openlog(3P, 3p) | open a connection to the logging facility |
syslog, closelog, openlog, vsyslog(3) | send messages to the system logger |
closelog, openlog, setlogmask, syslog(3P, 3p) | control system log |
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);
vsyslog():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_BSD_SOURCE
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.
The option argument to openlog() is a bit mask constructed by ORing together any of the following values:
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.
This determines the importance of the message. The levels are, in order of decreasing importance:
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 |
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)
2023-07-20 | Linux man-pages 6.05.01 |