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. If ident is NULL, the program name is used. (POSIX.1-2008 does not specify the behavior when ident is NULL.)
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 connection is opened when the first message is logged).
LOG_NOWAIT
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.)
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 or POSIX.1-2008.) 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
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 processes)
LOG_LOCAL0 through LOG_LOCAL7
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, POSIX.1-2001, and POSIX.1-2008. 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 or POSIX.1-2008, 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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