SYSLOG(3)

NAME
syslog, syslog_r, vsyslog, vsyslog_r, syslogp, syslogp_r, vsyslogp, vsyslogp_r, openlog, openlog_r, closelog, closelog_r, setlogmask, setlogmask_r - control system log

LIBRARY
libc

SYNOPSIS
#include <syslog.h>
void
syslog(int priority, const char *message, ...);
void
syslog_r(int priority, struct syslog_data *data, const char *message, ...);
void
syslogp(int priority, const char *msgid, const char *sdfmt, const char *message, ...);
void
syslogp_r(int priority, struct syslog_data *data, const char *msgid, const char *sdfmt, const char *message, ...);
void
openlog(const char *ident, int logopt, int facility);
void
openlog_r(const char *ident, int logopt, int facility, struct syslog_data *data);
void
closelog(void);
void
closelog_r(struct syslog_data *data);
int
setlogmask(int maskpri);
int
setlogmask_r(int maskpri, struct syslog_data *data);
#include <stdarg.h>
void
vsyslog(int priority, const char *message, va_list args);
void
vsyslog_r(int priority, struct syslog_data *data, const char *message, va_list args);
void
vsyslogp(int priority, const char *msgid, const char *sdfmt, const char *message, va_list args);
void
vsyslogp_r(int priority, struct syslog_data *data, const char *msgid, const char *sdfmt, const char *message, va_list args);

DESCRIPTION
The syslog() function writes message to the system message logger. The message is then written to the system console, log files, logged-in users, or forwarded to other machines as appropriate (see syslogd(8)).

The message is identical to a printf(3) format string, except that ‘%m’ is replaced by the current error message. (As denoted by the global variable errno; see strerror(3).) A trailing newline is added if none is present.

The syslog_r() function is a multithread-safe version of the syslog() function. It takes a pointer to a syslog_data structure which is used to store information. This parameter must be initialized before syslog_r() is called. The SYSLOG_DATA_INIT constant is used for this purpose. The syslog_data structure and the SYSLOG_DATA_INIT constant are defined as:

    struct syslog_data {
            int             log_file;
            int             connected;
            int             opened;
            int             log_stat;
            const char     *log_tag;
            int             log_fac;
            int             log_mask;
    };
    #define SYSLOG_DATA_INIT { \
        .log_file = -1, \
        .log_fac = LOG_USER, \
        .log_mask = 0xff, \
    }

The structure is composed of the following elements:

log_file
    contains the file descriptor of the file where the message is logged

connected
    indicates if connect has been done

opened
    indicates if openlog_r() has been called

log_stat
    status bits, set by openlog_r()

log_tag
    string to tag the entry with

log_fac
    facility code

log_mask
    mask of priorities to be logged

The vsyslog() function is an alternative form in which the arguments have already been captured using the variable-length argument facilities of stdarg(3).

The syslogp() variants take additional arguments which correspond to new fields in the syslog-protocol message format. All three arguments are evaluated as printf(3) format strings and any of them can be NULL. This enables applications to use message IDs, structured data, and UTF-8 encoded content in messages.

The message is tagged with priority. Priorities are encoded as a facility and a level. The facility describes the part of the system generating the message. The level is selected from the following ordered (high to low) list:

LOG_EMERG
    A panic condition. This is normally broadcast to all users.

LOG_ALERT
    A condition that should be corrected immediately, such as a corrupted system database.

LOG_CRIT
    Critical conditions, e.g., hard device errors.

LOG_ERR
    Errors.

LOG_WARNING
    Warning messages.

LOG_NOTICE
    Conditions that are not error conditions, but should possibly be handled specially.

LOG_INFO
    Informational messages.

LOG_DEBUG
    Messages that contain information normally of use only when debugging a program.

The vsyslog_r() is used the same way as vsyslog() except that it takes an additional pointer to a syslog_data structure. It is a multithread-safe version of the vsyslog() function described above.

The openlog() function provides for more specialized processing of the messages sent by syslog() and vsyslog(). The parameter ident is a string that will be prepended to every message. The logopt argument is a bit field specifying logging options, which is formed by OR'ing one or more of the following values:

LOG_CONS
    If syslog() cannot pass the message to syslogd(8) it will attempt to write the message to the console (“/dev/console”).

LOG_NDELAY
    Open the connection to syslogd(8) immediately. Normally the open is delayed until the first message is logged. Useful for programs that need to manage the order in which file descriptors are allocated.

