SYSLOG(3)

SYSLOG(3) | Library Functions Manual | 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

Standard C Library (libc, -lc)

# 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_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.)

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)

The BSD syslog Protocol, RFC, 3164, August 2001.

The syslog Protocol, Internet-Draft, draft-ietf-syslog-protocol-23, September 2007.

# 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 paranthesis and character quoting. If the msgfmt contains UTF-8 characters, then it has to start with a Byte Order Mark.

May 3, 2010 | NetBSD 7.1