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+# nng_log
+
+## NAME
+
+# nng_log --- log messages
+
+## SYNOPSIS
+
+```c
+#include <nng/nng.h>
+
+void nng_log_err(const char *msgid, const char *msg, ...);
+void nng_log_warn(const char *msgid, const char *msg, ...);
+void nng_log_notice(const char *msgid, const char *msg, ...);
+void nng_log_info(const char *msgid, const char *msg, ...);
+void nng_log_debug(const char *msgid, const char *msg, ...);
+
+void nng_log_auth(nng_log_level level, const char *msgid, const char *msg, ...);
+```
+
+## DESCRIPTION
+
+The {{i:`nng_log`}} functions are used to post a message to system or application {{i:logs}}.
+
+The first five forms all post a message at the severity indicated by the function name.
+The _msgid_ should be a short message identifier that should indicate the message in question.
+A `NULL` value for _msgid_ can be used as well.
+
+Message identifiers can be used to assist in filtering and classifying logged messages.
+These should uniquely identify the nature of the problem, whe possible, to assist in trouble-shooting.
+They should also be short.
+Eight characters or less is ideal, and more than sixteen is strongly discouraged.
+Message identifiers may not be displayed to human readers, or may not be displayed by default.
+Therefore, any information in the message identifier should also be in the log content.
+
+The message is formatted as if by `sprintf`, using `msg` as the format, and remaining arguments as arguments to the format.
+
+The final function, {{i:`nng_log_auth`}}, is used for posting authentication related messages which might be treated specially,
+such as be storing them in a separate (and presumably more secure) log file.
+It takes the severity as a level in _level_.
+The severity can be one of the following values:
+
+- `NNG_LOG_ERR`
+- `NNG_LOG_WARN`
+- `NNG_LOG_NOTICE`
+- `NNG_LOG_INFO`
+- `NNG_LOG_DEBUG`
+
+The message itself is handled according to the logging facility set up with [`nng_log_set_logger`][log_logger].
+Message delivery is best effort, and messages may be suppressed based on the priority set with [`nng_log_set_level`][log_level].
+
+Note that in order to get log messages, a suitable logger must be set using {{i:`nng_log_set_logger`}}.
+The default logger, {{i:`nng_null_logger`}} simply discards logged content.
+
+> [!TIP]
+> Applications should take care to limit the use of higher severity levels, as message logs
+> are potentially expensive, increase stress for end users and administrators, and further may
+> mask real problems if incorrectly over used.
+>
+> Warnings and error messages should be concise and actionable, and notices should only
+> really be those things that are worthy of attention.
+>
+> Informational and debug messages used during development should be removed when no longer
+> needed, as these messages can overwhelm logging subsystems and can reduce the
+> signal-to-noise value for the message logs, impairing the diagnostic value of the logs.
+
+## SEE ALSO
+
+[nng_log_facility][log_facility],
+[nng_log_level][log_level],
+[nng_log_logger][log_logger]
+
+[log_facility]: ./nng_log_facility.md
+[log_level]: ./nng_log_level.md
+[log_logger]: ./nng_log_logger.md