The logger API provides a common interface to process messages issued by developers. These messages are processed by active backends. Currently only the UART backend is available. However, a generic backend interface exists and more backends can be added and easily plugged in to the system.

Summary of logger features:

  • Deferred logging reduces the time needed to log a message by shifting time consuming operations to a known context instead of processing and sending the log message when called.
  • Multiple backends supported (up to 9 backends).
  • Compile time filtering on module level.
  • Run time filtering independent for each backend.
  • Additional run time filtering on module instance level.
  • Timestamping with user provided function.
  • Dedicated API for dumping data
  • Dedicated API for handling transient strings
  • Panic support - in panic mode logger switches to blocking, in-place processing.
  • Printk support - printk message can be redirected to the logger.
  • Design ready for multi-domain/multi-processor system

Logger API is highly configurable at compile time as well as at run time. Using Kconfig options (see Global Kconfig Options) logs can be gradually removed from compilation to reduce image size and execution time when logs are not needed. During compilation logs can be filtered out on module basis and severity level.

Logs can also be compiled in but filtered on run time using dedicate API. Run time filtering is independent for each backend and each source of log messages. Source of log messages can be a module or specific instance of the module.

There are four severity levels available in the system: error, warning, info and debug. For each severity level the logger API (include/logging/log.h) has set of dedicated macros. Logger API also has macros for logging data.

For each level following set of macros are available:

  • LOG_X for standard printf-like messages, e.g. LOG_ERR.
  • LOG_HEXDUMP_X for dumping data, e.g. LOG_HEXDUMP_WRN.
  • LOG_INST_X for standard printf-like message associated with the particular instance, e.g. LOG_INST_INF.
  • LOG_INST_HEXDUMP_X for dumping data associated with the particular instance, e.g. LOG_HEXDUMP_INST_DBG

There are two configuration categories: configurations per module and global configuration. When logging is enabled globally, it works for modules. However, modules can disable logging locally. Every module can specify its own logging level. The module must define the LOG_LEVEL macro before using the API. Unless a global override is set, the module logging level will be honored. The global override can only increase the logging level. It cannot be used to lower module logging levels that were previously set higher. It is also possible to globally limit logs by providing maximal severity level present in the system, where maximal means lowest severity (e.g. if maximal level in the system is set to info, it means that errors, warnings and info levels are present but debug messages are excluded).

Each module which is using the logger must specify its unique name and register itself to the logger. If module consists of more than one file, registration is performed in one file but each file must define a module name.

Logger is designed to be thread safe and minimizes time needed to log the message. Time consuming operations like string formatting or access to the transport are not performed by default when logger API is called. When logger API is called a message is created and added to the list. Dedicated, configurable pool of log messages is used. There are 2 types of messages: standard and hexdump. Each message contain source ID (module or instance ID and domain ID which might be used for multiprocessor systems), timestamp and severity level. Standard message contains pointer to the string and 32 bit arguments. Hexdump message contains copied data.

Global Kconfig Options

These options can be found in the following path subsys/logging/Kconfig.

CONFIG_LOG: Global switch, turns on/off the logger.

CONFIG_LOG_RUNTIME_FILTERING: Enables runtime reconfiguration of the logger.

CONFIG_LOG_MODE_OVERFLOW: When logger cannot allocate new message oldest one are discarded.

CONFIG_LOG_MODE_NO_OVERFLOW: When logger cannot allocate new message it is discarded.

CONFIG_LOG_DEFAULT_LEVEL: Default level, sets the logging level used by modules that are not setting their own logging level.

CONFIG_LOG_OVERRIDE_LEVEL: It overrides module logging level when it is not set or set lower than the override value.

CONFIG_LOG_MAX_LEVEL: Maximal (lowest severity) level which is compiled in.

CONFIG_LOG_FUNC_NAME_PREFIX_ERR: Prepend ERROR log messages with function name.

CONFIG_LOG_FUNC_NAME_PREFIX_WRN: Prepend WARNING log messages with function name.

CONFIG_LOG_FUNC_NAME_PREFIX_INF: Prepend INFO log messages with function name.

CONFIG_LOG_FUNC_NAME_PREFIX_DBG: Prepend DEBUG log messages with function name.

CONFIG_LOG_PRINTK: Redirect printk calls to the logger.

CONFIG_LOG_PRINTK_MAX_STRING_LENGTH: Maximal string length that can be processed by printk. Longer strings are trimmed.

CONFIG_LOG_INPLACE_PROCESS: Messages are processed in the context of the log macro call. Note that it can lead to errors when logger is used in the interrupt context.

CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD: When number of buffered log messages reaches the threshold dedicated thread (see log_thread_set()) is waken up. If CONFIG_LOG_PROCESS_THREAD is enabled then this threshold is used by the internal thread.

