syslog(3) -- Linux man page
NAMEcloselog, openlog, syslog - send messages to the system logger
void openlog(const char *ident, int option, int facility);
void syslog(int priority, const char *format, ...);
DESCRIPTIONcloselog() 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 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.
PARAMETERSThis section lists the parameters used to set the values of option, facility, and priority.
optionThe 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 specified.)
- (Not in SUSv3.) Print to stderr as well.
- Include PID with each message.
facilityThe 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 instead)
- security/authorization messages (private)
- clock daemon (cron and at)
- system daemons without separate facility value
- ftp daemon
- kernel messages
- LOG_LOCAL0 through LOG_LOCAL7
- reserved for local use
- line printer subsystem
- mail subsystem
- USENET news subsystem
- messages generated internally by syslogd
- LOG_USER (default)
- generic user-level messages
levelThis determines the importance of the message. The levels are, in order of decreasing importance:
- system is unusable
- action must be taken immediately
- critical conditions
- error conditions
- warning conditions
- normal, but significant, condition
- informational message
- debug-level message
The function setlogmask(3) can be used to restrict logging to specified levels only.
CONFORMING TOThe 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.
HISTORYA 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>.
NOTESThe 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 constant.
Never pass a string with user-supplied data as a format, use
- syslog(priority, "%s", string);
SEE ALSOlogger(1), setlogmask(3), syslog.conf(5), syslogd(8)