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log is implemented as a cloneable device, it clones itself without intervention from the system clone device. 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 using 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.
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struct trace_ids {
short ti_mid;
short ti_sid;
char ti_level;
};
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Each trace_ids structure specifies a mid, sid, and level from which messages will be accepted. strlog(9F) 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 using the ioctl call, log will begin sending up messages subject to the restrictions noted above. These messages
are obtained using the getmsg(2) function. 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.
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struct log_ctl {
short mid;
short sid;
char level; /* level of message for tracing */
short flags; /* message disposition */
#if defined(_LP64) || defined(_I32LPx)
clock32_t ltime; /* time in machine ticks since boot */
time32_t ttime; /* time in seconds since 1970 */
#else
clock_t ltime;
time_t ttime;
#endif
int seq_no; /* sequence number */
int pri; /* priority = (facility|level) */
};
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The priority consists 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 string.
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, 32-bits
each, on the next 32-bit boundary following the end of the format string.
ENXIO is returned for I_TRCLOG ioctls without any trace_ids structures, or for any unrecognized ioctl calls. The driver silently ignores incorrectly formatted log messages sent to the driver by a user process (no error results).
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).
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