syslog(3) Library Functions Manual syslog(3)

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

Standard C library (libc, -lc)

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


    Since glibc 2.19:
    glibc 2.19 and earlier:

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(). The values that may be specified for option and facility are described 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 together a facility value and a level value (described below). If no facility value is ORed into priority, then the default value set by openlog() is used, or, if there was no preceding openlog() call, a default of LOG_USER is employed.

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). The format string need not include a terminating newline character.

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.

closelog() closes the file descriptor being used to write to the system logger. The use of closelog() is optional.

Values for option

The option argument to openlog() is a bit mask constructed by ORing together any of the following values:

Write directly to the system console if there is an error while sending to the system logger.
Open the connection immediately (normally, the connection is opened when the first message is logged). This may be useful, for example, if a subsequent chroot(2) would make the pathname used internally by the logging facility unreachable.
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.) Also log the message to stderr.
Include the caller's PID with each message.

Values for facility

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.

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)
reserved for local use
line printer subsystem
mail subsystem
USENET news subsystem
messages generated internally by syslogd(8)
generic user-level messages
UUCP subsystem

Values for level

This 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.

For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
openlog (), closelog () Thread safety MT-Safe
syslog (), vsyslog () Thread safety MT-Safe env locale


4.2BSD, SUSv2, POSIX.1-2001.
4.3BSD, SUSv2, 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 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);

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

2024-05-02 Linux man-pages 6.9.1