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closelog, openlog, syslog - send messages to the system logger
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void vsyslog(int priority, const char *format, va_list ap);
closelog() closes the descriptor being used to write to the system
logger. The use of closelog() is optional.
openlog() opens a connection to the system logger for a program. The
string pointed to by ident is prepended to every message, and is
typically set to the program name. The option argument specifies flags
which control the operation of openlog() and subsequent calls to
syslog(). The facility argument establishes a default to be used if
none is specified in subsequent calls to syslog(). Values for option
and facility are given below. The use of openlog() is optional; it
will automatically be called by syslog() if necessary, in which case
ident will default to NULL.
syslog() generates a log message, which will be distributed by
syslogd(8). The priority argument is formed by ORing the facility and
the level values (explained below). The remaining arguments are a
format, as in printf(3) and any arguments required by the format,
except that the two character sequence %m will be replaced by the error
message string strerror(errno). A trailing newline is added when
The function vsyslog() performs the same task as syslog() with the
difference that it takes a set of arguments which have been obtained
using the stdarg(3) variable argument list macros.
This section lists the parameters used to set the values of option,
facility, and priority.
The option argument to openlog() is an OR of any of these:
Write directly to system console if there is an error while
sending to system logger.
Open the connection immediately (normally, the connection is
opened when the first message is logged).
Don’t wait for child processes that may have been created while
logging the message. (The GNU C library does not create a child
process, so this option has no effect on Linux.)
The converse of LOG_NDELAY; opening of the connection is delayed
until syslog() is called. (This is the default, and need not be
(Not in SUSv3.) Print to stderr as well.
Include PID with each message.
The facility argument is used to specify what type of program is
logging the message. This lets the configuration file specify that
messages from different facilities will be handled differently.
security/authorization messages (DEPRECATED Use LOG_AUTHPRIV
security/authorization messages (private)
clock daemon (cron and at)
system daemons without separate facility value
LOG_LOCAL0 through LOG_LOCAL7
reserved for local use
line printer subsystem
USENET news subsystem
messages generated internally by syslogd
generic user-level messages
This determines the importance of the message. The levels are, in
order of decreasing importance:
system is unusable
action must be taken immediately
normal, but significant, condition
The function setlogmask(3) can be used to restrict logging to specified
The functions openlog(), closelog(), and syslog() (but not vsyslog())
are specified in SUSv2 and POSIX 1003.1-2001. POSIX 1003.1-2001
specifies only the LOG_USER and LOG_LOCAL* values for facility.
However, with the exception of LOG_AUTHPRIV and LOG_FTP, the other
facility values appear on most Unix systems. The LOG_PERROR value for
option is not specified by POSIX 1003.1-2001, but is available in most
versions of Unix.
A syslog() function call appeared in 4.2BSD. 4.3BSD documents
openlog(), syslog(), closelog(), and setlogmask(). 4.3BSD-Reno also
documents vsyslog(). Of course early v* functions used the <varargs.h>
mechanism, which is not compatible with <stdarg.h>.
The parameter ident in the call of openlog() is probably stored as-is.
Thus, if the string it points to is changed, syslog() may start
prepending the changed string, and if the string it points to ceases to
exist, the results are undefined. Most portable is to use a string
Never pass a string with user-supplied data as a format, use
syslog(priority, "%s", string);
logger(1), setlogmask(3), syslog.conf(5), syslogd(8)