closelog, openlog, syslog, vsyslog - send messages to the system logger


#include <syslog.h>
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
void closelog(void);
#include <stdarg.h>
void vsyslog(int priority, const char *format, va_list ap);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
vsyslog(): _BSD_SOURCE


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 may be added if 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 list macros.

The subsections below list the parameters used to set the values of option, facility, and priority.
The option argument to openlog() is an OR of any of these:
LOG_CONS Write directly to system console if there is an error
while sending to system logger.
LOG_NDELAY Open the connection immediately (normally, the connec
tion is opened when the first message is logged).
LOG_NOWAIT Don't wait for child processes that may have been cre
ated while logging the message. (The GNU C library does not create a child process, so this option has no effect on Linux.)
LOG_ODELAY The converse of LOG_NDELAY; opening of the connection is
delayed until syslog() is called. (This is the default, and need not be specified.)
LOG_PERROR (Not in POSIX.1-2001.) Print to stderr as well.
LOG_PID 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.
LOG_AUTH security/authorization messages (DEPRECATED Use
LOG_AUTHPRIV security/authorization messages (private)
LOG_CRON clock daemon (cron and at)
LOG_DAEMON system daemons without separate facility value
LOG_FTP ftp daemon
LOG_KERN kernel messages (these can't be generated from user pro
reserved for local use
LOG_LPR line printer subsystem
LOG_MAIL mail subsystem
LOG_NEWS USENET news subsystem
LOG_SYSLOG messages generated internally by syslogd(8)
LOG_USER (default)
generic user-level messages
LOG_UUCP UUCP subsystem
This determines the importance of the message. The levels are, in order of decreasing importance:
LOG_EMERG system is unusable
LOG_ALERT action must be taken immediately
LOG_CRIT critical conditions
LOG_ERR error conditions
LOG_WARNING warning conditions
LOG_NOTICE normal, but significant, condition
LOG_INFO informational message
LOG_DEBUG debug-level message
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.1-2001. POSIX.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.1-2001, but is available in most versions of Unix.


The argument 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 constant.

Never pass a string with user-supplied data as a format, use the following instead:
syslog(priority, "%s", string);


logger(1), setlogmask(3), syslog.conf(5), syslogd(8)


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