Manpage of SYSLOG
SYSLOGSection: Linux Programmer's Manual (3)
NAMEcloselog, openlog, syslog, vsyslog - 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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
DESCRIPTIONcloselog() closes the file 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 identis prepended to every message, and is typically set to the program name. If identis NULL, the program name is used. (POSIX.1-2008 does not specify the behavior when identis NULL.)
The optionargument specifies flags which control the operation of openlog() and subsequent calls to syslog(). The facilityargument establishes a default to be used if none is specified in subsequent calls to syslog(). Values for optionand facilityare given below. The use of openlog() is optional; it will automatically be called by syslog() if necessary, in which case identwill default to NULL.
syslog() generates a log message, which will be distributed by syslogd(8). The priorityargument is formed by ORing the facilityand the levelvalues (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 %mwill 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.
optionThe optionargument 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 POSIX.1-2001 or POSIX.1-2008.) Print to stderr as well.
- Include PID with each message.
facilityThe facilityargument 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
- security/authorization messages (private)
- clock daemon (cron and at)
- system daemons without separate facility value
- ftp daemon
- kernel messages (these can't be generated from user processes)
- LOG_LOCAL0 through LOG_LOCAL7
- reserved for local use
- line printer subsystem
- mail subsystem
- USENET news subsystem
- messages generated internally by syslogd(8)
- LOG_USER (default)
- generic user-level messages
- UUCP subsystem
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.
ATTRIBUTESFor an explanation of the terms used in this section, see attributes(7).
|openlog(), closelog()||Thread safety||MT-Safe|
|syslog(), vsyslog()||Thread safety||MT-Safe env locale|
CONFORMING TOThe 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_USERand LOG_LOCAL*values for facility. However, with the exception of LOG_AUTHPRIVand LOG_FTP, the other facilityvalues appear on most UNIX systems. The LOG_PERRORvalue for optionis not specified by POSIX.1-2001 or POSIX.1-2008, but is available in most versions of UNIX.
NOTESThe argument identin 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);
SEE ALSOlogger(1), setlogmask(3), syslog.conf(5), syslogd(8)
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