The
syslog() function writes
message to the system message logger. The message is then written to the system console, log files, logged-in users, or forwarded to other machines as appropriate (see
syslogd(8)).
The message is identical to a
printf(3) format string, except that ‘%m' is replaced by the current error message. (As denoted by the global variable
errno; see
strerror(3).) A trailing newline is added if none is present.
The
syslog_r() function is a multithread-safe version of the
syslog() function. It takes a pointer to a
syslog_data structure which is used to store information. This parameter must be initialized before
syslog_r() is called. The
SYSLOG_DATA_INIT constant is used for this purpose. The
syslog_data structure and the
SYSLOG_DATA_INIT constant are defined as:
struct syslog_data {
int log_file;
int connected;
int opened;
int log_stat;
const char *log_tag;
int log_fac;
int log_mask;
};
#define SYSLOG_DATA_INIT { \
.log_file = -1, \
.log_fac = LOG_USER, \
.log_mask = 0xff, \
}
The structure is composed of the following elements:
log_file
contains the file descriptor of the file where the message is logged
connected
indicates if connect has been done
opened
indicates if openlog_r() has been called
log_stat
status bits, set by openlog_r()
log_tag
string to tag the entry with
log_mask
mask of priorities to be logged
The
vsyslog() function is an alternative form in which the arguments have already been captured using the variable-length argument facilities of
varargs(3).
The
syslogp() variants take additional arguments which correspond to new fields in the syslog-protocol message format. All three arguments are evaluated as
printf(3) format strings and any of them can be
NULL. This enables applications to use message IDs, structured data, and UTF-8 encoded content in messages.
The message is tagged with
priority. Priorities are encoded as a
facility and a
level. The facility describes the part of the system generating the message. The level is selected from the following
ordered (high to low) list:
LOG_EMERG
A panic condition. This is normally broadcast to all users.
LOG_ALERT
A condition that should be corrected immediately, such as a corrupted system database.
LOG_CRIT
Critical conditions, e.g., hard device errors.
LOG_WARNING
Warning messages.
LOG_NOTICE
Conditions that are not error conditions, but should possibly be handled specially.
LOG_INFO
Informational messages.
LOG_DEBUG
Messages that contain information normally of use only when debugging a program.
The
vsyslog_r() is used the same way as
vsyslog() except that it takes an additional pointer to a
syslog_data structure. It is a multithread-safe version of the
vsyslog() function described above.
The
openlog() function provides for more specialized processing of the messages sent by
syslog() and
vsyslog(). The parameter
ident is a string that will be prepended to every message. The
logopt argument is a bit field specifying logging options, which is formed by OR'ing one or more of the following values:
LOG_CONS
If
syslog() cannot pass the message to
syslogd(8) it will attempt to write the message to the console (“
/dev/console”).
LOG_NDELAY
Open the connection to
syslogd(8) immediately. Normally the open is delayed until the first message is logged. Useful for programs that need to manage the order in which file descriptors are allocated.
LOG_PERROR
Write the message to standard error output as well to the system log.
LOG_PID
Log the process id with each message: useful for identifying instantiations of daemons. (This PID is placed within brackets between the ident and the message.)
The
facility parameter encodes a default facility to be assigned to all messages that do not have an explicit facility encoded:
LOG_AUTHPRIV
The same as LOG_AUTH, but logged to a file readable only by selected individuals.
LOG_DAEMON
System daemons, such as
routed(8), that are not provided for explicitly by other facilities.
LOG_FTP
The file transfer protocol daemon:
ftpd(8).
LOG_KERN
Messages generated by the kernel. These cannot be generated by any user processes.
LOG_MAIL
The mail system.
LOG_NEWS
The network news system.
LOG_USER
Messages generated by random user processes. This is the default facility identifier if none is specified.
LOG_UUCP
The uucp system.
LOG_LOCAL0
Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.
The
openlog_r() function is the multithread-safe version of the
openlog() function. It takes an additional pointer to a
syslog_data structure. This function must be used in conjunction with the other multithread-safe functions.
The
closelog() function can be used to close the log file.
The
closelog_r() does the same thing as
closelog(3) but in a multithread-safe way and takes an additional pointer to a
syslog_data structure.
The
setlogmask() function sets the log priority mask to
maskpri and returns the previous mask. Calls to
syslog() with a priority not set in
maskpri are rejected. The mask for an individual priority
pri is calculated by the macro
LOG_MASK(
pri); the mask for all priorities up to and including
toppri is given by the macro
LOG_UPTO(
toppri). The default allows all priorities to be logged.
The
setlogmask_r() function is the multithread-safe version of
setlogmask(). It takes an additional pointer to a
syslog_data structure.