platform_system_core/liblog/README

LIBLOG(3)          Android Internal NDK Programming Manual           LIBLOG(3)



NAME
       liblog - Android Internal NDK logger interfaces

SYNOPSIS
       /*
        * 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, ...)
       IF_ALOGC()
       ALOGD(format, ...)
       SLOGD(format, ...)
       RLOGD(format, ...)
       ALOGD_IF(cond, format, ...)
       SLOGD_IF(cond, format, ...)
       RLOGD_IF(cond, format, ...)
       IF_ALOGD()
       ALOGI(format, ...)
       SLOGI(format, ...)
       RLOGI(format, ...)
       ALOGI_IF(cond, format, ...)
       SLOGI_IF(cond, format, ...)
       RLOGI_IF(cond, format, ...)
       IF_ALOGI()
       ALOGW(format, ...)
       SLOGW(format, ...)
       RLOGW(format, ...)
       ALOGW_IF(cond, format, ...)
       SLOGW_IF(cond, format, ...)
       RLOGW_IF(cond, format, ...)
       IF_ALOGW()
       ALOGE(format, ...)
       SLOGE(format, ...)
       RLOGE(format, ...)
       ALOGE_IF(cond, format, ...)
       SLOGE_IF(cond, format, ...)
       RLOGE_IF(cond, format, ...)
       IF_ALOGE()
       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_frontend.h>

       int android_set_log_frontend(int frontend_flag)
       int android_get_log_frontend()

       Link with -llog

DESCRIPTION
       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
       sub-log.

       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
       opened  with an  ANDROID_LOG_RDONLY  mode.   ANDROID_LOG_NONBLOCK  mode
       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
       entries.

       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
       reboot.

       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_frontend() selects frontend filters. Argument is either
       LOGGER_DEFAULT, LOGGER_LOGD, LOGGER_NULL or LOGGER_LOCAL. Log to logger
       daemon for default or logd,  drop contents on floor,  or log into local
       memory   respectively.          Both   android_set_log_frontend()   and
       android_get_log_frontend()  return  the  current  frontend mask,   or a
       negative errno for any problems.

ERRORS
       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.

SEE ALSO
       syslogd(8), klogd, auditd(8)



                                  08 Feb 2017                        LIBLOG(3)