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LIBLOG(3) Android Internal NDK Programming Manual LIBLOG(3)
liblog - Android Internal NDK logger interfaces
* Please limit to 24 characters for runtime is loggable,
* 16 characters for persist is loggable, and logcat pretty
* alignment with limit of 7 characters.
#define LOG_TAG "yourtag"
#include <log/log.h>
ALOG(android_priority, tag, format, ...)
IF_ALOG(android_priority, tag)
LOG_PRI(priority, tag, format, ...)
LOG_PRI_VA(priority, tag, format, args)
#define LOG_TAG NULL
ALOGV(format, ...)
SLOGV(format, ...)
RLOGV(format, ...)
ALOGV_IF(cond, format, ...)
SLOGV_IF(cond, format, ...)
RLOGV_IF(cond, format, ...)
ALOGD(format, ...)
SLOGD(format, ...)
RLOGD(format, ...)
ALOGD_IF(cond, format, ...)
SLOGD_IF(cond, format, ...)
RLOGD_IF(cond, format, ...)
ALOGI(format, ...)
SLOGI(format, ...)
RLOGI(format, ...)
ALOGI_IF(cond, format, ...)
SLOGI_IF(cond, format, ...)
RLOGI_IF(cond, format, ...)
ALOGW(format, ...)
SLOGW(format, ...)
RLOGW(format, ...)
ALOGW_IF(cond, format, ...)
SLOGW_IF(cond, format, ...)
RLOGW_IF(cond, format, ...)
ALOGE(format, ...)
SLOGE(format, ...)
RLOGE(format, ...)
ALOGE_IF(cond, format, ...)
SLOGE_IF(cond, format, ...)
RLOGE_IF(cond, format, ...)
LOG_FATAL(format, ...)
LOG_ALWAYS_FATAL(format, ...)
LOG_FATAL_IF(cond, format, ...)
LOG_ALWAYS_FATAL_IF(cond, format, ...)
ALOG_ASSERT(cond, format, ...)
LOG_EVENT_INT(tag, value)
LOG_EVENT_LONG(tag, value)
clockid_t android_log_clockid()
log_id_t android_logger_get_id(struct logger *logger)
int android_logger_clear(struct logger *logger)
int android_logger_get_log_size(struct logger *logger)
int android_logger_get_log_readable_size(struct logger *logger)
int android_logger_get_log_version(struct logger *logger)
struct logger_list *android_logger_list_alloc(int mode,
unsigned int tail,
pid_t pid)
struct logger *android_logger_open(struct logger_list *logger_list,
log_id_t id)
struct logger_list *android_logger_list_open(log_id_t id, int mode,
unsigned int tail,
pid_t pid)
int android_logger_list_read(struct logger_list *logger_list,
struct log_msg *log_msg)
void android_logger_list_free(struct logger_list *logger_list)
log_id_t android_name_to_log_id(const char *logName)
const char *android_log_id_to_name(log_id_t log_id)
android_log_context create_android_logger(uint32_t tag)
int android_log_write_list_begin(android_log_context ctx)
int android_log_write_list_end(android_log_context ctx)
int android_log_write_int32(android_log_context ctx, int32_t value)
int android_log_write_int64(android_log_context ctx, int64_t value)
int android_log_write_string8(android_log_context ctx,
const char *value)
int android_log_write_string8_len(android_log_context ctx,
const char *value, size_t maxlen)
int android_log_write_float32(android_log_context ctx, float value)
int android_log_write_list(android_log_context ctx,
log_id_t id = LOG_ID_EVENTS)
android_log_context create_android_log_parser(const char *msg,
size_t len)
android_log_list_element android_log_read_next(android_log_context ctx)
android_log_list_element android_log_peek_next(android_log_context ctx)
int android_log_destroy(android_log_context *ctx)
#include <log/log_transport.h>
int android_set_log_transport(int transport_flag)
int android_get_log_transport()
Link with -llog
liblog represents an interface to the volatile Android Logging system
for NDK (Native) applications and libraries. Interfaces for either
writing or reading logs. The log buffers are divided up in Main, Sys
tem, Radio and Events sub-logs.
