SYSLOG(3) Linux Programmer's Manual SYSLOG(3)
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 for-
mat, except that the two character sequence %m will be replaced by the error message
string strerror(errno). A trailing newline is added when needed.
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
This section lists the parameters used to set the values of option, facility, and prior-
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 log-
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 mes-
sage. (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 specified.)
(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
security/authorization messages (DEPRECATED Use LOG_AUTHPRIV instead)
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
system is unusable
action must be taken immediately
normal, but significant, condition
The function setlogmask(3) can be used to restrict logging to specified levels only.
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 spec-
ified by POSIX 1003.1-2001, but is available in most versions of Unix.
A syslog function call appeared in BSD 4.2. BSD 4.3 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
Never pass a string with user-supplied data as a format, use
logger(1), setlogmask(3), syslog.conf(5), syslogd(8)
Linux 2002-01-03 SYSLOG(3)