CONFIG_LOG_PROCESS_THREAD: When enabled, logger is creating own thread which handles log processing.

CONFIG_LOG_BUFFER_SIZE: Number of bytes dedicated for the logger message pool. Single message capable of storing standard log with up to 3 arguments or hexdump message with 12 bytes of data take 32 bytes.

CONFIG_LOG_STRDUP_MAX_STRING: Longest string that can be duplicated using log_strdup().

CONFIG_LOG_STRDUP_BUF_COUNT: Number of buffers in the pool used by log_strdup().

CONFIG_LOG_DOMAIN_ID: Domain ID. Valid in multi-domain systems.

CONFIG_LOG_BACKEND_UART: Enabled build-in UART backend.

CONFIG_LOG_BACKEND_SHOW_COLOR: Enables coloring of errors (red) and warnings (yellow).

CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP: If enabled timestamp is formatted to hh:mm:ss:mmm,uuu. Otherwise is printed in raw format.


Logging in a module

In order to use logger in the module, a unique name of a module must be specified and module must be registered with the logger core using LOG_MODULE_REGISTER. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (CONFIG_LOG_DEFAULT_LEVEL) is used if custom log level is not provided.

#include <logging/log.h>

If the module consists of multiple files, then LOG_MODULE_REGISTER() should appear in exactly one of them. Each other file should use LOG_MODULE_DECLARE to declare its membership in the module. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (CONFIG_LOG_DEFAULT_LEVEL) is used if custom log level is not provided.

#include <logging/log.h>
/* In all files comprising the module but one */

In order to use logger API in a function implemented in a header file LOG_MODULE_DECLARE macro must be used in the function body before logger API is called. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (CONFIG_LOG_DEFAULT_LEVEL) is used if custom log level is not provided.

#include <logging/log.h>

static inline void foo(void)


Dedicated Kconfig template (subsys/logging/Kconfig.template.log_config) can be used to create local log level configuration.

Example below presents usage of the template. As a result CONFIG_FOO_LOG_LEVEL will be generated:

module = FOO
module-str = foo
source "subsys/logging/Kconfig.template.log_config"

Logging in a module instance

In case of modules which are multi-instance and instances are widely used across the system enabling logs will lead to flooding. Logger provide the tools which can be used to provide filtering on instance level rather than module level. In that case logging can be enabled for particular instance.

In order to use instance level filtering following steps must be performed:

  • a pointer to specific logger structure is declared in instance structure. LOG_INSTANCE_PTR_DECLARE is used for that.
#include <logging/log_instance.h>

struct foo_object {
     u32_t id;
  • module must provide macro for instantiation. In that macro, logger instance is registered and log instance pointer is initialized in the object structure.
#define FOO_OBJECT_DEFINE(_name)                             \
     struct foo_object _name = {                             \
             LOG_INSTANCE_PTR_INIT(log, foo, _name)          \

Note that when logger is disabled logger instance and pointer to that instance are not created.

In order to use the instance logging API in a source file, a compile-time log level must be set using LOG_LEVEL_SET.


void foo_init(foo_object *f)
     LOG_INST_INF(f->log, "Initialized.");

In order to use the instance logging API in a header file, a compile-time log level must be set using LOG_LEVEL_SET.

static inline void foo_init(foo_object *f)

     LOG_INST_INF(f->log, "Initialized.");

Controlling the logger

Logger can be controlled using API defined in include/logging/log_ctrl.h. Logger must be initialized before it can be used. Optionally, user can provide function which returns timestamp value. If not provided, k_cycle_get_32 is used for timestamping. log_process() function is used to trigger processing of one log message (if pending). Function returns true if there is more messages pending.

Following snippet shows how logger can be processed in simple forever loop.

#include <log_ctrl.h>

void main(void)

     while (1) {
             if (log_process() == false) {
                     /* sleep */

Logger controlling API contains also functions for run time reconfiguration of the logger. If run time filtering is enabled the log_filter_set() can be used to change maximal severity level for given module. Module is identified by source ID and domain ID. Source ID can be retrieved if source name is known by iterating through all registered sources.

If logger is processed from a thread then it is possible to enable a feature which will wake up processing thread when certain amount of log messages are buffered (see CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD). It is also possible to enable internal logger thread (see CONFIG_LOG_PROCESS_THREAD). In that case logger thread is initialized and log messages are processed implicitly.

Logger panic

In case of error condition system usually can no longer rely on scheduler or interrupts. In that situation deferred log message processing is not an option. Logger controlling API provides a function for entering into panic mode (log_panic()) which should be called in such situation.

When log_panic() is called, logger sends _panic_ notification to all active backends. It is backend responsibility to react. Backend should switch to blocking, synchronous mode (stop using interrupts) or disable itself. Once all backends are notified, logger flushes all buffered messages. Since that moment all logs are processed in a blocking way.