LOG_NLOG
    Stops syslog from writing to the system log. Only useful with LOG_PERROR.

LOG_PERROR
    Write the message to standard error output as well to the system log.

LOG_PID
    Log the process id with each message: useful for identifying instantiations of daemons. (This PID is placed within brackets between the ident and the message.)

LOG_PTRIM
    Trim anything syslog added to the message before writing to standard error output.

The facility parameter encodes a default facility to be assigned to all messages that do not have an explicit facility encoded:

LOG_AUTH
    The authorization system: login(1), su(1), getty(8), etc.

LOG_AUTHPRIV
    The same as LOG_AUTH, but logged to a file readable only by selected individuals.

LOG_CRON
    The cron daemon: cron(8).

LOG_DAEMON
    System daemons, such as routed(8), that are not provided for explicitly by other facilities.

LOG_FTP
    The file transfer protocol daemon: ftpd(8).

LOG_KERN
    Messages generated by the kernel. These cannot be generated by any user processes.

LOG_LPR
    The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.

LOG_MAIL
    The mail system.

LOG_NEWS
    The network news system.

LOG_SYSLOG
    Messages generated internally by syslogd(8).

LOG_USER
    Messages generated by random user processes. This is the default facility identifier if none is specified.

LOG_UUCP
    The uucp system.

LOG_LOCAL0
    Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.

The openlog_r() function is the multithread-safe version of the openlog() function. It takes an additional pointer to a syslog_data structure. This function must be used in conjunction with the other multithread-safe functions.

The closelog() function can be used to close the log file.

The closelog_r() does the same thing as closelog(3) but in a multithread-safe way and takes an additional pointer to a syslog_data structure.

The setlogmask() function sets the log priority mask to maskpri and returns the previous mask. Calls to syslog() with a priority not set in maskpri are rejected. The mask for an individual priority pri is calculated by the macro LOG_MASK(pri); the mask for all priorities up to and including toppri is given by the macro LOG_UPTO(toppri). The default allows all priorities to be logged.

The setlogmask_r() function is the multithread-safe version of setlogmask(). It takes an additional pointer to a syslog_data structure.

RETURN VALUES
The routines closelog(), closelog_r(), openlog(), openlog_r(), syslog(), syslog_r(), vsyslog(), vsyslog_r(), syslogp(), syslogp_r(), vsyslogp(), and vsyslogp_r() return no value.

The routines setlogmask() and setlogmask_r() always return the previous log mask level.

EXAMPLES
    syslog(LOG_ALERT, "who: internal error 23");
    openlog("ftpd", LOG_PID | LOG_NDELAY, LOG_FTP);
    setlogmask(LOG_UPTO(LOG_ERR));
    syslog(LOG_INFO, "Connection from host %d", CallingHost);
    syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
    syslogp(LOG_INFO|LOG_LOCAL2, NULL, NULL, "foobar error: %m");
    syslogp(LOG_INFO, "ID%d", "[meta language=\"en-US\"]",
            "event: %s", 42, EventDescription);

For the multithread-safe functions:

    struct syslog_data sdata = SYSLOG_DATA_INIT;
    syslog_r(LOG_INFO|LOG_LOCAL2, &sdata, "foobar error: %m");

SEE ALSO
logger(1), syslogd(8) RFC, 3164, August 2001, The BSD syslog Protocol. Internet-Draft, draft-ietf-syslog-protocol-23, September 2007, The syslog Protocol.

HISTORY
These non-multithread-safe functions appeared in 4.2BSD. The multithread-safe functions appeared in OpenBSD 3.1 and then in NetBSD 4.0. The async-signal-safe functions appeared in NetBSD 4.0. The syslog-protocol functions appeared in NetBSD 5.0.

CAVEATS
It is important never to pass a string with user-supplied data as a format without using ‘%s’. An attacker can put format specifiers in the string to mangle your stack, leading to a possible security hole. This holds true even if you have built the string “by hand” using a function like snprintf(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by syslog().

Always be sure to use the proper secure idiom:

    syslog(priority, "%s", string);

With syslogp() the caller is responsible to use the right formatting for the message fields. A msgid must only contain up to 32 ASCII characters. A sdfmt has strict rules for parenthesis and character quoting. If the msgfmt contains UTF-8 characters, then it has to start with a Byte Order Mark.