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Man Pages
SYSLOG(3) FreeBSD Library Functions Manual SYSLOG(3)

syslog, vsyslog, openlog, closelog, setlogmask
control system log

Standard C Library (libc, -lc)

#include <syslog.h>
#include <stdarg.h>
void
syslog(int priority, const char *message, ...);
void
vsyslog(int priority, const char *message, va_list args);
void
openlog(const char *ident, int logopt, int facility);
void
closelog(void);
int
setlogmask(int maskpri);

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 vsyslog() function is an alternate form in which the arguments have already been captured using the variable-length argument facilities of stdarg(3).
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:
A panic condition. This is normally broadcast to all users.
A condition that should be corrected immediately, such as a corrupted system database.
Critical conditions, e.g., hard device errors.
Errors.
Warning messages.
Conditions that are not error conditions, but should possibly be handled specially.
Informational messages.
Messages that contain information normally of use only when debugging a program.
The openlog() function provides for more specialized processing of the messages sent by syslog() and vsyslog(). The ident argument 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:
If syslog() cannot pass the message to syslogd(8) it will attempt to write the message to the console (“/dev/console”).
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.
Write the message to standard error output as well to the system log.
Log the process id with each message: useful for identifying instantiations of daemons. On FreeBSD, this option is enabled by default.
The facility argument encodes a default facility to be assigned to all messages that do not have an explicit facility encoded:
The authorization system: login(1), su(1), getty(8), etc.
The same as LOG_AUTH, but logged to a file readable only by selected individuals.
Messages written to /dev/console by the kernel console output driver.
The cron daemon: cron(8).
System daemons, such as routed(8), that are not provided for explicitly by other facilities.
The file transfer protocol daemons: ftpd(8), tftpd(8).
Messages generated by the kernel. These cannot be generated by any user processes.
The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.
The mail system.
The network news system.
The network time protocol system.
Security subsystems, such as ipfw(4).
Messages generated internally by syslogd(8).
Messages generated by random user processes. This is the default facility identifier if none is specified.
The uucp system.
Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.
The closelog() function can be used to close the log file.
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 routines closelog(), openlog(), syslog() and vsyslog() return no value.
The routine setlogmask() always returns the previous log mask level.

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_ERR|LOG_LOCAL2, "foobar error: %m");

logger(1), syslogd(8)

These functions appeared in 4.2BSD.

Never 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 the string was built using a function like snprintf(), as the resulting string may still contain user-supplied conversion specifiers for later interpolation by syslog().
Always use the proper secure idiom:
syslog(priority, "%s", string);
April 12, 2018 FreeBSD 12.0-RELEASE

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