to change how GLib treats messages of the various levels using `g_log_set_handler()` and `g_log_set_fatal_mask()`.

#### Members

G_LOG_FLAG_RECURSION | internal flag |
G_LOG_FLAG_FATAL | internal flag |
G_LOG_LEVEL_ERROR | log level for errors, see `g_error()`. This level is also used for messages produced by `g_assert()`. |
G_LOG_LEVEL_CRITICAL | log level for critical warning messages, see `g_critical()`. This level is also used for messages produced by `g_return_if_fail()` and `g_return_val_if_fail()`. |
G_LOG_LEVEL_WARNING | log level for warnings, see `g_warning()` |
G_LOG_LEVEL_MESSAGE | log level for messages, see `g_message()` |
G_LOG_LEVEL_INFO | log level for informational messages, see `g_info()` |
G_LOG_LEVEL_DEBUG | log level for debug messages, see `g_debug()` |
G_LOG_LEVEL_MASK | a mask including all log levels |

### struct GLogField

    struct GLogField {
      const gchar *key;
      gconstpointer value;
      gssize length;
    };

Structure representing a single field in a structured log entry. See `g_log_structured()` for details.

Log fields may contain arbitrary values, including binary with embedded nul bytes. If the field contains a string, the string must be UTF-8 encoded and have a trailing nul byte. Otherwise, `length` must be set to a non-negative value.

#### Members

const gchar *`key`; | field name (UTF-8 string) |
gconstpointer `value`; | field value (arbitrary bytes) |
gssize `length`; | length of `value` , in bytes, or -1 if it is nul-terminated |

Since: 2.50

### enum GLogWriterOutput

Return values from GLogWriterFuncs to indicate whether the given log entry was successfully handled by the writer, or whether there was an error in handling it (and hence a fallback writer should be used).

If a GLogWriterFunc ignores a log entry, it should return `G_LOG_WRITER_HANDLED`.

#### Members

G_LOG_WRITER_HANDLED | Log writer has handled the log entry. |
G_LOG_WRITER_UNHANDLED | Log writer could not handle the log entry. |

Since: 2.50