Logger consists of 3 main parts:

  • Frontend
  • Core
  • Backends

Log message is generated by a source of logging which can be a module or instance of a module.


Frontend is engaged when logger API is called in a source of logging (e.g. LOG_INF) and is responsible for filtering a message (compile and run time), allocating buffer for the message, creating the message and putting that message into the list of pending messages. Since logger API can be called in an interrupt, frontend is optimized to log the message as fast as possible. Each log message consists of one or more fixed size chunks. Message head chunk contains log entry details like: source ID, timestamp, severity level and the data (string pointer and arguments or raw data). Message contains also a reference counter which indicates how many users still uses this message. It is used to return message to the pool once last user indicates that it can be freed. If more than 3 arguments or 12 bytes of raw data is used in the log then log message is formed from multiple chunks which are linked together.

It may happen that frontend cannot allocate message. It happens if system is generating more log messages than it can process in certain time frame. There are two strategies to handle that case:

  • Overflow - oldest pending messages are freed, before backends process them, until new message can be allocated.
  • No overflow - new log is dropped if message cannot be allocated.

Second option is simpler however in many case less welcomed. On the other hand, handling overflows degrades performance of the logger since allocating a message requires freeing other messages which degrades logger performance. It is thus recommended to avoid such cases by increasing logger buffer or filtering out logs.

If run-time filtering is enabled, then for each source of logging a filter structure in RAM is declared. Such filter is using 32 bits divided into ten 3 bit slots. Except slot 0, each slot stores current filter for one backend in the system. Slot 0 (bits 0-2) is used to aggregate maximal filter setting for given source of logging. Aggregate slot determines if log message is created for given entry since it indicates if there is at least one backend expecting that log entry. Backend slots are examined when message is process by the logger core to determine if message is accepted by given backend.

In the example below backend 1 is set to receive errors (slot 1) and backend 2 up to info level (slot 2). Slots 3-9 are not used. Aggregated filter (slot 0) is set to info level and up to this level message from that particular source will be buffered.

slot 0 slot 1 slot 2 slot 3 slot 9


When log processing is triggered, a message is removed from the list of pending messages. If runtime filtering is disabled, the message is passed to all active backends, otherwise the message is passed to only those backends that have requested messages from that particular source (based on the source ID in the message), and severity level. Once all backends are iterated, the message is considered processed by the logger, but the message may still be in use by a backend.

Logging strings

Logger stores the address of a log message string argument passed to it. Because a string variable argument could be transient, allocated on the stack, or modifiable, logger provides a mechanism and a dedicated buffer pool to hold copies of strings. The buffer size and count is configurable (see CONFIG_LOG_STRDUP_MAX_STRING and CONFIG_LOG_STRDUP_BUF_COUNT).

If a string argument is transient, the user must call cpp:func:log_strdup to duplicate the passed string into a buffer from the pool. See the examples below. If a strdup buffer cannot be allocated, a warning message is logged and an error code returned indicating CONFIG_LOG_STRDUP_BUF_COUNT should be increased. Buffers are freed together with the log message.

char local_str[] = "abc";

LOG_INF("logging transient string: %s", log_strdup(local_str));
local_str[0] = '\0'; /* String can be modified, logger will use duplicate."

Logger backends

Logger supports up to 9 concurrent backends. Logger backend interface consists of two functions:

  • log_backend_put() - backend gets log message.
  • log_backend_panic() - on that call backend is notified that it must switch to panic (synchronous) mode. If backend cannot support synchronous, interrupt-less operation (e.g. network) it should stop any processing.

The log message contains a reference counter tracking how many backends are processing the message. On receiving a message backend must claim it by calling log_msg_get() on that message which increments a reference counter. Once message is processed, backend puts back the message (log_msg_put()) decrementing a reference counter. On last log_msg_put(), when reference counter reaches 0, message is returned to the pool. It is up to the backend how message is processed. If backend intends to format message into the string, helper function for that are available in include/logging/log_output.h.

Example message formatted using log_output_msg_process().

[00:00:00.000,274] <info> sample_instance.inst1: logging message


The message pool can be starved if a backend does not call log_msg_put() when it is done processing a message. The logger core has no means to force messages back to the pool if they’re still marked as in use (with a non-zero reference counter).

#include <log_backend.h>

void put(const struct log_backend *const backend,
         struct log_msg *msg)

     /* message processing */


Logger backends are registered to the logger using LOG_BACKEND_DEFINE macro. The macro creates an instance in the dedicated memory section. Backends can be dynamically enabled (log_backend_enable()) and disabled.


The Logger architecture has the following limitations:

  • Strings as arguments (%s) require special treatment (see Logging strings).
  • Logging double floating point variables is not possible because arguments are 32 bit values.
  • Number of arguments in the string is limited to 9.