log - interface to STREAMS error logging and event tracing
log is a STREAMS software device driver that provides an interface for
console logging and for the STREAMS error logging and event tracing
processes [strerr(1M), strace(1M)]. log presents two separate
interfaces: a function call interface in the kernel through which STREAMS
drivers and modules submit log messages; and a subset of ioctl(2) system
calls and STREAMS messages for interaction with a user level console
logger, an error logger, a trace logger, or processes that need to submit
their own log messages.
Kernel Interface [Toc] [Back]
log messages are generated within the kernel by calls to the function
strlog(mid, sid, level, flags, fmt, arg1, ...)
short mid, sid;
Required definitions are contained in <sys/strlog.h>, <sys/log.h>, and
<sys/syslog.h>. mid is the STREAMS module id number for the module or
driver submitting the log message. sid is an internal sub-id number
usually used to identify a particular minor device of a driver. level is
a tracing level that allows for selective screening out of low priority
messages from the tracer. flags are any combination of SL_ERROR (the
message is for the error logger), SL_TRACE (the message is for the
tracer), SL_CONSOLE (the message is for the console logger), SL_FATAL
(advisory notification of a fatal error), and SL_NOTIFY (request that a
copy of the message be mailed to the system administrator). fmt is a
printf(3S) style format string, except that %s, %e, %E, %g, and %G
conversion specifications are not handled. Up to NLOGARGS (currently 3)
numeric or character arguments can be provided.
log is opened via the clone interface, /dev/log. Each open of /dev/log
obtains a separate stream to log. In order to receive log messages, a
process must first notify log whether it is an error logger, trace
logger, or console logger via a STREAMS I_STR ioctl call (see below).
For the console logger, the I_STR ioctl has an ic_cmd field of I_CONSLOG,
with no accompanying data. For the error logger, the I_STR ioctl has an
ic_cmd field of I_ERRLOG, with no accompanying data. For the trace
logger, the ioctl has an ic_cmd field of I_TRCLOG, and must be
accompanied by a data buffer containing an array of one or more struct
trace_ids elements. Each trace_ids structure specifies an mid, sid, and
level from which message will be accepted. strlog will accept messages
whose mid and sid exactly match those in the trace_ids structure, and
whose level is less than or equal to the level given in the trace_ids
structure. A value of -1 in any of the fields of the trace_ids structure
indicates that any value is accepted for that field.
Once the logger process has identified itself via the ioctl call, log
will begin sending up messages subject to the restrictions noted above.
These messages are obtained via the getmsg(2) system call. The control
part of this message contains a log_ctl structure, which specifies the
mid, sid, level, flags, time in ticks since boot that the message was
submitted, the corresponding time in seconds since Jan. 1, 1970, a
sequence number, and a priority. The time in seconds since 1970 is
provided so that the date and time of the message can be easily computed,
and the time in ticks since boot is provided so that the relative timing
of log messages can be determined.
The priority is comprised of a priority code and a facility code, found
in <sys/syslog.h>. If SL_CONSOLE is set in flags, the priority code is
set as follows. If SL_WARN is set, the priority code is set to
LOG_WARNING. If SL_FATAL is set, the priority code is set to LOG_CRIT.
If SL_ERROR is set, the priority code is set to LOG_ERR. If SL_NOTE is
set, the priority code is set to LOG_NOTICE. If SL_TRACE is set, the
priority code is set to LOG_DEBUG. If only SL_CONSOLE is set, the
priority code is set to LOG_INFO. Messages originating from the kernel
have the facility code set to LOG_KERN. Most messages originating from
user processes will have the facility code set to LOG_USER.
Different sequence numbers are maintained for the error and trace logging
streams, and are provided so that gaps in the sequence of messages can be
determined (during times of high message traffic some messages may not be
delivered by the logger to avoid hogging system resources). The data
part of the message contains the unexpanded text of the format string
(null terminated), followed by NLOGARGS words for the arguments to the
format string, aligned on the first word boundary following the format
A process may also send a message of the same structure to log, even if
it is not an error or trace logger. The only fields of the log_ctl
structure in the control part of the message that are accepted are the
level, flags, and pri fields; all other fields are filled in by log
before being forwarded to the appropriate logger. The data portion must
contain a null terminated format string, and any arguments (up to
NLOGARGS) must be packed one word each, on the next word boundary
following the end of the format string.
ENXIO is returned for I_TRCLOG ioctls without any trace_ids structures,
or for any unrecognized I_STR ioctl calls. Incorrectly formatted log
messages sent to the driver by a user process are silently ignored (no
Processes that wish to write a message to the console logger may direct
their output to /dev/conslog, using either write(2) or putmsg(2).
Example of I_ERRLOG notification.
struct strioctl ioc;
ioc.ic_cmd = I_ERRLOG;
ioc.ic_timout = 0; /* default timeout (15 secs.) */
ioc.ic_len = 0;
ioc.ic_dp = NULL;
ioctl(log, I_STR, &ioc);
Example of I_TRCLOG notification.
struct trace_ids tid;
tid.ti_mid = 2;
tid.ti_sid = 0;
tid.ti_level = 1;
tid.ti_mid = 1002;
tid.ti_sid = -1; /* any sub-id will be allowed */
tid.ti_level = -1; /* any level will be allowed */
ioc.ic_cmd = I_TRCLOG;
ioc.ic_timout = 0;
ioc.ic_len = 2 * sizeof(struct trace_ids);
ioc.ic_dp = (char *)tid;
ioctl(log, I_STR, &ioc);
Example of submitting a log message (no arguments).
struct strbuf ctl, dat;
struct log_ctl lc;
char *message = "Don't forget to pick up some milk
on the way home";
ctl.len = ctl.maxlen = sizeof(lc);
ctl.buf = (char *)&lc;
dat.len = dat.maxlen = strlen(message);
dat.buf = message;
lc.level = 0;
lc.flags = SL_ERROR|SL_NOTIFY;
putmsg(log, &ctl, &dat, 0);
strace(1M), strerr(1M), getmsg(2), intro(2), putmsg(2), write(2),
The log driver high and low water marks are tunable via the master file.
PPPPaaaaggggeeee 4444 [ Back ]