The logging interfaces are a series of macros, all of which can be
overridden individually in order to control the verbosity of the appli
cation or library. [ASR]LOG[VDIWE] calls are used to log to BAsic,
System or Radio sub-logs in either the Verbose, Debug, Info, Warning or
Error priorities. [ASR]LOG[VDIWE]_IF calls are used to perform thus
based on a condition being true. IF_ALOG[VDIWE] calls are true if the
current LOG_TAG is enabled at the specified priority. LOG_ALWAYS_FATAL
is used to ALOG a message, then kill the process. LOG_FATAL call is a
variant of LOG_ALWAYS_FATAL, only enabled in engineering, and not
release builds. ALOG_ASSERT is used to ALOG a message if the condition
is false; the condition is part of the logged message.
LOG_EVENT_(INT|LONG) is used to drop binary content into the Events
The log reading interfaces permit opening the logs either singly or
multiply, retrieving a log entry at a time in time sorted order,
optionally limited to a specific pid and tail of the log(s) and finally
a call closing the logs. A single log can be opened with android_log
ger_list_open; or multiple logs can be opened with android_log
ger_list_alloc, calling in turn the android_logger_open for each log
id. Each entry can be retrieved with android_logger_list_read. The
log(s) can be closed with android_logger_list_free. The logs should be
will report when the log reading is done with an EAGAIN error return
code, otherwise the android_logger_list_read call will block for new
The ANDROID_LOG_WRAP mode flag to the android_logger_list_alloc_time
signals logd to quiesce the reader until the buffer is about to prune
at the start time then proceed to dumping content.
The ANDROID_LOG_PSTORE mode flag to the android_logger_open is used to
switch from the active logs to the persistent logs from before the last
The value returned by android_logger_open can be used as a parameter to
the android_logger_clear function to empty the sub-log. It is recom
mended to only open log ANDROID_LOG_WRONLY in that case.
The value returned by android_logger_open can be used as a parameter to
the android_logger_get_log_(size|readable_size|version) to retrieve the
sub-log maximum size, readable size and log buffer format protocol ver
sion respectively. android_logger_get_id returns the id that was used
when opening the sub-log. It is recommended to open the log
ANDROID_LOG_RDONLY in these cases.
android_set_log_transport() selects transport filters. Argument is
logger daemon for default or logd, drop contents on floor, or log into
local memory respectively. Both android_set_log_transport()
and android_get_log_transport() return the current transport mask, or
a negative errno for any problems.
If messages fail, a negative error code will be returned to the caller.
The -ENOTCONN return code indicates that the logger daemon is stopped.
The -EBADF return code indicates that the log access point can not be
opened, or the log buffer id is out of range.
For the -EAGAIN return code, this means that the logging message was
temporarily backed-up either because of Denial Of Service (DOS) logging
pressure from some chatty application or service in the Android system,
or if too small of a value is set in /proc/sys/net/unix/max_dgram_qlen.
To aid in diagnosing the occurence of this, a binary event from liblog
will be sent to the log daemon once a new message can get through
indicating how many messages were dropped as a result. Please take
action to resolve the structural problems at the source.
It is generally not advised for the caller to retry the -EAGAIN return
code as this will only make the problem(s) worse and cause your
application to temporarily drop to the logger daemon priority, BATCH
scheduling policy and background task cgroup. If you require a group of
messages to be passed atomically, merge them into one message with
embedded newlines to the maximum length LOGGER_ENTRY_MAX_PAYLOAD.
Other return codes from writing operation can be returned. Since the
library retries on EINTR, -EINTR should never be returned.
syslogd(8), klogd, auditd(8)
08 Feb 2017 LIBLOG(3)